Man oh man! You’re explaining a lesson I wished that I had learned the easy way. Close to 20 years ago I built a SBC for a 1969 SS Camaro convertible. Very nice restoration. Engine had to be built to look exactly like it was off of the show room. So no HEI conversion. What the customer elected to do was a Pertronics conversion in a dual point housing. The distributor was a “rebuilt” brand from a popular parts store here in the U.S. The conversion kit was straight forward and looked very clean. Where I got into trouble as the engine builder….. When I built the engine, degreeing the cam off of top dead center from the positive stop method, I noticed that the timing reference on the balancer was off near 45 degrees from the factory timing pointer. The harmonic balancer was brand new but some stock form of aftermarket. I did not purchase this for the build it was supplied with the rest of the parts. I’ll further explain that it’s been my long term policy to lock the crankshaft down after the cam is degreed in and verified. Then I will close everything up around the front cover and install the balancer and verify timing reference. Just for this reason. Ensuring proper ignition timing. So in this case. I taped the balancer to paint a mark in line with the timing pointer and painted a nice bright white stripe on the balancer to correct. Big mistake!! So the engine is fired up. Brand new transmission from a reputable transmission shop that supplied everything surrounding. When we filled the transmission with fluid. A very loud noise began. It was the flex plate being pushed into the transmission inspection cover and the shift linkage had issues to be worked out. The car owner was standing right there and elected to take the car to the transmission shop right then and there. We managed to set timing and check for leaks but the engine did not get much running time. Maybe 5 minutes and I was not comfortable with the situation either way. A few days later I get a call at work from “the transmission guru”. He just wanted to tell me that my timing mark was off about 15 degrees. So he fixed that for me. I was busy with work and didn’t really have time to realize just what had been done. When it gets out of the shop the car owner called me to inform me that the engine was running hot. No problem, I’ll set the timing and check everything out. When time comes to set timing and I’m looking for my painted timing mark, it’s gone. That transmission guy took it upon himself to strip the painted timing reference off of the balancer and repaint the balancer. He then hastily remarked the balancer with a piece of chalk you could barely see. The explanation on how transmission guru determined top dead center??? Not worth mention the guy was screwed up in the head and just screwed me over big time. So now I’m trying to set timing based roughly off of feeling for compression and doing my best to get a mark on the balancer. It was mess everything in the way. All culminated into uncertainty. When I tried to set timing the initial was set around 8 degrees and when I looked at the total timing I was well passed 45 degrees. I thought the vacuum advance was still hooked up. It wasn’t. Where is all of this advance coming from? I was in a hand wringing situation. The car owner was very understanding. They could see and knew enough about the situation to realize that we were out in left field for referencing. They took the car home with them with the plans to disassemble the front end of the engine and reestablish TDC reference later. I took my problems to work and explained them to my boss. A very wise engine builder. He immediately asks me why I didn’t use the mill or something else to make a permanent mark on the balancer. I had the tools available to do so and didn’t. But where was all of this extra advance coming from. It’s all brand new store bought rebuilt?? The answer came a few weeks later on the engine dyno at work. Another engine. This time a 327. Same popular parts store “rebuild” dual point with an electronic conversion kit. This time the timing marks were right and not tampered with. This engine was hitting near 52 degrees with no vacuum advance. Mr. Boyd calls me into the dyno room and asks me if this is what I was seeing with my project?? Exactly. Same brand distributor from the same parts store doing the same thing. So he has me pull this distributor and he goes at disassembly. The mechanical advance plates!!! The stop pins had deep wear grooves. The plates the stop pins were mating to, worn out! He calls popular parts store and explains. Their response….. No way! For sure for certain. It was. It was the same on the distributor that had plagued me with issues. Moral of the story. Permanently mark your timing references. The uncertainty that little detail created was more than I want to ever deal with again. For those that set timing by ear. It isn’t happening when your mechanical advance is in this condition. You’re flat lost with a slow boiler that will never cool down. My apologies for the long winded this one I felt was just too important.
Easy to find TDC with a spark plug style piston stop. Turn motor each direction till piston stops, mark the balancer at those points. TDC is right in the middle of your marks. Found many spun balancers this way from 5° to 30° off
It’s all easy until it’s in a fully dressed engine compartment. Not every valve combustion chamber and valve diameter will allow for the method you described. Screw a positive stop down a spark plug hole deep enough with a valve head diameter large enough and you’re going to bend a valve. It’s always easy until details are missed or ignored. Then it’s a real problem. I’ve seen this bite the best of them. It can and has destroyed engines.
@@hughobrien4139 Usually the positive stop method works well, you just to use a little common sense and not use the longest ratchet you have and then use all of your force on it. In this example being a SBC you can use either cylinder 1 or 6 which should take care of most clearance issues. At the extreme worst case situation buy a quality aftermarket balancer that is fully marked (not something from ebay, speedmaster, or the interweb at all, any decent engine builder can supply you with a good source for this. Also its pretty easy to visually inspect the balancer, the keyway lines up with TDC.
Great explanation! Thank you Mr. Gold for the walk-through on vacuum advance with the visuals (excellent graph.) Side note for anyone interested: David Vizard has a very recent video that touches on the ignition subject and expands on a couple points, (manifold vacuum for the win). Again, thank you for helping us less experienced guys.
You are 100% correct on vacuum advance. It was changed to ported for emissions purposes only. It is interesting that a more efficient engine doesn't always produce the "cleanest" emissions. The only difference between manifold and ported is at idle and deceleration. At cruise they are about the same. It is alarming how many guys don't understand how vacuum advance works or why we use it.
How can you say that he is “100% correct” when he doesn’t understand how ported vacuum actually works? I agree that using manifold vacuum is the way to go, but he thinks that having the vacuum canister connected to a ported source will ADD vacuum advance at wide open throttle. Completely wrong.
@@tonysingleton8340 it's undebatable because neither side will ever change. When changing to direct manifold vacuum, it's done as part of a series of changes designed with a purpose. If a guy has a stock emissions car, retains the stock vacuum can, keeps the factory timing specs, leaves the advance springs alone.......... I'd say that guy ought to keep the vacuum ported.
TIMING AND VACUUM ADVANCE 101 by GM Engineer. The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to burn than rich mixtures; idle in particular, as idle mixture is affected by exhaust gas dilution. This requires that lean mixtures have "the fire lit" earlier in the compression cycle (spark timing advanced), allowing more burn time so that peak cylinder pressure is reached just after TDC for peak efficiency and reduced exhaust gas temperature (wasted combustion energy). Rich mixtures, on the other hand, burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency. The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the autocam mechanism. The amount of advance added by the distributor, combined with initial static timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most SBC's like). Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation. At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing setting (i.e., if your static timing is at 10 degrees, at idle it's actually around 25 degrees with the vacuum advance connected). The same thing occurs at steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway, you'd see about 50 degrees advance (10 degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum advance) at steady-state cruise (it only takes about 40 horsepower to cruise at 50mph). When you accelerate, the mixture is instantly enriched (by the accelerator pump, power valve, etc.), burns faster, doesn't need the additional spark advance, and when the throttle plates open, manifold vacuum drops, and the vacuum advance can returns to zero, retarding the spark timing back to what is provided by the initial static timing plus the centrifugal advance provided by the distributor at that engine rpm; the vacuum advance doesn't come back into play until you back off the gas and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean. The key difference is that centrifugal advance (in the distributor autocam via weights and springs) is purely rpm-sensitive; nothing changes it except changes in rpm. Vacuum advance, on the other hand, responds to engine load and rapidly-changing operating conditions, providing the correct degree of spark advance at any point in time based on engine load, to deal with both lean and rich mixture conditions. By today's terms, this was a relatively crude mechanical system, but it did a good job of optimizing engine efficiency, throttle response, fuel economy, and idle cooling, with absolutely ZERO effect on wide-open throttle performance, as vacuum advance is inoperative under wide-open throttle conditions. In modern cars with computerized engine controllers, all those sensors and the controller change both mixture and spark timing 50 to 100 times per second, and we don't even HAVE a distributor any more - it's all electronic. Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it. If you look at the centrifugal advance calibrations for these "ported spark, late-timed" engines, you'll see that instead of having 20 degrees of advance, they had up to 34 degrees of advance in the distributor, in order to get back to the 34-36 degrees "total timing" at high rpm wide-open throttle to get some of the performance back. The vacuum advance still worked at steady-state highway cruise (lean mixture = low emissions), but it was inoperative at idle, which caused all manner of problems - "ported vacuum" was strictly an early, pre-converter crude emissions strategy, and nothing more. What about the Harry high-school non-vacuum advance polished billet "whizbang" distributors you see in the Summit and Jeg's catalogs? They're JUNK on a street-driven car, but some people keep buying them because they're "race car" parts, so they must be "good for my car" - they're NOT. "Race cars" run at wide-open throttle, rich mixture, full load, and high rpm all the time, so they don't need a system (vacuum advance) to deal with the full range of driving conditions encountered in street operation. Anyone driving a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don't understand what vacuum advance is, how it works, and what it's for - there are lots of long-time experienced "mechanics" who don't understand the principles and operation of vacuum advance either, so they're not alone. Vacuum advance calibrations are different between stock engines and modified engines, especially if you have a lot of cam and have relatively low manifold vacuum at idle. Most stock vacuum advance cans aren’t fully-deployed until they see about 15” Hg. Manifold vacuum, so those cans don’t work very well on a modified engine; with less than 15” Hg. at a rough idle, the stock can will “dither” in and out in response to the rapidly-changing manifold vacuum, constantly varying the amount of vacuum advance, which creates an unstable idle. Modified engines with more cam that generate less than 15” Hg. of vacuum at idle need a vacuum advance can that’s fully-deployed at least 1”, preferably 2” of vacuum less than idle vacuum level so idle advance is solid and stable; the Echlin #VC-1810 advance can (about $10 at NAPA) provides the same amount of advance as the stock can (15 degrees), but is fully-deployed at only 8” of vacuum, so there is no variation in idle timing even with a stout cam. For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively.
Great timing curve, mechanical and vacuum advance tutorial. I was a tune up mechanic in my youth and quickly learned to move vacuum advance from ported to manifold. The smog laws in 1966 in California moved them back to ported to lower emissions. Engines ran hotter and got poor mileage and had sluggish street performance. I've converted several friends' engines to manifold vacuum advance with great results. Some are harder to start when warm since there's more advance while cranking. Opening the throttle stops that. Street driving with this makes for cooler running and better MPG. Last note: I lived in Denver for a decade, and we could run nearly 40 degrees total there with no detonation. Higher altitude makes the difference. I'm in Texas now at 700 ft and 35 degrees is my maximum total now. Thans for the fine video.
