WHAT IS THE BEST COMPRESSION FOR BOOST? (8.5:1 VS 10:1 540 BBC STROKER TEST)
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- Опубликовано: 10 авг 2021
- HOW MUCH POWER DO WE GAIN WITH HIGHER COMPRESSION? WE HAVE ALL BEEN TOLD THAT WE NEED LOW COMPRESSION WHEN BUILDING POWER ADDER (SUPERCHARGED, NITROUS AND TURBOCHARGED) MOTORS, BUT IS THAT STILL TRUE? IF YOU LOOK AT THE VARIOUS PRODUCTION TURBO AND SUPERCHARGED MOTORS, ALL OF THEM HAVE LOW COMPRESSION, BUT WHAT HAPPENS IF WE RUN HIGH COMPRESSION WITH BOOST? CHECK OUT THIS VIDEO COMPARING A PAIR OF 540 BBC STROKERS (8.5:1 AND 10.0:1) TO SEE HOW THEY DIFFER IN POWER. GOOD STUFF!
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I thought we were going see what the blower on the 10 to 1 motor did.
Yeah wtf lol
lol i was thinking the same thing! i rewound and watched back to see if i missed the numbers somewhere lol
Ditto. I was expecting to see what power it made in the 4-6# boost range... maybe one more pull at 8-9.
Rich prob didn't do it bc he knew what was prob going to happen! ;)
@@nabilguffey3201 No way he could have run the same blower speed on the higher compression motor with the same fuel. Less boost, less power.
I don't think this comparison is terribly helpful. The question I think we're all wanting answered is: if you're OCTANE LIMITED, say building a pump gas motor, which is a better approach, low-compression/high-boost, or higher-compression/low-boost.
Even if your octane limited you can always pull timing out to keep.it happy with the higher compression.
@@chadcollyer4957 yeah, but will you pull so much, you would have made more power from the low compression engine?
@@chadcollyer4957 to a point.
all of you are wanting?
get a 9:5.1 compression, high compression will develop knock eventually if a mistake is made, then you need a rebuild, plus you'll always need high octane gas, no exceptions.. High compression is like having a gf who is only happy when you kiss her ass, the day you don't, she dumps you n you're stuck with pink eye..lol
If you have race gas, high compression and high boost is best. Lol
Even on pump gas I think the higher compression would be better. Off boost performance would be better and if you are tuning for maximum power for a given fuel the maximum output should be the same because the cylinder pressure where knock occurs would be the same.
@@christianmeeks4430 pump gas and high compression means your ignition system and timing needs to be dead on lol
@@rcairforceone the difference is that "dead on" is much more attainable today than it was in 1990, let alone 1980.
High compression vs low compression. High compression runs better out of boost.. but runs the same in boost but with a smaller tuning window.
Also, having Richard Holdener making motor sounds when your motor is under load is worth a few more octane points
The only reason to run lower compression in my mind is if you don't have access to quality high octane fuels. Otherwise, more compression, the better, assuming you can time it/tune it correctly
Even in that situation you can always pull timing out to keep it happy with high compression.
This is exactly my issue
@@chadcollyer4957 until you run out of timing Dx what I'm dealing with.
Higher SC and TC MAP.
Even then that's what AEM's meth kits are for. $700 for their biggest kit and $2 a bottle for winter washer fluid.
Richard I think you're videos are fantastic my only issue is you rarely use pump fuel i understand the limitations and possible engine damage when testing but I think its what a large percentage of viewers run. I would love to see more of it. The power you get on hi octane would be unobtainable for a lot of us.
The issue we had back in the day with high static CR's was timing, the quality of the head gaskets and the carb technology. Today, you can pull timing out of the motor exactly where you want it, tunable closed loop MPI with knock sensors and MLS head gaskets, so yes, I feel you can run higher static ratio's and pick up torque down in the lower RPM band, especially with turbo's.
Ethanol baby!
and aluminum heads help reduce the tendency to detonate as well as offer better shaped combustion chambers.