The distributor on my 67 gto had a single advance stop on the underside of the weight plate and the weights had a different shape. For some reason the gto had a rubber bushing on the stop. Aftermarket timing kits supplied a steel bushing. Chevys that I saw had the same type. Your chart shows the total timing graphed at 35 degrees not 30. In the late 60's and early 70's the recommended total timing on both small and big block Chevys was 38-42 as I remember.......
Liked the video. I'm somewhat passionate about ignition timing and have studied it extensively over the years. FTR, I've a manifold vacuum preference too. The ported vacuum as you know was a crutch used in the early smog days and was a way to try and clean up emissions. I noticed the engines you referenced with your "good book" were all pre-smog examples. Keep up the good work.
Thanks Kevin and thanks for inspiring the latest video! Unfortunately the presentation quality wasn't the best but we got the reference info out there for anyone interested to see it. AG
amazing finally after countless videos you finally made it make sense vac advance on ported because it keeps timing at idle and when there is no vac the centrifugal advance takes over the work .... never seen it that way so much easier to understand thankyou i keep carbon fouling plugs on my 383 stroker after about 30 miles so i am trying to dial it in and you helped me on this mission thankyou !
I found that sbc with comp cams 268h liked 14° base 14°mechanical 10° vacuum. Yes both advance mechinisms were modified to only allow that much. Then i used weaker springs and a adjustable vacuum can set at about 12". So total combined would never be more than 38°. I used a vacuum gauge to determin best amount of vacuum advance every 100 rpm up to about 2800 rpm. Beyond that im not concerned with economy just torque. Read your spark plugs they will tell you lots.
I agree about using the knowledge from the past. If you're building a regular production engine, naturally aspirated, someone in the past already designed one, all you have to do is follow their blueprint. I remember the old "Chevy Bible" and the Chevrolet interchange manuals, they were worth their weight in gold......no pun intended.
Nice lesson... My experience with vac advance has been that connecting to manifold provides great cruise efficiency and smoothness, but the vac cannister cannot respond quickly enough to throttle input. It fails to pull timing out quickly enough under load. You get a brief moment of spark knock while the advance plate moves back. Connected to ported vac doesn't give such issues. I have since gone to MSD programmable, which uses a GM MAP sensor and has a fully programmable curve in addition to the run curve ie: "mechanical". My 302C has about 45 degrees total dialled in at cruise, and around 25 at idle. It's a happy engine.
The vacuum advance really is important in a street vehicle, they spend most of their time at something less than half throttle and at a steady state like highway use may be considerably less. People need to realize this and be truthful with themselves and the actual application of their engine. A part throttle dyno run may show the benefits of layering more vacuum advance on top of the total mechanical in such cases, there may be free torque and fuel economy to be had and better driveability. You did hint at this, but there are numbers to be had. In modern engines there are ECUs, multiple sensors, and advance tables that play the game, but really it's not that much more functionally advanced than the good old distributor. Anyway thanks for another interesting video and congratulations on breaking the magic 10k subscribers, it must feel good for you and your team.
And GM took the ported vacuum party to the next level with Transmission Controlled Spark (TCS) in 1970. This system completely shut off vacuum advance until the engine was in top gear or 3rd gear on a 4-speed manual. Usually found on vehicles without A.I.R. (smog pump) it was combined with a 195 degree thermostat to elevate combustion temperatures to lower emissions. There was a temp sensor in the pass side cylinder to give full advance if the engine got too hot. In 71 it was modified into another system called CEC and for 72 the TCS prevented vacuum advance on the 4-speed in all gears except 4th. The good old 70' emissions!
Not to go too far off topic but this got me thinking about the other parts of the ignition system; plug wires, distributor cap, rotor, spark plugs. Recurving the distributor would be good too. Heat range of spark plugs and adjustable vacuum diaphragm. Maybe with an engine on the test stand or dyno.
I’m a 73 year old drag racer and have not been able to find a vacuum advance diaphragm that will last very long on my HEI distributor. I’m about to ready to order a HEI distributor from Progression Ignition that features the ability to program everything with your phone app.
On another note, I never knew the factory Chevy timing curves were ridiculously slow and had too much mechanical advance and not enough initial advance. I didn't know that they were this bad until you showed the factory specs. Modifying it like you have makes a big difference in getting off the line 💪
If you think that one is bad, should see a stock TBI 350 or L31 Vortec 350 advance curve. They start out negative at WOT at low rpm. The TBIs only run about 16-20* WOT total depending on applicaton and the L31s about 23-25* WOT total. Total advance on both are not in until the 4,000+ rpm range. As the engine heats up from 180F to 210F they pull another 3-4* of timing. Vortecs also lose up to another 7* in high IAT conditions. The Vortec 350 can have NEGATIVE 10* of timing at WOT at 1,000 rpm in hot weather.
I really like your take on manifold vacuum I am going to go that route, so I am going to set my total advance to 34 degrees, my question is when I read my initial timing it should be quite a bit different when the vacuum is hooked up to the distributor compared to when I unplug it, for example since I don’t have my truck with right now, let’s say I have 14 degrees of initial timing, then I plug the vacuum in I would expect to see around 19-23, keeping in mind not to let my total advance go over 34 degrees,and I should be ok to hot start, if you reply it would be much appreciated, I really like your videos, and I’m with you the guy’s from GM were no dummies 😀😀
Thanks for the question. If you have 14 degrees of initial timing and 15-20" of vacuum, then you connect your distributor to the vacuum port, your timing should increase about 20 degrees plus/minus. Remember, when the engine is under load, the vacuum will drop out and your total will be the initial plus the centrifugal/mechanical advance. Your engine loves lots of timing at low load because it takes longer for combustion. Hope this helps. AG
@@goldsgarage8236 thank you, so at 3000rpm with vacuum line on what would you expect to see timing at, with everything we said factored in, will I be over 34degrees
Very good, very interesting. I assumed you were going to discuss whether ported or manifold vacuum is correct.I have seen the issue argued in both directions.the one thing I might add is when you are at steady cruise, manifold vacuum will be a fuel milage improvement.
Congratulations on the 10K subs, it is all a testament to your great content!! Back in the 80's I worked at The Carb Shop here in BC. As such, I have a Sun distributor machine as well as a Sun osilliscope/gas analyzer too!
It’s great when you go into detail. I really learned something on that youtube about bolts. Not sure if you done one yet! But a you tube on decreeing a cam
Even emissions engines that utilize “ported” vacuum also utilize “manifold” vacuum. A thermal switch plumbed into engine coolant switches between the two depending on engine temperature. If the engine starts running hot it switches to manifold vacuum because the engine runs more efficient therefore cooler...
.. the huge variety of vacuum cans that were available from the factory and also Standard Motor Products. The amount of vacuum required to start the rod advancing varied widely. Some would be fully advanced with only 5” of vacuum. The one you need will not start advancing until 8 to 10. Also , the travel of the rod was almost always too much so you needed to make or buy a limiter tab that would go under the inner most screw the attaches the can to the distributor body. You cannot go wrong with the curves in the How to Hotrod Small Block Chevies. They are straight from the engineers. This stuff was figured out in the later 50s.
My street curve had 10* initial, 10* vacuum hooked to direct manifold vacuum and the centrifugal advance would begin just above idle and be all in by 3000 rpm. 12* in the distributor which is 24*#at the crank. 10 initial 24* centrifugal ---- 34 total Plus 10* vacuum Equals 44* at cruise. Very safe usually. There will always be some engines that defy conventional wisdom. They are called Infernal Combustion Engines for a reason.
Great information. My 65 c10 truck hooked up to manifold only but has terrible gas mileage. I wanted to hook up to vaccum but has cylinder diaphragm which has no suction at all. Was thinking of buying that hex diaphragm that you are able to adjust vaccum. My question is at idle what should i set my timing with vaccum and what should be my total timing at 3000rpm. Im currently running manifold vaccum at 12, 12 degrees before tdc and 35 total timing. My spark plug smell like fuel and gas mileage is poor. Would love to hear your suggestions. Thanks
Thanks for the question Nick. Try this, set your total timing with vacuum disconnected, say 35-36 degrees all in by 3000 RPM. Your initial timing should now be somewhere between 12-16 degrees with vacuum still disconnected, depending on your distributor. Now when you re-connect to your MANIFOLD vacuum port, you should have more than 30 degrees at idle. Your engine needs lots of advance at idle because the fuel air mixture is not dense and combustion takes longer. If the other conditions you mentioned still exist, look for other causes. I hop I understood your question correctly. AG
Good tip I've been throwing lock washers away for years, 30 years ago my boces teacher told me the same thing, friction holds bolts tight, and there not used in engines and those bolts will stay tight forever.
I love this subject, nobody ever mentions the vacuum canisters are calibrated differently, for the application. Optimum peak combustion chamber pressure occurs at 14 degrees atdc.
I added an aluminum plate to the arm of my vacuum can limiting the vacuum advance to 8 degrees. I think it originally was s 22ish degrees. With my throttle plate virtually closed, my idle was slways higher than i wanted. I was finally able to get a lower idle by redicing my vacuum advance. I think it was actually too low after the mod, so i actually opened my throttle plates slightly. I ran 13 to 14 advance, 34 total and using the stock GM weights. I used a Mr Gadket or Accell spring kit. I had heavily modified some new World Products heads, installed a mild Edelbrock performer cam and manifold. To make it pretty, i added some cast iron Sanderson block hugger manifolds and 2 1/2 dual exhaust. I spent months playing with and learning to get the timing curve right. Yup, manifold vacuum is the right way.... I don't care what other internet professionals say. It may be devicive, but it really doesnt bother me if they do it the wrong way. 😁 I had this 350 in a 73 Chevy stepside. I had it bored 30 over. I dont recall my time slips best et, but i managed 76mph in the 8th with some TA50 tires. Those didnt grip well and my converter grabbed pretty early. With so little weight in the back, this combo was so hard to launch without smoking the tires. From my et and online calcuulators, i think it was 294 HP. No race truck for sure. I never built it to race, just took it out to the track twice for curiosity. I miss my old truck. It never let me down. Did I mention happy 10,000? Yeah, but ill do it again. 😁 🎉🎉🎉🎉🎉🎉🎉🎉🎉🎉
Excellent...agreed on manifold vacuum source...working a 67 Ford 289....question...if running manifold vacuum source, do I still need to disconnect and plug vacuum source to set initial timing?...thank you in advance.
We have to find for you some better audio settings. The Mic is actually shutting off as you come to the end of a word.I don't know if it's the noise blanking or the hands free activation..love the videos and the subjects. Thanks.
I see so many guys saying to use manifold vacuum, and that ported didn't come into use until the 70s to help with smog, but all the 40s - 60s cars I have owned and/or worked on came stock with ported vacuum, except for the early 60s aluminum 215 Buick V8 and, if memory is correct, the 64 - 66 Nailheads.
Those who claim that ported vaccum was purely for emissions miss understand the whole emissions thing. Vaccum advanced emissions was first started by Ford in 1968 with the dual vaccum advanced distributor (2 vaccum advance hookups + mechanical. The primary & secondary vaccum lines hooked up to it's own diaphragm. Other then two vaccum nipples the vaccum advance can looked the same as a standard vaccum advance can externally. the dual advanced distributor (vaccum + mechanical) only had one vaccum nipple and diaphragm. The secondary vaccum advance was hooked up to ported vaccum & only kicked in during part throttle & deceleration. It would retard the timing slightly during these conditions. At WOT manifold vaccum goes to zero and tranfers over mechanical advance. The primary vaccum line was hooked up to manifold. Pre 1968 the one & only vaccum line was hooked up to ported vaccum in the vast majority of cases from the factory. Some were hooked up to manifold vaccum. Your initial timing & and carburetor adjustments handled your idling conditions pre 1968.
Hmm,I'm running a stock l31 motor with a Eddy carb. I locked the mechanical advance to keep the timing down to 36 and hooked the vacuum advance up to ported. It feels like it's got a flat spot around 40 to 50. And I can't rev past 4000.
That is a problem Old, but I doubt that it is caused by the ported vacuum advance. One way to find out, if you know you have 36 degrees total, try removing the vacuum line and see what happens. Then you could try connecting it to a manifold vacuum source. Just a suggestion. AG
I figured out what it was there's an intake leak in the back. where the china wall meets the head. I Did hook up the Eddy 750 to the full manifold port though and I noticed the motor idles alot quieter.
I didn't know factory HEI ignitions had way more than 20 degrees of mechanical advance built into them, the aftermarket HEI ignitions on the market seem to have only 20 or so, I THINK! I have had a hard time getting info on built in mechanical advance on anything. I have been using manifold vacuum advance on my mild GM performance 350, and it been working out great, but I need access to a distributor machine for the remainder. Good video.
Thanks Mike, try this! Disconnect your vacuum advance. Set your total timing at all in 3000 RPM or more. Then check your timing at idle with the vacuum still disconnected. The difference will be your mechanical or centrifugal advance. AG
So, the vacuum advance, when manifold vacuum, actually allows the engine to act much like it has a race distributor locked at 34 degrees, but without the hard starting issue because before it fires its at 4 degrees, after it starts its at 30 and the time between the drop in pressure from WOT until max mechanical advance is about a second or two at most?
Your example for vacuum can had about 16 degree of advance. I think my can had more. My car had a misfire at cruise with the can hooked up. My whole curve may have been quicker. Where do you find specs for the different cans? My 68 has a Q-jet. A lot of HEI's have a lot of advance due to EGR.
Thanks for the question Kraschs. i have a 1968 Manual, if you can tell me the specific engine i will look it up for you. Most small blocks had 15 degrees and most BB had 12 degrees. AG
My car with a 4:11 gears runs 3K RPM @ 55 MPH at part throttle crusing on the highway. How would that work with a vacuum advance? I have 17 initial advance with 18 Mechanical all in by 3K. What would your advice be?
Absolutely fine with vac advance in that scenario. It allows you to run a leaner light throttle cruise mixture and helps keep engine temp in check. You wont need as much vac advance as you would if cruising at 2000 or 2500 for eg so you need an adjustment(many have this feature, but its not difficult to make a stop/limiter of some type) The big advantage is you will also get a little better mileage when cruising which saves fuel for when you want to mash the throttle👍 there is no downside to vac advance at all.
My 73 Camaro Z28 has a mild cammed 350 in it with 3:73 gears. With the turbo 400 auto transmission it turns 3000 RPM at 60 MPH. I have the timing set up so that it has 36 degrees total mechanical advance all in by about 2900 RPM. It idles with about 14 degrees of advance with the vacuum disconnected. On top of that the vacuum advance when connected to a manifold source is now limited to about 9 degrees so the total advance at part throttle cruising is 45 to 46 degree max and the engine runs perfect. I believe the rule is that you never want to exceed 50 degrees of total advance in any scenario. It is notable that when I bought the car several years ago the timing setup was all wrong. Since I took a day to make the corrections the car runs and drives so much better and the fuel economy has significantly improved. Best part is that it was practically free to do that. Has a 180 degree thermostat with a stock radiator and it never exceeds that temperature while driving on the hottest days. Only exception is if it sits idling for a long period then it might get to 190. Can has a typical stock Hayden clutch fan setup. If you were to add about 10 degrees of vacuum advance to your setup you should see some improvements over what you are running now. There really is no down side to this.
12:24 You're one of the few that say "flame propagation" or a burn rate. So many people believe the combustion mixture explodes. As a side note: engine "ping" is a shock wave in the cylinder bouncing back and forth at the speed of sound.
I haven't ever seen a reason to lockout a distributor the total advance is the only thing to worry about always has a better response from idle starts easier and still holds accurate timing at full throttle
Good points whether you use ported or manifold vacuum for advancing, I found for my 340 that initial set at 22 degrees allowed the max advance go to 34 and using the manifold vacuum stalls the engine when I stomp it from stop. If I move it to ported, no more falling flat, trouble is that my centrifugal needs to advance quicker like about 1800 rpm. Can turning the set screw in the vacuum pot do that or what will it take?
Congratulations on reaching 10K subs! I knew that this channel would grow if you kept up the interesting content that really isn't explained elsewhere on YT.
I actually have a 1968 327 bored .04 over with a Melling Stage 3 cam(MC22398), and I'm a little confused about this timing thing. Should I set base time w/o the vacuum advance? ie., plug it off. But then what about setting total time? Should that also be done w/o vacuum advance? Because I've set the base time with the vacuum advance attached. Which is manifold vacuum. Thanks. Love the show and hope to have your input soon. Lol. It's New Years Eve. 12/31/24
Happy New Year JB. To set total timing, disconnect your vacuum advance, rev the engine until it stops advancing, (this should be about 3000 RPM) and set your total here. Usually the best results will be at about 34-36 degrees. Then let the engine idle and observe the initial timing, usually about 12-16 degrees. When you reconnect your vacuum advance you may have 30-40 degrees at idle, that is fine. Your engine needs lots of advance at idle because the flame travel is slow. When the engine is under load, the vacuum advance will drop out and your mechanical and initial advance will be in play. Hope this helps. Thanks for the question.AG
@@goldsgarage8236 so then your base time goes out the window at this point? The manufacturer suggest 12 to 14 base timing. Interesting. This is the first aftermarket cam I've involved in a build. I've done like 5. Lol. I know, rookie. It's still a hell of a challenge to get everything right. Especially when the engine doesn't belong where it's going. Haha
I have a rebuilt 350 engine and im trying to install the distributor in place but even after i macht the oil pump drive with the distributor one still have a 1/4 inch gap for some reason the distributor doesn't want to drop down all the way in????
Okay i will try that also forgot to told you the distributor it was on a 70s 350 the rebuild one im working on is a 80s still the same probably???@@goldsgarage8236
The ported vacume goes away with throttle opening just like manifold vacume. The only thing different between the two is the ported vacume is zero at idle
Put a T in the hose and run a vacume gage into the the dash area and take a drive. Both sources of vacume go away at the same rate when you get on the throttle.
Absolutely correct. Only difference between the two is one is providing a vacuum at idle and one isn’t. Once the throttle plates are cracked open they both provide the same vacuum. At wide open throttle they both stop providing vacuum. Where this nonsense started about a ported source providing vacuum at WOT I’ll never know…
I VERY MUCH prefer NO vacuum advance at idle on a modified engine, ESPECIALLY with an automatic trans. I tune for a solid idle and maximum throttle response which usually involves initial timing being just short of straining the starter motor, reaching full advance as early as possible without gas ping usually being determined by the fuel being used, and the amount of mechanical advance being determined by an advance travel limiter to achieve the amount of total advance desired. When all of that is accomplished and if I want to attempt to achieve the best fuel economy possible, then I will tinker with making use of vacuum advance. If the vehicle that you're working on is for the most part factory stock and uses a factory stock type carburetor, then it very well may work best with the additional ignition timing that manifold sourced vacuum supplies at idle speed because of lean idle/low speed a/f calibration. Not looking for argument. If your methods work for you, that's good. I have been satisfied with mine for a long time.
@mickangio16 I do the same...find the best idle quality and off idle response with the best base timing setting-provides the smallest throttle blade opening and the best idle vacume number. Then I tune the mechanical advance curve up to total advance. When the base and curve is optimized then add in about 10 degrees of vacume advance and see if it likes it at cruise. Add more if it wants it.
Good tip on so called lock washers. When these things spread open [as they like to do], surface area under the bolt head begins to disappear and trouble has arrived. .
the problem I have noticed with manifold vacuum is during tip in the timing gets retarded,throttle response seems better with ported vacuum but not every engine responds the same
A sbc 350 can make 400 hp. But a 383 can make 400 hp with more idle vacuum, and because of the increased stroke, make more low end torque. So if you're not limited by any racing rules, and you're changing the crank and pistons anyways, I would go with a 383, unless I already had the parts to do a 350.
Good points.@@yurimodin7333 though if changing the crank and pistons anyways, I would also get stroker clearanced 6" connecting rods so as to not use a small base circle cam.
Yes indeed. And that a good thing at part throttle/cruise speed, better gas mileage. I have my advance at 45 degrees for part throttle. Under wide open throttle the addition vacuum advance goes away
So a normal engine always works better with more than initial timing? Does that mean that initial timing alone is only good for starting a cold engine ?
Thanks for the question Run. Well, initial also makes up part of total timing as well. The more initial you have, the less centrifugal advance is required. AG
Thanks for the question Joe. Always set timing with he vacuum hose disconnected. Capping the port is ok to do but it doesn't change the timing setting. AG
Another point, always set total timing all in and let the idle timing be what may. For example, 36 degree at 3,000 RPM all in, If the distributor has 20 degrees of centrifugal that will give you 16 degrees initial at idle, however hook your vacuum back up and you will have 40-50 degrees at idle which is fine. Your engine need lots of timing at idle. AG
! u r right about the washers. I wondrd y they went out of shape and found loose, later. Thank you. Now, I need to go back and get rid of a few... Ha! Cheers!