@@albertgaspar627 Yeah, I'm doing a Saturn 1.9 LDOHC build on mine with 10 to1 comp with low boost 8-10 PSI and an E85 blend. Building the most velocity instead of volume, JE forged pistons, ARP main and head studs, knife edged 3rd gen crank and lower tie plate, port and polished head, mild intake deshrouding, mild bowl blending, dual intake cams, ported polished intake manifold, bigger TBI and low boost E85 like I mentioned.I figure it'll be better if the machine shop builds it well. The car is light with the polymer panels, carbon fiber hood from Seibon, and the 15X7 Einkei RPF1's that are 9.8LBS each, and Mfactory limited slip differential. Should be a fun ride to surprise some people as well with my little baby car, ha ha...lol My engine is light too at 220 LBS, half the weight almost of a normal 4 banger from the lost foam casting..lol
@@junechris sounds like a nice design, check out David Vizard's channel too. he does dyno work with headers, you might find some tech there to help the velocity you want by clearing out the combustion chamber as fast as you want to fill it in. As you know, the joy of DOHC is you can play around with overlap and lobe separation by moving the individual cams around instead of having to buy a new re-grind.
I have a naturally aspired GM EcoTec 2.2 litre in the "pile" of project cars, would love to play with its potential since GM already went ahead and had a supercharged version (like the Saturn Ion). As you know, in a light car it can be a sweet piece when done right.
@@albertgaspar627 Yeah, I've heard about the cam options you mentioned about. Right before u messaged me, I was happy to find Ross makes forged pistons right in their catalog for the Saturns. And one of the other guys I spoke to on here was a 30+ year machinist that seems quite fond of the Ross ones in regard to the ring gap recommendations being more or less right on the money so i'm gonna relay that to my friend doing the build for me tomorrow.
Before even watching it I know the high compression one will make more power with boost because boost just enhances the NA curve of that engine. The better NA the better with boost.
Man, this is a timely video. I am looking at these exact motors and wondering the same things discussed here. Thanks Richard for the comparisons.
I like stock compression + ring gap + boost because well, pistons are expensive and junkyard engines are cheap!
The noise you made for each engine at the beginning made my day! Well done sir! 👏
You said it, the roots application is probably best off with the 8.5:1 long block. By my rough calculation, the 11 psi boost would have given you well over 1100 hp if not for the power needed to spin the blower, about 195 hp if the formula held true. That’s basically lots of heat added to the intake charge. If I had a couple GT-45s making that same 11 psi, I’d want the 10:1 or even more, EFI, and E85, too! That 540 would be a beast either way, I think.
If one wants to run high rpm and 30-40 psi boost, then 9:1 would be as low as I would ever go... Tuning becomes critical with higher compression, but will always make more power, and efficiency on daily drivers... The way the compression is made is also important, like having a dome piston with it down in the hole 60 thou would be asking for detonation and/or pre-ignition. Proper design combustion chamber is imperative for the proper platform.
Great Information Rich
For running on pump gas, I think it makes a difference how much boost you can run without running into detonation. The dynamic compression ratio based on cam specs is a big factor, too. Even though the static compression may be 10:1, the dynamic compression may be a lot lower with a big cam, so lots of variables there, tough to do a true apples to apples. Good stuff to talk about, though!
Imo for street usage lower CR sounds nice to run regular pump gas. Go on the track and pump up the boost with better gas.
Finally some quality content coming across my notifications…
Very similar to combo I have right now working on an na tune with a terminator 4500 to break it in. Winter comes turbo, 10.3-1
E85 is hard to find. What would the minimum octane you would run on boosted applications that high of compression
For anyone with little to no experience with tuning any boosted applications I would always lean towards the lower compression but anyone with experience that can ballpark a base tune pretty close to where it needs to be for the engine to be happy then the only question would be do you have a trans or rear end that can handle it with out grenading one or the other if not both
To answer the CR question just look at diesel tractor pulling. They use lower CR pistons so that more boost can be put into the engine. So yes, depending on the extra air to be added, lower static compression can be a benefit.
Pump 91/93= lower compression. No lower than 9 to 1 tho.
E85= all of it.
Richard, you tease! Lil, thanks for the content
Higher compression with boost increases the likelihood of detonation. Lower compression gives more wiggle room to withstand that issue. If the tuning is spot on and the fuel has really high octane (91+), then the higher compression will not cause an issue. Lower compression is often recommended for boosted engines because it just allows for more error.
I think this is an excellent question. Most people are going to be limited by the fuel octane availability, and I also dont see anyone else providing any data on this subject.