I really like the lock washer lesson! I fully agree! I think those were designed back when harmonics were not fully understood or there was a knowledge deficit on how to mitigate certain applications where harmonics came into play.” So just add a spring into the equation and maybe that bolt won’t back out because of harmonic resonance.” That’s all I can reasonably figure on.
@@yurimodin7333 my timing light is flashing about 11 o'clock on the balancer. I have no timing marks on the balancer but the pointer on the chain cover only reads to 24 degrees btdc. I do have a new balancer and timing tab ordered.
New to your channel, but you should do a video on vacuum secondaries and how many people think if they dont feel their secondaries opening up it means its not working
My last bracket car was a 78 Camaro lightened a lot. Strip only so no street equipment. Pretty light. Std bore 350, Cam Dynamics 234* at .050, .480 lift hyd, cast flat tops, dbl hump heads ported per Vizard’s book, Performer intake, Holley 600 vac sec, T350, 3000 stall, 4.88 gears, 9” slicks. 12.63 at 105 was it’s best time on a t&t nite. The foot brake class was 13.00 and slower so I had to retard the time and limit the throttle pedal travel to slow it down to 13.00. - Vacuum secondaries CAN do the business !!
You are correct Shelly, and 40-50 is fine at idle or part throttle. When the throttle opens, the vacuum drops and the advance drops out.. Hope this makes sense to you. AG
you need to take your car with similar ignition timing like the one in your chart hast to a cruise and add a vacuum gage with a long hose inside the cockpit. you will see when your rpm is on 3000 where the centrifugal advance is on 34 and you close the throttle, your manifiold vacuum will go to max, therefore the vacuum advanced will add 15+ and you are at around 49 degrees btdc. BUT this seems normal and should not go over 50-52° in total. saying NOT MORE THAN 34° seems to be wrong for me....
Thanks for the comment Motor. i had to go back and watch the video to reply. I agree with you in that situation where you have max vacuum when the throttle plates are closed, and max centrifugal due to RPM. AG.
Alan, I noticed that you use generic branded Bravex HEI distributors, my question is do you find that the curve is right on the money for your builds? Thanks and keep up the good informative videos. :)
You are 100% INCORRECT about vacuum advance…Ported vacuum does NOT add ANY timing when it is at WOT because at WOT there is NO vacuum at either port UNLESS the carb is too small in which case it creates a restriction and can pull in a little timing with PORTED and MANIFOLD vacuum…and there are times when a motor idles better with PORTED vacuum… Having said that, YES, the engineers working on STOCK motors know more than you do, but they weren’t making distributors with curves for motors with as much camshaft as we run now, so you CAN’T use a 1960’s book for tuning a 2020’s motor combo. 🤦♂️ You have to be smart enough to let the motor tell you what it wants and then make the curve do that.
I tune LOTS of carbureted motors for customers and about half of them are fairly rowdy or are low compression and over cam’d and those types of combinations typically want 20-36° INITIAL timing just so they’ll idle right…vacuum advance hooked to manifold vacuum doesn’t work well on some of those motors because if the motor won’t start with any more than say 20° timing but the motor “wants” to be at 25-30° at idle, the advance will pull in more timing and make it idle higher and then when you put the trans in gear, the RPM will drop a little and the vacuum will drop a little and then the vacuum advance will pull timing and then the RPM goes down even more all of which causes a huge RPM swing from idling in Park/Neutral to being put into gear and then people blame the converter for being too tight…on rowdy combos, it’s critical that the initial timing not move when idling to putting the trans into gear.
That is an interesting question Bob. Most high performance street cams should still provide 10-15" of mercury vacuum at idle which is sufficient to advance the timing. Most high performance distributors have adjustable vacuum settings so i think it should work ok. AG
Funny story I was installing a GPS system on a CAT dozer one time and asked a shop mechanic where they kept the lock washers. He laughed out load and told me "we don't use lock washers and don't have any in the building" that was when I learned how wrong they are. LOL
They sell them split washers in grade 8, don't buy the cheap ones did not realize that until recently so will keep throwing them away unless I know they are grade 8.
Ported is more metered for the throttle. My 1984 doesn't need vacuum advance at idle. Manifold vacuum thru the dist. Will actually retard timing during acceleration.
I have always been a full port vacuum guy. I’ve worked on Small Block Chevy’s for 40 years now. The advantage is at idle. Engines will always idle better regardless of the cam with the distributor connected to the manifold vacuum on the carb. At WOT who cares neither port is worth a darn. At half throttle or cruise mode both ports will be putting out some vacuum so we are now back to the whole advantage of manifold port vacuum, idle. The tuned port was designed for emission compliance and hoping to get better gas mileage. That is it. Most of us are building street and muscle cars so this is really irrelevant. Go manifold vacuum port and your engine will idle so much better and be happier.
I do not agree with the lock washer episode I learned a lot about lock washers from motorcycle mechanics aluminum parts versus steel parts. Nowadays most lock washers are very cheap.
If you are going with 350 then why don't you go with a factory roller cam block with factory lifters ect! you could save 400+ dollars than using a link roller lifters and special front timing cover and that money saved can be for upgrading the crank for 383 or better heads.. Over achieve is never a bad thing!
Vacuum advance is primarily to fire lean stretched out molecules at steady state light part throttle . The fuel mixture is lean and needs more time to fire the fuel This benefits fuel economy . Where does light throttle occur typically 37mph to 55mph approx Vacuum needs to be measured and known at every 5mph How much advance , manifold vacuum at mph and get the highest reading u can ,at approx 14.5--15:1 air fuel ratio . Primarily vac advance manifold or ported can also depend on auto or manual g/box . The auto can have more vac adv because of the stall in the convertor ie helps engine thru stall and engine detonation is less likely . VS a manual =ported Is there some emissions reduced on deceleration using manifold connection ?? Many emissions distributors have both advance and retard cans
I recently saw Junkerup's video about this. I have nothing to add on the subject, but it's funny to see people rehash things like this and argue in the comments
Regardless of what this video tells you - subscribe as it has good content.
David vizard
I’m honoured David, an endorsement from DV is the ultimate psychic income! Thank you very much. AG
@@goldsgarage8236 An endorsement by DV, now that is something!!!!
Well if dv says to subscribe it ought to be a knowledgeable person and so I will thanks DV.
Thanks
No.
Man oh man! You’re explaining a lesson I wished that I had learned the easy way.
Close to 20 years ago I built a SBC for a 1969 SS Camaro convertible. Very nice restoration. Engine had to be built to look exactly like it was off of the show room. So no HEI conversion.
What the customer elected to do was a Pertronics conversion in a dual point housing.
The distributor was a “rebuilt” brand from a popular parts store here in the U.S.
The conversion kit was straight forward and looked very clean.
Where I got into trouble as the engine builder…..
When I built the engine, degreeing the cam off of top dead center from the positive stop method, I noticed that the timing reference on the balancer was off near 45 degrees from the factory timing pointer.
The harmonic balancer was brand new but some stock form of aftermarket. I did not purchase this for the build it was supplied with the rest of the parts.
I’ll further explain that it’s been my long term policy to lock the crankshaft down after the cam is degreed in and verified. Then I will close everything up around the front cover and install the balancer and verify timing reference. Just for this reason. Ensuring proper ignition timing.
So in this case. I taped the balancer to paint a mark in line with the timing pointer and painted a nice bright white stripe on the balancer to correct. Big mistake!!
So the engine is fired up. Brand new transmission from a reputable transmission shop that supplied everything surrounding.
When we filled the transmission with fluid. A very loud noise began. It was the flex plate being pushed into the transmission inspection cover and the shift linkage had issues to be worked out.
The car owner was standing right there and elected to take the car to the transmission shop right then and there.
We managed to set timing and check for leaks but the engine did not get much running time. Maybe 5 minutes and I was not comfortable with the situation either way.
A few days later I get a call at work from “the transmission guru”.
He just wanted to tell me that my timing mark was off about 15 degrees. So he fixed that for me.
I was busy with work and didn’t really have time to realize just what had been done.
When it gets out of the shop the car owner called me to inform me that the engine was running hot.
No problem, I’ll set the timing and check everything out.
When time comes to set timing and I’m looking for my painted timing mark, it’s gone. That transmission guy took it upon himself to strip the painted timing reference off of the balancer and repaint the balancer. He then hastily remarked the balancer with a piece of chalk you could barely see.
The explanation on how transmission guru determined top dead center??? Not worth mention the guy was screwed up in the head and just screwed me over big time.
So now I’m trying to set timing based roughly off of feeling for compression and doing my best to get a mark on the balancer. It was mess everything in the way. All culminated into uncertainty.
When I tried to set timing the initial was set around 8 degrees and when I looked at the total timing I was well passed 45 degrees. I thought the vacuum advance was still hooked up. It wasn’t.
Where is all of this advance coming from?
I was in a hand wringing situation. The car owner was very understanding. They could see and knew enough about the situation to realize that we were out in left field for referencing.
They took the car home with them with the plans to disassemble the front end of the engine and reestablish TDC reference later.
I took my problems to work and explained them to my boss. A very wise engine builder. He immediately asks me why I didn’t use the mill or something else to make a permanent mark on the balancer. I had the tools available to do so and didn’t.
But where was all of this extra advance coming from. It’s all brand new store bought rebuilt??
The answer came a few weeks later on the engine dyno at work.
Another engine. This time a 327. Same popular parts store “rebuild” dual point with an electronic conversion kit.
This time the timing marks were right and not tampered with. This engine was hitting near 52 degrees with no vacuum advance.
Mr. Boyd calls me into the dyno room and asks me if this is what I was seeing with my project??
Exactly. Same brand distributor from the same parts store doing the same thing.
So he has me pull this distributor and he goes at disassembly.
The mechanical advance plates!!! The stop pins had deep wear grooves. The plates the stop pins were mating to, worn out!
He calls popular parts store and explains. Their response….. No way!
For sure for certain. It was.
It was the same on the distributor that had plagued me with issues.
Moral of the story. Permanently mark your timing references. The uncertainty that little detail created was more than I want to ever deal with again.
For those that set timing by ear. It isn’t happening when your mechanical advance is in this condition. You’re flat lost with a slow boiler that will never cool down.
My apologies for the long winded this one I felt was just too important.
Thanks for sharing your experience Hugh. There are many ways to get in trouble around engines. AG
Easy to find TDC with a spark plug style piston stop. Turn motor each direction till piston stops, mark the balancer at those points. TDC is right in the middle of your marks. Found many spun balancers this way from 5° to 30° off
It’s all easy until it’s in a fully dressed engine compartment.
Not every valve combustion chamber and valve diameter will allow for the method you described.