I would never consider 8.5:1 with that cam size and a limit of 11 psi. Flat torque curves are what win races and generate efficiency on the streets, not peak numbers. I once watched a video of BMW m7, 770hp 4.4 v8 outrun a late model Dodge Challenger at 1100 hp w/ blower from a 60 mph drag. 1100 hp is nothing to cough at, but the Bimmer just ate his lunch.
Personally, I am about efficiency, and it is more about building what works than cool dyno digits. That's how I got an 11,000lb dually 5.9 Cummins to outrun an Audi A6, on a 62mm turbo, w/o melting down...
And with 300k mi on the clock...
i'd take the 8.5 for longevity alone, but i'm not a competitor.
and considering the price per gallon of 93 octane, that 540's gonna gulp down the gallons. buy the cheaper gas, still have 600 + hp
All my best performing boosted 4 cylinders are 10.1 or more compression with good flowing cylinder heads ! E85 is key to keeping them alive
For the street on pump gas low compression for the win. Easier to control detonation.
Why though would you recommend low compression for boat motors at wide open throttle say on E85? Is it the extra heat from the higher compression motor that causes problems????
I'm REALLY GLAD to see this comparison. specifically the low compression comparison to hi compression. ive ben considering purchasing bbc pistons... sound i go for h581cp100@$390/set? (my compression limit with 10.3-1. ... OR save some $$ to select the h426cp100 set @$250. with lower comp at 8.9-1 ?? ill be useing pump gas...preferably 87. so i think you just helped me figure it out... thx for the video. i was wondering about this VERY THING
I totally agree with you, they both have their place. You just have to build what it is you are building for. In a W.A. Street Machine, I'd run low CR for the "just in case" factor.
W.A?
Great video
My friend has a side mount whipple supercharger on his 03 v6 mustang it has a air to air intercooler hes making 605 / 645 at the wheels on a forged 4.3 v6
What fuel octane are you using? It is my understanding that if you have enough octane, you can have high compression and high boost.
The shop that built the 4.3L for my Typhoon bumped up the compression to 8.7:1 but they also built it to handle 30psi.
the test that needed to be done would be to boost the both motors to the same cylinder pressure at TDC, the lower compression motor motor is in theory larger then. All compression done in the motor creates heat in the charge, compression done outside the motor with boost and intercoolers, results in more mass flow rate.
Thats why I watched the video. I thought he tested both combinations. I am kind of pissed he didn't.
I originally built my 540 now 555 with (8.5) but when measured after it was really 8.2 so that I could run 13-14 lbs boost on 93 octane with a carb and water/meth injection non intercooled , but now I have a dedicated second tank with 114 for the track and pound 20 lbs down its throat with a intercooler into it so I say if you have to run pump gas then low compression but in my case I would do 9.5 . Also Steve Morris still builds some boosted BBC’s with 8.5 so what does that tell you ?
It tells you that Power Tour setups need to be compatible with pump gas...LMFAO
I have recently taken a low boost paxton motor that was 8:5:1 to 11:1 in a corvette. I left 100hp on table for years by not running higher compression. Motor runs better, idles better, all drivability better. BUT it is compensated on boost by methanol injection … however it was at 8:5:1 also. So was winning combo foe me
Would less ring gap be required if you used boosted the low compression option vs the high compression boosted version? Could you get away with not using forged components using the low vs high? Example, could you run 12-18 pounds of boost on a 5.3LS with 8.3:1 on stock internals and stock ring gap..... properly intercooled.
you can run high comp and boost on stock internals with ring gap with enough octane
if you have e85 avaiable i see no reason to run at 8.5:1, a roots blown pump gas street deal, the lower compression might be good, gets hot low quality fuel hot day ect. iv been running a blower on 9.5:1 cr 8lbs of boost with e85 for years im pretty sure if i turned it up higher i would find the horsepower limit of my rotating assembly before id run into detonation.
I was hoping to see the same blower put on the higher cr motor but driven slower to make the same power as the highly boosted low cr motor and than figure out witch way was better.
This is only theoretical, but I think more compression and less boost would heat the incoming air less, while creating equal cylinder pressure. Thus MAYBE the air would denser. Sure, there's intercoolers, but that's more expense.
Hey Richard I got a L77 6.0ltr & I'll be doing a heads & cam package with a stage 3 ls3 intake "standard intake port" with an expected output of 500whp on ron98, it's staying N/A with future goal of going Boost. I've got the option of decking the heads to increase compression. How would this effect (torque, power & fuel consumption)?
I've did searches but can't find direct answers, just answers that beat around the bush.
compression adds power through the entire rev range-let me know if it makes 500 whp
I'm just here for the awesome engine sounds
Hey Richard, didn’t Freiburger and Brule do a swap of solid roller lifters onto a hydraulic roller cam and had a power increase therefore the 540 solid cam likely had the edge?
you have you conclusion backward
I think lowering compression for boost is an old thing, fuel back in the day was trash, so they lower the compression to avoid nock or something like that, but now just getting a better fuel is probably enough that's what I think.
Still very relevant today, in a hot environment daily driving a boosted high compression engine is asking for a disaster despite high octane fuel
@@Airman.. that's why you don't rock an eBay intercooler, and in the video he is using carbs on top and that doesn't really help with knock.
@@donkeykiller69 ECS intercooler with 100% methanol injection 95 Octane, CR 10.6 @106F summer day engine under medium load rattles like a rattle snake
For DD bring the CR down, for track\NOS○race fuel take the CR to the moon it doesn't matters as boost/cr dynamic pressures will exceed static pressures
As always great content...
Haven’t seen you in a while, Hope all is well..
Really bummed you didn’t run the Higher compression motor with Boost?
I think everyone was expecting that.
Just Say’n
As always great videos.
Again, truly hope all is well with the family, etc...
the 10:1 wasn't mine-but we can guess what happens by now...right?
Not sure what Happens Now?
I do know quite a few mustangs running stock compression with Boost as well as LT1’s
How much power have you made on a big block 2 bolt main?
I’ve ran 9.25 :1 on 6 to 8 psi in a marine application , the problem is the load that exist in a boat to put that into perspective have you seen a brake pedal in a boat ? But we also have the biggest radiator there is that being lake water , so cool engine temps say 140 to 160 combined with aluminum heads , coated pistons , allows us to get away with a lil higher compression, but 10:1 in a marine app I mean I’d doit but wouldn’t recommend it to a customer
Good video
A missing element in your test: the cylinders exchange the same vol of air NA. Not so on boost. As a result, can you run more boost on the PA motor while achieving the SAME dynamic compression or peak cylinder pressures? It stands to reason you can. The net effect of using boost may be the larger compression volume of the PA motor would effectively act like a larger displacement and has the potential to make more power at the same peak cylinder pressures.
Higher static compression just requires less boost for the same power. This is why my LSA motor is 11:1 static. I don't have to spin the blower above its max efficiency to make the number I need...
I was told to go with lower compression, and I was told a supercharger raises static compression. Moroever, for a lot of boost. I was told to consider alcohol, but I rather run pump gas 91 octane, so I was told not to run too much boost for a supercharger roots style application. I am curious what you think.
supercharger has no effect on static compression. Pump gas and low compression are a good match with a roots blower
What fuel will it be running is the biggest factor in my opinion.
When you factor in limited octane, it comes down to the detonation limit of the compressed charge. 8.5 to 1 at 14lb of boost will have 250psi compressed where 10 to 1 will have that same compression pressure at 10lb of boost. the difference is the lower compression engine will have a larger combustion chamber with a larger charge at the same pressure. obviously at the same boost the more efficient higher compression ratio will make more power. But that only works with fuel that will tolerate it. Run the same engine on pump gas boosted to the detonation limit and I think the lower compression combo will come out on top.
This is a realy interesting test! I have never seen a similar one before, but always wanted to! Thanks for delivering 👍
If your hard parts can handle the stress the higher compression version will always make more power.
Agreed with proper fuel
Is 11.8:1 big block on e85 with boost crazy? I’m thinking about it
As long as the crown on the piston is rated for a boosted application feel free.
My 540 is 14 to 1 compression with a 284 298 at 50 cam 112 degree lobe separation. Going to add twin 88 s with . what do you think?
SURE
If you’re running two turbos individually ( non compound) I would think that 10.5:1 cr would work well with 13 lbs-14 boost with a 245@.050 .600 lift roller compression. Am I correct? I want to build a 416 ci engine with a 1000 hp at 6000 rpm peak. It is an FE. I would think that it would be a streetable E85 engine.
yes-alum heads would be a good idea
I run 10:1 compression on my 429SBC with 26 pounds of boost from an F2 Procharger on E85. Runs great in my street car.