Screw a positive stop down a spark plug hole deep enough with a valve head diameter large enough and you’re going to bend a valve.
It’s always easy until details are missed or ignored. Then it’s a real problem.
I’ve seen this bite the best of them.
It can and has destroyed engines.
@@hughobrien4139 Usually the positive stop method works well, you just to use a little common sense and not use the longest ratchet you have and then use all of your force on it. In this example being a SBC you can use either cylinder 1 or 6 which should take care of most clearance issues. At the extreme worst case situation buy a quality aftermarket balancer that is fully marked (not something from ebay, speedmaster, or the interweb at all, any decent engine builder can supply you with a good source for this. Also its pretty easy to visually inspect the balancer, the keyway lines up with TDC.
@@fastone371
Thanks
Great explanation! Thank you Mr. Gold for the walk-through on vacuum advance with the visuals (excellent graph.) Side note for anyone interested: David Vizard has a very recent video that touches on the ignition subject and expands on a couple points, (manifold vacuum for the win). Again, thank you for helping us less experienced guys.
Thanks for for watching and commenting. AG
You are 100% correct on vacuum advance. It was changed to ported for emissions purposes only. It is interesting that a more efficient engine doesn't always produce the "cleanest" emissions. The only difference between manifold and ported is at idle and deceleration. At cruise they are about the same. It is alarming how many guys don't understand how vacuum advance works or why we use it.
I once hooked up two vacuum guages and took my truck for a few laps around town.
Exactly the same until you let off the gas pedal.
How can you say that he is “100% correct” when he doesn’t understand how ported vacuum actually works? I agree that using manifold vacuum is the way to go, but he thinks that having the vacuum canister connected to a ported source will ADD vacuum advance at wide open throttle. Completely wrong.
@@tonysingleton8340 it's undebatable because neither side will ever change.
When changing to direct manifold vacuum, it's done as part of a series of changes designed with a purpose.
If a guy has a stock emissions car, retains the stock vacuum can, keeps the factory timing specs, leaves the advance springs alone..........
I'd say that guy ought to keep the vacuum ported.
@@tonysingleton8340typical RUclips expert, at least you understand.
You are correct Tony. Sorry for the misunderstanding. I have addressed this in a follow up video. AG
TIMING AND VACUUM ADVANCE 101 by GM Engineer.
The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to burn than rich mixtures; idle in particular, as idle mixture is affected by exhaust gas dilution. This requires that lean mixtures have "the fire lit" earlier in the compression cycle (spark timing advanced), allowing more burn time so that peak cylinder pressure is reached just after TDC for peak efficiency and reduced exhaust gas temperature (wasted combustion energy). Rich mixtures, on the other hand, burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency.
The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the autocam mechanism. The amount of advance added by the distributor, combined with initial static timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most SBC's like). Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation.
At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing setting (i.e., if your static timing is at 10 degrees, at idle it's actually around 25 degrees with the vacuum advance connected). The same thing occurs at steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway, you'd see about 50 degrees advance (10 degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum advance) at steady-state cruise (it only takes about 40 horsepower to cruise at 50mph).
When you accelerate, the mixture is instantly enriched (by the accelerator pump, power valve, etc.), burns faster, doesn't need the additional spark advance, and when the throttle plates open, manifold vacuum drops, and the vacuum advance can returns to zero, retarding the spark timing back to what is provided by the initial static timing plus the centrifugal advance provided by the distributor at that engine rpm; the vacuum advance doesn't come back into play until you back off the gas and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean.
The key difference is that centrifugal advance (in the distributor autocam via weights and springs) is purely rpm-sensitive; nothing changes it except changes in rpm. Vacuum advance, on the other hand, responds to engine load and rapidly-changing operating conditions, providing the correct degree of spark advance at any point in time based on engine load, to deal with both lean and rich mixture conditions. By today's terms, this was a relatively crude mechanical system, but it did a good job of optimizing engine efficiency, throttle response, fuel economy, and idle cooling, with absolutely ZERO effect on wide-open throttle performance, as vacuum advance is inoperative under wide-open throttle conditions. In modern cars with computerized engine controllers, all those sensors and the controller change both mixture and spark timing 50 to 100 times per second, and we don't even HAVE a distributor any more - it's all electronic.
Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it.
If you look at the centrifugal advance calibrations for these "ported spark, late-timed" engines, you'll see that instead of having 20 degrees of advance, they had up to 34 degrees of advance in the distributor, in order to get back to the 34-36 degrees "total timing" at high rpm wide-open throttle to get some of the performance back. The vacuum advance still worked at steady-state highway cruise (lean mixture = low emissions), but it was inoperative at idle, which caused all manner of problems - "ported vacuum" was strictly an early, pre-converter crude emissions strategy, and nothing more.
What about the Harry high-school non-vacuum advance polished billet "whizbang" distributors you see in the Summit and Jeg's catalogs? They're JUNK on a street-driven car, but some people keep buying them because they're "race car" parts, so they must be "good for my car" - they're NOT. "Race cars" run at wide-open throttle, rich mixture, full load, and high rpm all the time, so they don't need a system (vacuum advance) to deal with the full range of driving conditions encountered in street operation. Anyone driving a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don't understand what vacuum advance is, how it works, and what it's for - there are lots of long-time experienced "mechanics" who don't understand the principles and operation of vacuum advance either, so they're not alone.
Vacuum advance calibrations are different between stock engines and modified engines, especially if you have a lot of cam and have relatively low manifold vacuum at idle. Most stock vacuum advance cans aren’t fully-deployed until they see about 15” Hg. Manifold vacuum, so those cans don’t work very well on a modified engine; with less than 15” Hg. at a rough idle, the stock can will “dither” in and out in response to the rapidly-changing manifold vacuum, constantly varying the amount of vacuum advance, which creates an unstable idle. Modified engines with more cam that generate less than 15” Hg. of vacuum at idle need a vacuum advance can that’s fully-deployed at least 1”, preferably 2” of vacuum less than idle vacuum level so idle advance is solid and stable; the Echlin #VC-1810 advance can (about $10 at NAPA) provides the same amount of advance as the stock can (15 degrees), but is fully-deployed at only 8” of vacuum, so there is no variation in idle timing even with a stout cam.
For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively.
Explained like an engineer and I agree with everything you are saying. Thanks for contributing to my channel AG
Great timing curve, mechanical and vacuum advance tutorial. I was a tune up mechanic in my youth and quickly learned to move vacuum advance from ported to manifold. The smog laws in 1966 in California moved them back to ported to lower emissions. Engines ran hotter and got poor mileage and had sluggish street performance. I've converted several friends' engines to manifold vacuum advance with great results. Some are harder to start when warm since there's more advance while cranking. Opening the throttle stops that. Street driving with this makes for cooler running and better MPG. Last note: I lived in Denver for a decade, and we could run nearly 40 degrees total there with no detonation. Higher altitude makes the difference. I'm in Texas now at 700 ft and 35 degrees is my maximum total now. Thans for the fine video.
Great information, thanks for the input. AG
I've always ran manifold vacuum.
The distributor on my 67 gto had a single advance stop on the underside of the weight plate and the weights had a different shape. For some reason the gto had a rubber bushing on the stop. Aftermarket timing kits supplied a steel bushing. Chevys that I saw had the same type.
Your chart shows the total timing graphed at 35 degrees not 30. In the late 60's and early 70's the recommended total timing on both small and big block Chevys was 38-42 as
I remember.......
the fuel is different now, we can't get away with that much timing anymore.
The mechanical advance was 30 degrees, add 4 degrees initial=34 degrees.
Liked the video. I'm somewhat passionate about ignition timing and have studied it extensively over the years.
FTR, I've a manifold vacuum preference too. The ported vacuum as you know was a crutch used in the early smog days and was a way to try and clean up emissions. I noticed the engines you referenced with your "good book" were all pre-smog examples. Keep up the good work.
Thanks Kevin and thanks for inspiring the latest video! Unfortunately the presentation quality wasn't the best but we got the reference info out there for anyone interested to see it. AG
30+ years in heavy industrial transportation and you are 100% correct on split washers👍
Great affirmation from someone with real life hands on experience. AG
You’re a very good instructor and do a great job explaining the details.
Thanks for the nice compliment Rich. AG
amazing finally after countless videos you finally made it make sense vac advance on ported because it keeps timing at idle and when there is no vac the centrifugal advance takes over the work .... never seen it that way so much easier to understand thankyou i keep carbon fouling plugs on my 383 stroker after about 30 miles so i am trying to dial it in and you helped me on this mission thankyou !
Thanks for the info Shado. glad it was helpful.AG
great video, ported vs manifold each have their place depending on what your timing curve needs.
Great point! AG
I found that sbc with comp cams 268h liked 14° base 14°mechanical 10° vacuum. Yes both advance mechinisms were modified to only allow that much. Then i used weaker springs and a adjustable vacuum can set at about 12". So total combined would never be more than 38°. I used a vacuum gauge to determin best amount of vacuum advance every 100 rpm up to about 2800 rpm. Beyond that im not concerned with economy just torque. Read your spark plugs they will tell you lots.
I agree about using the knowledge from the past. If you're building a regular production engine, naturally aspirated, someone in the past already designed one, all you have to do is follow their blueprint. I remember the old "Chevy Bible" and the Chevrolet interchange manuals, they were worth their weight in gold......no pun intended.
Thanks Al. The pun is ok. AG
Nice lesson... My experience with vac advance has been that connecting to manifold provides great cruise efficiency and smoothness, but the vac cannister cannot respond quickly enough to throttle input. It fails to pull timing out quickly enough under load. You get a brief moment of spark knock while the advance plate moves back. Connected to ported vac doesn't give such issues. I have since gone to MSD programmable, which uses a GM MAP sensor and has a fully programmable curve in addition to the run curve ie: "mechanical". My 302C has about 45 degrees total dialled in at cruise, and around 25 at idle. It's a happy engine.
Thanks for the info Gergatron. Sounds like you have taken this to the next level. Congratulations. AG
The vacuum advance really is important in a street vehicle, they spend most of their time at something less than half throttle and at a steady state like highway use may be considerably less.
People need to realize this and be truthful with themselves and the actual application of their engine.
A part throttle dyno run may show the benefits of layering more vacuum advance on top of the total mechanical in such cases, there may be free torque and fuel economy to be had and better driveability. You did hint at this, but there are numbers to be had.
In modern engines there are ECUs, multiple sensors, and advance tables that play the game, but really it's not that much more functionally advanced than the good old distributor.
Anyway thanks for another interesting video and congratulations on breaking the magic 10k subscribers, it must feel good for you and your team.
Very nice compliment UncleVom. Thanks so much! AG
And GM took the ported vacuum party to the next level with Transmission Controlled Spark (TCS) in 1970. This system completely shut off vacuum advance until the engine was in top gear or 3rd gear on a 4-speed manual. Usually found on vehicles without A.I.R. (smog pump) it was combined with a 195 degree thermostat to elevate combustion temperatures to lower emissions. There was a temp sensor in the pass side cylinder to give full advance if the engine got too hot. In 71 it was modified into another system called CEC and for 72 the TCS prevented vacuum advance on the 4-speed in all gears except 4th. The good old 70' emissions!
Great info, Thanks Thud. Emission control was more difficult and complicated prior to the introduction of digital technology. AG
Not to go too far off topic but this got me thinking about the other parts of the ignition system; plug wires, distributor cap, rotor, spark plugs. Recurving the distributor would be good too. Heat range of spark plugs and adjustable vacuum diaphragm. Maybe with an engine on the test stand or dyno.
Thanks for the suggestion Eric, we will get to all that. AG
I’m a 73 year old drag racer and have not been able to find a vacuum advance diaphragm that will last very long on my HEI distributor. I’m about to ready to order a HEI distributor from Progression Ignition that features the ability to program everything with your phone app.
Thanks Noah. The Joe Fernandez Pontiac that i will be doing next has Progression ignition. I will se if Joe can give us a demo on it. AG
Congratulations on 10K👍
Thank you 🙌
Great helpful content.....Thx for Detailed explanation on the "Ported" vacuum advance issue...
Thanks Tom, always good to see your comments. AG
On another note, I never knew the factory Chevy timing curves were ridiculously slow and had too much mechanical advance and not enough initial advance. I didn't know that they were this bad until you showed the factory specs. Modifying it like you have makes a big difference in getting off the line 💪
Thanks Robert. AG
If you think that one is bad, should see a stock TBI 350 or L31 Vortec 350 advance curve. They start out negative at WOT at low rpm. The TBIs only run about 16-20* WOT total depending on applicaton and the L31s about 23-25* WOT total. Total advance on both are not in until the 4,000+ rpm range. As the engine heats up from 180F to 210F they pull another 3-4* of timing. Vortecs also lose up to another 7* in high IAT conditions. The Vortec 350 can have NEGATIVE 10* of timing at WOT at 1,000 rpm in hot weather.
Thank you for explaining the vacuum advance ported manifold vacuum an how it works with mechanical advance
Thanks you for watching Robert. AG
I really like your take on manifold vacuum I am going to go that route, so I am going to set my total advance to 34 degrees, my question is when I read my initial timing it should be quite a bit different when the vacuum is hooked up to the distributor compared to when I unplug it, for example since I don’t have my truck with right now, let’s say I have 14 degrees of initial timing, then I plug the vacuum in I would expect to see around 19-23, keeping in mind not to let my total advance go over 34 degrees,and I should be ok to hot start, if you reply it would be much appreciated, I really like your videos, and I’m with you the guy’s from GM were no dummies 😀😀
Thanks for the question. If you have 14 degrees of initial timing and 15-20" of vacuum, then you connect your distributor to the vacuum port, your timing should increase about 20 degrees plus/minus. Remember, when the engine is under load, the vacuum will drop out and your total will be the initial plus the centrifugal/mechanical advance. Your engine loves lots of timing at low load because it takes longer for combustion. Hope this helps. AG
@@goldsgarage8236 thank you, so at 3000rpm with vacuum line on what would you expect to see timing at, with everything we said factored in, will I be over 34degrees
I've, always hooked my distriburor vacuum hose, to the base of the carburetor.
That is correct Stroker. AG
Very good, very interesting. I assumed you were going to discuss whether ported or manifold vacuum is correct.I have seen the issue argued in both directions.the one thing I might add is when you are at steady cruise, manifold vacuum will be a fuel milage improvement.
Good point David, I should have thought to mention that. AG
I have old massey ferguson tractors from the 50s and 60s. They have lock washers on 1/2" bolts. And smaller.
Congratulations on the 10K subs, it is all a testament to your great content!! Back in the 80's I worked at The Carb Shop here in BC. As such, I have a Sun distributor machine as well as a Sun osilliscope/gas analyzer too!
Thanks alleyoop. You have some great real life experience. AG
It’s great when you go into detail. I really learned something on that youtube about bolts. Not sure if you done one yet! But a you tube on decreeing a cam
Thanks Derrick, we are planning to video degreeing the Pontiac 400 cam now in progress. Watch for it. AG
Thanks for the explanation. I now understand how it all works. I was unsure about the vacuum advance.
Thanks for commenting Sunset. AG
Even emissions engines that utilize “ported” vacuum also utilize “manifold” vacuum. A thermal switch plumbed into engine coolant switches between the two depending on engine temperature. If the engine starts running hot it switches to manifold vacuum because the engine runs more efficient therefore cooler...
Thanks for the comment, I installed one of my 604 crate engine builds in an 87 Monte Carlo a few years ago that had that set up. AG
I recurved a lot of distributors from 73 to 94. One thing nit mentioned is the huge variety of vacuum cans uu
.. the huge variety of vacuum cans that were available from the factory and also Standard Motor Products. The amount of vacuum required to start the rod advancing varied widely. Some would be fully advanced with only 5” of vacuum. The one you need will not start advancing until 8 to 10.
Also , the travel of the rod was almost always too much so you needed to make or buy a limiter tab that would go under the inner most screw the attaches the can to the distributor body.
You cannot go wrong with the curves in the How to Hotrod Small Block Chevies. They are straight from the engineers. This stuff was figured out in the later 50s.
My street curve had 10* initial, 10* vacuum hooked to direct manifold vacuum and the centrifugal advance would begin just above idle and be all in by 3000 rpm. 12* in the distributor which is 24*#at the crank.
10 initial
24* centrifugal
----
34 total
Plus 10* vacuum
Equals
44* at cruise. Very safe usually. There will always be some engines that defy conventional wisdom. They are called Infernal Combustion Engines for a reason.
Great information. My 65 c10 truck hooked up to manifold only but has terrible gas mileage. I wanted to hook up to vaccum but has cylinder diaphragm which has no suction at all. Was thinking of buying that hex diaphragm that you are able to adjust vaccum. My question is at idle what should i set my timing with vaccum and what should be my total timing at 3000rpm. Im currently running manifold vaccum at 12, 12 degrees before tdc and 35 total timing. My spark plug smell like fuel and gas mileage is poor. Would love to hear your suggestions. Thanks
Thanks for the question Nick. Try this, set your total timing with vacuum disconnected, say 35-36 degrees all in by 3000 RPM. Your initial timing should now be somewhere between 12-16 degrees with vacuum still disconnected, depending on your distributor.
Now when you re-connect to your MANIFOLD vacuum port, you should have more than 30 degrees at idle.
Your engine needs lots of advance at idle because the fuel air mixture is not dense and combustion takes longer.
If the other conditions you mentioned still exist, look for other causes.
I hop I understood your question correctly. AG
Good tip I've been throwing lock washers away for years, 30 years ago my boces teacher told me the same thing, friction holds bolts tight, and there not used in engines and those bolts will stay tight forever.
Great affirmation for me William. Thanks AG
I love this subject, nobody ever mentions the vacuum canisters are calibrated differently, for the application.
Optimum peak combustion chamber pressure occurs at 14 degrees atdc.
Thanks for the comment Captian.AG
I added an aluminum plate to the arm of my vacuum can limiting the vacuum advance to 8 degrees.
I think it originally was s 22ish degrees. With my throttle plate virtually closed, my idle was slways higher than i wanted.
I was finally able to get a lower idle by redicing my vacuum advance. I think it was actually too low after the mod, so i actually opened my throttle plates slightly.
I ran 13 to 14 advance, 34 total and using the stock GM weights. I used a Mr Gadket or Accell spring kit.
I had heavily modified some new World Products heads, installed a mild Edelbrock performer cam and manifold. To make it pretty, i added some cast iron Sanderson block hugger manifolds and 2 1/2 dual exhaust.
I spent months playing with and learning to get the timing curve right.
Yup, manifold vacuum is the right way.... I don't care what other internet professionals say.
It may be devicive, but it really doesnt bother me if they do it the wrong way. 😁
I had this 350 in a 73 Chevy stepside. I had it bored 30 over.
I dont recall my time slips best et, but i managed 76mph in the 8th with some TA50 tires. Those didnt grip well and my converter grabbed pretty early. With so little weight in the back, this combo was so hard to launch without smoking the tires. From my et and online calcuulators, i think it was 294 HP. No race truck for sure. I never built it to race, just took it out to the track twice for curiosity. I miss my old truck. It never let me down.
Did I mention happy 10,000?
Yeah, but ill do it again. 😁
🎉🎉🎉🎉🎉🎉🎉🎉🎉🎉
Thanks for all that Randy! AG
@@goldsgarage8236 I'm not a man of few words. 😂
Thanks for doing what you do. 👍
Excellent...agreed on manifold vacuum source...working a 67 Ford 289....question...if running manifold vacuum source, do I still need to disconnect and plug vacuum source to set initial timing?...thank you in advance.
For sure Greg. Thanks for the question. AG
We have to find for you some better audio settings. The Mic is actually shutting off as you come to the end of a word.I don't know if it's the noise blanking or the hands free activation..love the videos and the subjects. Thanks.
Thanks for your comments Mike, we will work at getting better.AG
Correction, thanks for your comments Jim. AG
I see so many guys saying to use manifold vacuum, and that ported didn't come into use until the 70s to help with smog, but all the 40s - 60s cars I have owned and/or worked on came stock with ported vacuum, except for the early 60s aluminum 215 Buick V8 and, if memory is correct, the 64 - 66 Nailheads.
Thanks for the information James.AG
Really
I'm 65yrs old and all my cars back in those days were manifold.
@@bigpump9483Not mine
Those who claim that ported vaccum was purely for emissions miss understand the whole emissions thing. Vaccum advanced emissions was first started by Ford in 1968 with the dual vaccum advanced distributor (2 vaccum advance hookups + mechanical. The primary & secondary vaccum lines hooked up to it's own diaphragm. Other then two vaccum nipples the vaccum advance can looked the same as a standard vaccum advance can externally. the dual advanced distributor (vaccum + mechanical) only had one vaccum nipple and diaphragm. The secondary vaccum advance was hooked up to ported vaccum & only kicked in during part throttle & deceleration. It would retard the timing slightly during these conditions. At WOT manifold vaccum goes to zero and tranfers over mechanical advance. The primary vaccum line was hooked up to manifold. Pre 1968 the one & only vaccum line was hooked up to ported vaccum in the vast majority of cases from the factory. Some were hooked up to manifold vaccum. Your initial timing & and carburetor adjustments handled your idling conditions pre 1968.