You CAN run a high boost on a high compression engine, IF!!! you have an inter-cooler. The problem with a super charger on a high compression engine, is temperature rise during compression. Too much compression could mean reaching temperatures high enough, for fuel to "auto-ignite", much like a diesel engine. The fuel may not wait for the spark. That pre ignition will destroy the engine. If you're going to boost, without inter-cooling, lower compression is a MUST! No matter what, I do recommend a high octane fuel on a boosted engine, because of its higher ignition temperature.
You can but, what's going to produce more power?
OK I'm new to this vehicle 1976 Chevy luv pickup with A ZZ502 bought this off my brother in has A671 blower I'm assuming that compression ratio is marginal at 9.6.1but still safe! I took it out the other day drove it around a the block got on it pretty hard it had back fired and apparently I blew the intake gaskets out what would cause that?
wrong timing or fuel
My ls 6.0 was told to go 9.5:1 with E85 and over 15 psi boost 1200 hp.
Now if I switch to methanol do I need to change anything compression, ring gap, bore hone, I am unable to find anything related.
Maybe a Q&A on switching fuels.
switching fuels is fine-nothing needed to change but the tune and possibly injector size and fuel pump to handle the increased fuel flow
is it typical for low compression builds to overlook and not optimize quench/squish when choose piston height? I wonder how big a factor that is in the power levels.
you should always look at quench-sometimes factory stuff just put the piston way down in the hole (not good)
I'm here researching combinations for a 400 sbc build. Street & strip, going in a 62 chevy C10. Motor's out of a 1972 Cheyenne. I believe I'm going with Dart heads, with as big of valves that will fit, 206?. I love blowers, but think turbo will be my power adder, for the boost control. I'm having the bottom fully built. Stroker. I'd love to be 1100 HP, without lifting the heads, or hurting the block. Any guidance would be appreciated.
that is a lot of power for a stock 400 block
@@richardholdener1727 What hp number would you feel comfortable with. I've decided not to put it in the truck, because of the curb weight, and high drag coefficient. I'll save the truck for something bigger. I have a lightweight 1983Toyota Corolla wagon, I'm thinking of putting it in, so the power to weight ratio will be much better, and the drag coefficient is .34
I been wrestling with the question of what cr to use on my 4v modular with an m122 eaton and an engine builder told me no more than 9:1 with 91 octane. (Best fuel I can get) I worry that it will be kind of a dog at part throttle but without high octane fuel i don’t think I have a choice.
9:1 is more than it has stock so it will be better than stock.
Lower compression would be good in a drag week application to run pump gas on the street. I like E85 but no telling what availability would be on rou te. I would probably go 9:1-9.5:1. As you've proven Richie.....more efficient n/a the better the power will be boosted.
yeah i agree too, going too high in compression will eventually develop "knock" no matter what on a boosted application, n that's the reason many ppl end up selling their "builds"..lol i rather have a nice 9:5.1 compression as i have in my 2v n run pump gas or e85 n be good on boost..
With my 427 LS with an F1-A, I built it to 9.65:1 as there is not 1 E85 pump in my state. My combination with the blower maxed out makes 12lbs of boost, if I would have known this in advance I probably would have built it to 10.65:1.
The advantage of high compression with boost is the reduced effects of boost lag. The advantage of lower compression is you can run higher boost levels before grenading the engine. I think a low compression engine with the same components will live longer with high boost pressures.
That is a nice engine. I like it.
Always been curious that a camshaft can dictate the dcr of motor. Example a a 8:1 compression motor can have a dcr of 7:8:1 but a 12:1 motor can have preferably 8:2 to 8:4:1 dcr. Both motors will crank 180-190 psi. Even though different compression ratio.
How does dcr come in affect when boosting 🤔.
does equal dcr a actually manifest itself in identical cranking compression
I went with middle of the road 9.5 to 1 with my turbo coyote motor for longevity and forgiveness. I know the stock compression ratio is higher and guys boost them but they're not out there every weekend at 7500rpm either.
The higher the compression the lower the Boost. I want to try 12.5:1 at 4-8lb of boost in a LS3 or K24/20 head. Twin Garrett 3776's or single Garrett 3776 for the K, electronic wastegate. Start at 1lb. and work up to the max. I'd stop at 8lb. even if it took it like a champ. 4 boost settings, 2lb. 4lb. 6lb and 8lb.
From my experiences with low compression motors, they do an excellent job in colder places. Ohio, Minnesota etc. Low compression motors are amazing for torque gains. Gives you a different feeling when you put your foot on the pedal.
I've seen the exact opposite. The high CR starts easier and makes more Tq overall.
The 8 5 to 1 would last a lot longer due to the lower compression ratio,and you could run it a tad fat on E85 and drop the oil a 10K or 5K,be an easy one to love
No reason to run compression that low with e85. I’m using 10.6 to 1 static with 20 psi from a Procharger on e85. Been 4 years without issue.
Low CR is dinosaur inefficiency. Engine control systems and engine design in 2022 do not require low CR for boost.
First, I enjoy your videos and like your lowest common denominator style delivery free of arrogance, it's refeshing. I've subscribed. I'd like to make a suggestion. To truly understand the difference between the two combinations presented, you also should provide the basic tuning settings on each combo when optimized. This leaves me wondering if your NA test and boosted test on your low compression engine was at the same timing and fueling settings. Without that information, power could be anywhere in the range presented...I can easily tune down my 7.88:1 compression centrifugally blown BBF with nothing but spark and lose 100hp. What were the settings? Did you optimize each to the edge of detonation then back it off a few degrees? What is your process?
To respond to you about which I think is better, I need to also know what you mean by "better". If "better" means durable, the reason my 7.88:1 combo is what it is is because you CAN NOT detonate it at the limit of the supercharger, even if you have a brain fart and dial in 40deg total advance. It will just stop making power and nose over, it won't ping. I make all my power with flow and leave a ton on the table for safety margin. I don't need more than 1000hp, and I don't care about efficiency.
na and boosted timing values the same? no
I believe we have reached the next plateau in internal combustion engines. Now the parts are stronger can we get the cooling and lubrication systems to meet these higher demands???
what do you think max comp ratio would be before you start getting self ignition?
Depends on the fuel and combustion chamber design
The size of the booster can determine lower compression but cam crutching is the key.
Through my years I've found you will make more power on a higher compression engine. A lower compression engine really shines at very high boost 35psi + and large spark advance. You will detonate a higher compression engine allot easier. I would rather have low compression and high boost then higher compression and low boost.
Richard. Looking at their 408LS, is the 10.8 compression too high for boost?
NO
I'm running 11.3:1 compression
E85 and 5 lbs of boost
With the cam I'm using
My effected compression is 15.47:1
I've done a few things to help
Heat coated tops and dry lube on skirts Total seal gappless second ring gapped at .028 4.31 bore 4.25 stroke 496ci AFR heads with mirror polished combustion chambers.
I don't have any problems
To me I like higher compression and lower boost levels. 10:1 with about 4 to 8 lbs of boost. With an intercooler. I think a street engine would do fine with 93 octane at this combo. Of course if needed timing can be pulled out or octane booster can be used or e85 for sure would make it very streetable. I like twin turbo best then the procharger is good but I wouldn't run a roots blower for the street unless it's for a street rod to look cool and then I would cut back the boost of it lower the compression and it would be more show than go.
Compression is great for power and if you have detonation issues add a camshaft with overlap to help I like compression with less boost then lower compression high boost
What’s the best static compression ratio for a pump gas only ls7 427 ci engine with a hp goal of apx 1000 hp???
boost?
@@richardholdener1727 not more than 20 psi. With a pd blower probably a whipple
I thought they were going to put the blower on the 10:1 motor lol
The book I recall reading - mid-late seventies era; I think the title was something like “super power” - spoke of 20% *underdrive* and 8 to 1 compression when running a Roots blower. (For routine-level street use, I think.)
Is the lower compression 540 giving up it's tight quench to achieve 8.5:1?
mostly big chamber
Back in the day it was 3 things, 1 fuel. Didn't have e85 at the pump, 2 didnt have computer software that could control things properly. 3 we didn't have you Richard.
well, 2 out of 3
So you're saying stock compression is good for boost? I wanna run twins on a sbc but i dont know where to start
octane, boost, compression and timing are all related
1bar seems safe. What if you're one of of those guys that needs a 5bar map sensor? Would the story be the same at 30 or 45psi?