@@williamallen7836 THANK YOU.
Hmm,I'm running a stock l31 motor with a Eddy carb. I locked the mechanical advance to keep the timing down to 36 and hooked the vacuum advance up to ported. It feels like it's got a flat spot around 40 to 50. And I can't rev past 4000.
That is a problem Old, but I doubt that it is caused by the ported vacuum advance. One way to find out, if you know you have 36 degrees total, try removing the vacuum line and see what happens. Then you could try connecting it to a manifold vacuum source. Just a suggestion. AG
I figured out what it was there's an intake leak in the back. where the china wall meets the head. I Did hook up the Eddy 750 to the full manifold port though and I noticed the motor idles alot quieter.
We have had great luck with tool bags, much like Paul has. From bikes to sleds to all offroad toys except we put the bags inside amo cans.
Thanks for the comment Darrin. AG
I didn't know factory HEI ignitions had way more than 20 degrees of mechanical advance built into them, the aftermarket HEI ignitions on the market seem to have only 20 or so, I THINK! I have had a hard time getting info on built in mechanical advance on anything. I have been using manifold vacuum advance on my mild GM performance 350, and it been working out great, but I need access to a distributor machine for the remainder. Good video.
Thanks Mike, try this! Disconnect your vacuum advance. Set your total timing at all in 3000 RPM or more. Then check your timing at idle with the vacuum still disconnected. The difference will be your mechanical or centrifugal advance. AG
@@goldsgarage8236 I'm going to make a chart like you did, and with that technique you described, I should nail it down, thanks.
So, the vacuum advance, when manifold vacuum, actually allows the engine to act much like it has a race distributor locked at 34 degrees, but without the hard starting issue because before it fires its at 4 degrees, after it starts its at 30 and the time between the drop in pressure from WOT until max mechanical advance is about a second or two at most?
i think you got it right Crazy. AG
Your example for vacuum can had about 16 degree of advance. I think my can had more. My car had a misfire at cruise with the can hooked up. My whole curve may have been quicker. Where do you find specs for the different cans? My 68 has a Q-jet. A lot of HEI's have a lot of advance due to EGR.
Thanks for the question Kraschs. i have a 1968 Manual, if you can tell me the specific engine i will look it up for you. Most small blocks had 15 degrees and most BB had 12 degrees. AG
68 corvette 327 350hp. It has the original distributor. I thought I set it at 15 btc and 36 total timing. I dont remember the curve.
My car with a 4:11 gears runs 3K RPM @ 55 MPH at part throttle crusing on the highway. How would that work with a vacuum advance? I have 17 initial advance with 18 Mechanical all in by 3K. What would your advice be?
Absolutely fine with vac advance in that scenario. It allows you to run a leaner light throttle cruise mixture and helps keep engine temp in check.
You wont need as much vac advance as you would if cruising at 2000 or 2500 for eg so you need an adjustment(many have this feature, but its not difficult to make a stop/limiter of some type)
The big advantage is you will also get a little better mileage when cruising which saves fuel for when you want to mash the throttle👍 there is no downside to vac advance at all.
Craig is correct, you should still use vacuum advance. AG
My 73 Camaro Z28 has a mild cammed 350 in it with 3:73 gears. With the turbo 400 auto transmission it turns 3000 RPM at 60 MPH. I have the timing set up so that it has 36 degrees total mechanical advance all in by about 2900 RPM. It idles with about 14 degrees of advance with the vacuum disconnected. On top of that the vacuum advance when connected to a manifold source is now limited to about 9 degrees so the total advance at part throttle cruising is 45 to 46 degree max and the engine runs perfect. I believe the rule is that you never want to exceed 50 degrees of total advance in any scenario. It is notable that when I bought the car several years ago the timing setup was all wrong. Since I took a day to make the corrections the car runs and drives so much better and the fuel economy has significantly improved. Best part is that it was practically free to do that. Has a 180 degree thermostat with a stock radiator and it never exceeds that temperature while driving on the hottest days. Only exception is if it sits idling for a long period then it might get to 190. Can has a typical stock Hayden clutch fan setup. If you were to add about 10 degrees of vacuum advance to your setup you should see some improvements over what you are running now. There really is no down side to this.
Thanks for sharing your experience Captain. AG
So if you have a adjustable vacuum advance how do you know where to adjust it to?
id love to hear this sort of input on a 1969 sbc 350.
Glad you enjoyed it. AG
12:24 You're one of the few that say "flame propagation" or a burn rate.
So many people believe the combustion mixture explodes.
As a side note: engine "ping" is a shock wave in the cylinder bouncing back and forth at the speed of sound.
Thanks Richie. AG
Actually it's not.
But who cares.
I haven't ever seen a reason to lockout a distributor the total advance is the only thing to worry about always has a better response from idle starts easier and still holds accurate timing at full throttle
Thanks for the comment Krazy! AG
Good points whether you use ported or manifold vacuum for advancing, I found for my 340 that initial set at 22 degrees allowed the max advance go to 34 and using the manifold vacuum stalls the engine when I stomp it from stop. If I move it to ported, no more falling flat, trouble is that my centrifugal needs to advance quicker like about 1800 rpm. Can turning the set screw in the vacuum pot do that or what will it take?
How about lighter advance springs ...
Jack is right about the springs, if ported works for you, then you should use it. AG
Congrats on reaching 10k subs 👍👍👍
Thanks! 😃AG
Congratz on 10k subs! Great content as always. Another video idea would be to going into points and HEI details.
Thanks Shanman. WE will get to that.
from kuwait and soo happy to follow and here you sir and Mr Vizard
Welcome Kuwait. I lived in Saudi Arabia for 2 years, you would be my neighbor. AG
Thanks for the follow up on this subject. Congrats on 10K subscribers.
Thanks you Al. AG
Congratulations on reaching 10K subs! I knew that this channel would grow if you kept up the interesting content that really isn't explained elsewhere on YT.
Very nice compliment. Thanks Robert. AG
What a great video. It really helped solidify my understanding of vacuum advance!
Thanks for the comment Owning. I'm glad it was helpful.AG
I actually have a 1968 327 bored
.04 over with a Melling Stage 3 cam(MC22398), and I'm a little confused about this timing thing. Should I set base time w/o the vacuum advance? ie., plug it off. But then what about setting total time? Should that also be done w/o vacuum advance? Because I've set the base time with the vacuum advance attached. Which is manifold vacuum. Thanks. Love the show and hope to have your input soon. Lol. It's New Years Eve. 12/31/24
Happy New Year JB. To set total timing, disconnect your vacuum advance, rev the engine until it stops advancing, (this should be about 3000 RPM) and set your total here. Usually the best results will be at about 34-36 degrees. Then let the engine idle and observe the initial timing, usually about 12-16 degrees. When you reconnect your vacuum advance you may have 30-40 degrees at idle, that is fine. Your engine needs lots of advance at idle because the flame travel is slow. When the engine is under load, the vacuum advance will drop out and your mechanical and initial advance will be in play. Hope this helps. Thanks for the question.AG
@goldsgarage8236 thank you. Best advice I've got...
@@goldsgarage8236 so then your base time goes out the window at this point? The manufacturer suggest 12 to 14 base timing. Interesting. This is the first aftermarket cam I've involved in a build. I've done like 5. Lol. I know, rookie. It's still a hell of a challenge to get everything right. Especially when the engine doesn't belong where it's going. Haha
Correct, your total timing is more important than base timing. AG
I have a rebuilt 350 engine and im trying to install the distributor in place but even after i macht the oil pump drive with the distributor one still have a 1/4 inch gap for some reason the distributor doesn't want to drop down all the way in????
point the rotor to # 1 cylinder in the engine, then rotate the engine. It will drop in. Time it from there. AG
Okay i will try that also forgot to told you the distributor it was on a 70s 350 the rebuild one im working on is a 80s still the same probably???@@goldsgarage8236
All BBC and SBC Gen 1 distributors are the same. AG
The ported vacume goes away with throttle opening just like manifold vacume. The only thing different between the two is the ported vacume is zero at idle
Put a T in the hose and run a vacume gage into the the dash area and take a drive. Both sources of vacume go away at the same rate when you get on the throttle.
Absolutely correct. Only difference between the two is one is providing a vacuum at idle and one isn’t. Once the throttle plates are cracked open they both provide the same vacuum. At wide open throttle they both stop providing vacuum. Where this nonsense started about a ported source providing vacuum at WOT I’ll never know…
I VERY MUCH prefer NO vacuum advance at idle on a modified engine, ESPECIALLY with an automatic trans. I tune for a solid idle and maximum throttle response which usually involves initial timing being just short of straining the starter motor, reaching full advance as early as possible without gas ping usually being determined by the fuel being used, and the amount of mechanical advance being determined by an advance travel limiter to achieve the amount of total advance desired. When all of that is accomplished and if I want to attempt to achieve the best fuel economy possible, then I will tinker with making use of vacuum advance. If the vehicle that you're working on is for the most part factory stock and uses a factory stock type carburetor, then it very well may work best with the additional ignition timing that manifold sourced vacuum supplies at idle speed because of lean idle/low speed a/f calibration. Not looking for argument. If your methods work for you, that's good. I have been satisfied with mine for a long time.
That is correct Robert, I apologize for the confusion and have done a follow up on this point in the next video. AG
@mickangio16 I do the same...find the best idle quality and off idle response with the best base timing setting-provides the smallest throttle blade opening and the best idle vacume number. Then I tune the mechanical advance curve up to total advance. When the base and curve is optimized then add in about 10 degrees of vacume advance and see if it likes it at cruise. Add more if it wants it.
With regard to split type lockwashers; you don't see them used with connecting rod fasteners or main cap fasteners which should tell you something.
Good point Ken, you are right on. You won't find them any where else either. AG
Good tip on so called lock washers. When these things spread open [as they like to do], surface area under the bolt head begins to disappear and trouble has arrived. .
Thanks Flinch. AG
the problem I have noticed with manifold vacuum is during tip in the timing gets retarded,throttle response seems better with ported vacuum but not every engine responds the same
Thanks for the comment Ted. AG
A sbc 350 can make 400 hp. But a 383 can make 400 hp with more idle vacuum, and because of the increased stroke, make more low end torque. So if you're not limited by any racing rules, and you're changing the crank and pistons anyways, I would go with a 383, unless I already had the parts to do a 350.
BUT keep in mind you have to clearance the block and use a small base circle cam in some cases.......that's why I am going 350 on my 1st build.
Good points.@@yurimodin7333 though if changing the crank and pistons anyways, I would also get stroker clearanced 6" connecting rods so as to not use a small base circle cam.
Thanks for the comments. AG
I have been told that under part throttle high vac you could have over 40 degrees advance at the higher RPM because of the vac advance.
Yes indeed. And that a good thing at part throttle/cruise speed, better gas mileage. I have my advance at 45 degrees for part throttle. Under wide open throttle the addition vacuum advance goes away
Tony is correct Cuzz, AG
I agree with lock washers in most cases if you get the bolt or nut tight it wont come loose anyway.
Right on. Thanks for the comment John. AG
So a normal engine always works better with more than initial timing? Does that mean that initial timing alone is only good for starting a cold engine ?
Thanks for the question Run. Well, initial also makes up part of total timing as well. The more initial you have, the less centrifugal advance is required. AG
Excellent video and a fantastic explanation, Allan. Thanks!
Thanks Todd. AG
Great info 👍 msd instructions say to put the vacuum on a ported source. But the video says manifold. Even theyre confused
Thanks for the comment 357. AG
Im s newbie...do you set your timing with the vacuum advance hooked up or disconnected with the manifold port capped?
Thanks for the question Joe. Always set timing with he vacuum hose disconnected. Capping the port is ok to do but it doesn't change the timing setting. AG
Another point, always set total timing all in and let the idle timing be what may. For example, 36 degree at 3,000 RPM all in, If the distributor has 20 degrees of centrifugal that will give you 16 degrees initial at idle, however hook your vacuum back up and you will have 40-50 degrees at idle which is fine. Your engine need lots of timing at idle. AG
@goldsgarage8236 Thank you for replying back. Great info . Much appreciated!
! u r right about the washers. I wondrd y they went out of shape and found loose, later. Thank you. Now, I need to go back and get rid of a few... Ha! Cheers!
Glad you found it helpful Stuck. AG
I really like the lock washer lesson! I fully agree!
I think those were designed back when harmonics were not fully understood or there was a knowledge deficit on how to mitigate certain applications where harmonics came into play.” So just add a spring into the equation and maybe that bolt won’t back out because of harmonic resonance.”
That’s all I can reasonably figure on.
Thanks Hugh! AG
Could that lock washer explanation be the same as a crush washer (I.e. Honda recommends on oil drain plug)?
Thanks for the question Mark, no if Honda recommends it, then I would trust them and use it. AG
used to work at an auto parts store, the crush washer is like a gasket.
Excellent video AG. Thank you. 👍
Thanks you Steve. AG
Your right on about lock washers, the guys at work look at me like I got two heads when I talk about lock washers
Thanks for the comment Richard, a little humor is always good. AG
Will a motor need more initial timing for a intake leak? Mine seems to need quite a bit of timing to start and idle.
how much is quite a bit?
@@yurimodin7333 my timing light is flashing about 11 o'clock on the balancer. I have no timing marks on the balancer but the pointer on the chain cover only reads to 24 degrees btdc. I do have a new balancer and timing tab ordered.
Thank you so much sir. I just found your channel and I'm subscribed. Great explanation on the vacuum advance question. 👍
Thank you for the comment Rip. Ag
Love your content so glad I found this channel.
Thanks you Saj. So am I. Thanks for the compliment. AG
Wow! Congratulations great channel! Take care
Thanks Brian. AG
New to your channel, but you should do a video on vacuum secondaries and how many people think if they dont feel their secondaries opening up it means its not working
My last bracket car was a 78 Camaro lightened a lot. Strip only so no street equipment. Pretty light. Std bore 350, Cam Dynamics 234* at .050, .480 lift hyd, cast flat tops, dbl hump heads ported per Vizard’s book, Performer intake, Holley 600 vac sec, T350, 3000 stall, 4.88 gears, 9” slicks. 12.63 at 105 was it’s best time on a t&t nite. The foot brake class was 13.00 and slower so I had to retard the time and limit the throttle pedal travel to slow it down to 13.00.
-
Vacuum secondaries CAN do the business !!
Thanks Steven, good suggestion. We will get to that. AG
I thought the vacuum adds to the total bringing it to 40 and 50 degree total.
You are correct Shelly, and 40-50 is fine at idle or part throttle. When the throttle opens, the vacuum drops and the advance drops out.. Hope this makes sense to you. AG
you need to take your car with similar ignition timing like the one in your chart hast to a cruise and add a vacuum gage with a long hose inside the cockpit. you will see when your rpm is on 3000 where the centrifugal advance is on 34 and you close the throttle, your manifiold vacuum will go to max, therefore the vacuum advanced will add 15+ and you are at around 49 degrees btdc. BUT this seems normal and should not go over 50-52° in total.
saying NOT MORE THAN 34° seems to be wrong for me....
Thanks for the comment Motor. i had to go back and watch the video to reply. I agree with you in that situation where you have max vacuum when the throttle plates are closed, and max centrifugal due to RPM. AG.
Alan, I noticed that you use generic branded Bravex HEI distributors, my question is do you find that the curve is right on the money for your builds? Thanks and keep up the good informative videos. :)
Thanks Jimyee. The ones I have used get all the advance in by 3000RPM or so on the dyno. They can be custom curved to suit a specific application. AG
You are 100% INCORRECT about vacuum advance…Ported vacuum does NOT add ANY timing when it is at WOT because at WOT there is NO vacuum at either port UNLESS the carb is too small in which case it creates a restriction and can pull in a little timing with PORTED and MANIFOLD vacuum…and there are times when a motor idles better with PORTED vacuum…
Having said that, YES, the engineers working on STOCK motors know more than you do, but they weren’t making distributors with curves for motors with as much camshaft as we run now, so you CAN’T use a 1960’s book for tuning a 2020’s motor combo. 🤦♂️ You have to be smart enough to let the motor tell you what it wants and then make the curve do that.
You are correct about WOT and I apologize for the for the misunderstanding. I have addressed this in a subsequent video. AG
I tune LOTS of carbureted motors for customers and about half of them are fairly rowdy or are low compression and over cam’d and those types of combinations typically want 20-36° INITIAL timing just so they’ll idle right…vacuum advance hooked to manifold vacuum doesn’t work well on some of those motors because if the motor won’t start with any more than say 20° timing but the motor “wants” to be at 25-30° at idle, the advance will pull in more timing and make it idle higher and then when you put the trans in gear, the RPM will drop a little and the vacuum will drop a little and then the vacuum advance will pull timing and then the RPM goes down even more all of which causes a huge RPM swing from idling in Park/Neutral to being put into gear and then people blame the converter for being too tight…on rowdy combos, it’s critical that the initial timing not move when idling to putting the trans into gear.
Thanks for sharing your knowledge and experience. AG
@@SmothersSupercars you got it, my man👍
Wouldn't a high performance, low vacuum cam negate the benefit of vacuum advance in a street car?
That is an interesting question Bob. Most high performance street cams should still provide 10-15" of mercury vacuum at idle which is sufficient to advance the timing. Most high performance distributors have adjustable vacuum settings so i think it should work ok. AG
Funny story I was installing a GPS system on a CAT dozer one time and asked a shop mechanic where they kept the lock washers. He laughed out load and told me "we don't use lock washers and don't have any in the building" that was when I learned how wrong they are. LOL
good story Ronnie, even the manufacturers don't believe in lock washers.AG
They sell them split washers in grade 8, don't buy the cheap ones did not realize that until recently so will keep throwing them away unless I know they are grade 8.
Good advice, thanks Shelly. AG
Ported is more metered for the throttle. My 1984 doesn't need vacuum advance at idle. Manifold vacuum thru the dist. Will actually retard timing during acceleration.
Thanks for the info Gary.AG
400hp comes at lower revs and has a smoother idle on a bigger motor so it's a how high a rpm do you want your 400hp question.
Thanks for the input Tom. AG
Awesome channel Alan
Thanks you for the nice compliment Paul AG
I have always been a full port vacuum guy. I’ve worked on Small Block Chevy’s for 40 years now. The advantage is at idle. Engines will always idle better regardless of the cam with the distributor connected to the manifold vacuum on the carb. At WOT who cares neither port is worth a darn. At half throttle or cruise mode both ports will be putting out some vacuum so we are now back to the whole advantage of manifold port vacuum, idle. The tuned port was designed for emission compliance and hoping to get better gas mileage. That is it. Most of us are building street and muscle cars so this is really irrelevant. Go manifold vacuum port and your engine will idle so much better and be happier.
Agreed! Thanks kk. AG
THANK YOU VERY VERY MUCH FOR THIS VIDEO ❤🔥💪👌👍🏻
You are welcome, thanks for watching and commenting James. AG
Awsome explanation
Thanks Mark. AG
I do not agree with the lock washer episode I learned a lot about lock washers from motorcycle mechanics aluminum parts versus steel parts. Nowadays most lock washers are very cheap.
Thanks for your comment Paul, I think i mentioned that lock washers may be helpful in a connection that is too soft to torque. AG
Thanks You Very Much Sir❤
Thanks you for watching James. AG
34 degrees... Is that with manifold vacuum disconnected?
Correct, thanks for the question Eric. AG
Thank you for your knowledge and video's USA 🇺🇸 USA 🇺🇸
Thanks again Patrick. AG
How about saying “ I disagree” ?
Thanks for the comment Peter.AG
If you are going with 350 then why don't you go with a factory roller cam block with factory lifters ect! you could save 400+ dollars than using a link roller lifters and special front timing cover and that money saved can be for upgrading the crank for 383 or better heads.. Over achieve is never a bad thing!
That would be ideal Larry but factory roller blocks with mechanical fuel pump access are hard to find. AG
Vacuum advance is primarily to fire lean stretched out molecules at steady state light part throttle . The fuel mixture is lean and needs more time to fire the fuel This benefits fuel economy . Where does light throttle occur typically 37mph to 55mph approx
Vacuum needs to be measured and known at every 5mph
How much advance , manifold vacuum at mph and get the highest reading u can ,at approx 14.5--15:1 air fuel ratio .
Primarily vac advance manifold or ported can also depend on auto or manual g/box . The auto can have more vac adv because of the stall in the convertor ie helps engine thru stall and engine detonation is less likely . VS a manual =ported
Is there some emissions reduced on deceleration using manifold connection ?? Many emissions distributors have both advance and retard cans
Thanks for your thoughts and comments Paul. AG
I recently saw Junkerup's video about this. I have nothing to add on the subject, but it's funny to see people rehash things like this and argue in the comments
Good point. AG
A great wife, is key!
For sure Stroker, I will pass it on. AG