Probably not, but if you're running a 540 up at 30 or 45 psi of boost you are probably on alcohol, and at that point you have so much octane and charge air cooling it isn't an issue.
What I want to see is a 13-1 compression with low boost
I don’t play with old v8 stuff. But on my 4g63 I run 10:1 and any e85 only car I build the motors 10:1. Motor has seen 47psi from a 67mm turbo and 788whp. E85 is amazing stuff. Performance is on par with c16.
If the engine is not in a race only (or boat) application where a certain amount of practicality goes out the window, higher compression is the way to go, you'll make more power for a given amount of fuel, so the fuel system can be smaller and you don't have to stop at the pump as often. You will also make more power off boost which will make the thing easier/nicer to drive in a few different ways; it can compensate (a bit) for a wilder cam at low engine speeds making any low speed driving much nicer, it also means you might be able to shave a few 100 rpm off your stall on your torque converter.
Modern ECUs can do closed loop boost control, knock retard, IAT retard, exhaust backpressure retard and table blending based on ethanol content; people are talking about driving on pump gas and lower comp being better, but really all you would need to do is either change map with slightly lower boost and less timing, or do nothing and let the ecu pull timing if your ECU is tuned correctly.
Not exactly. Knock sensors are not exactly scientific level sensors and some setups can suffer catastrophic damage almost instantly if they encounter detonation. It takes a dyno and good tuner to find that line in the sand. You add tiny amounts of timing until the motor no longer produces increased output. Then you back it off, to each tuners opinion, on what they feel is adequate safety margin...
Jesus christ those mouth noiseds killed me dude 🤣🤣🤣🤣
Good tune good gas good to go
If you trailer your car to the track, yeah, go for the compression, because it's not gonna get hot enough. For a street car, choosing the lower compression offers your a better portfolio of benefits with boost. That said, 8.5 may not be necessary. 9.0-9.5 is fine with a big cam.
I like the lower compression because it is more forgiving. More power is always better, until you break something because it's temperamental.
I think the question is what the goal is. Absolute max power? Low CR and lots of boost wins. More return on boost than CR. Driveability with a flatter curve without spikey boost curves? Higher CR and slightly less PSI from the whistle bits.
Yes on low compression for a blower motor on pump gas.
Cylinder pressure is cylinder pressure right
4500 flanges offer better cylinder-to-cylinder distribution and a little straighter path down the intake runners over an equivalent 4150 intake, right?
Let's be honest, fuel quality(and, later, emissions) is what has driven compression ratios since the earliest engines, and, today, fuel quality is as good as it's ever been. A better NA engine is a better power adder engine all said and done, it's better off of the power adder(minimal boost or not spraying it) and it's better on the power adder.
This essentially goes back to "should you build your boosted engine, or just increase the boost?" - at the end of the day, 1000HP is still 1000HP, and an appropriate torque converter & gearing can band-aid any power curve(I mean, if they can make 1000lb-ft@1500RPM diesels work... Lol), so this may not be particularly relevant to the junkyard LS + Amazon turbo crowd - but, if you're building an engine, you're going to have to give me a hell of a reason why not to go high comp. There's applications where you want the greater safety margin from detonation for sure, but fuel quality is so good today, nevermind the availability of an alcohol fuel at the pump(E85), that the risk has fallen. It wasn't that long ago that everyone thought these Coyotes were going to be blowing up with boost because of their 11:1 stock compression - so much for that theory LOL.
Maybe I'm biased though, we've been spec'ing out an engine build for my nephew's Mustang and, having grown up with the Modular scene, I was influenced by high comp + boost earlier. Needless to say, right from the beginning we've been planning on ~14:1 comp with softened chambers, and moderate boost in the future. Probably a 438" Windsor, probably high ports, and probably a Holley Hi-ram, controlled by a Terminator X... ...but nothing is set in stone until the money is spent of course, but we've already started with the TermX.
Oh yea, and for an E85 'street car' of course lol
I think you hit the nail right on the head. I think back to how many bone stock LS2’s were running around with 10+ psi of boost on daily driven vehicles. Mostly Corvettes. 11:1 CR and stock ring gaps. That was before E85 was super popular too.
It's been proven that with a roots blower, lower compression with higher boost will outdo a higher compression and lower boost.