Backpressure is bad, but exhaust velocity is good for an N/A engine. An exhaust that is "too large" will reduce exhaust gas velocity AND back pressure. The loss of back pressure isn't the issue, but the loss in exhaust gas velocity IS a problem, because it reduces scavenging. This is only applicable to non-turbo engines.
It also very much applies to turbo engines. It's harder to target an appropriate runner length and diameter and the effect isn't as dramatic but you can definitely modify your VE at various RPMs both on and off boost with a properly designed turbo header. The improved VE off boost can actually help low rpm spool time as well as provide a bit more part throttle response without hindering higher flow performance (within reason).
Agreed 100 %. Just change your small cam 283 exhaust from 2 to 2-1/2, you will notice right away lost low rpm torque. Big pipe only benefits high rpm power.
I could be wrong, but if you reduce the exhaust gas velocity and there's no scavenging there should be technically more back pressure than an exhaust that has higher flow exhaust and scavenging. Since you need a pressure differential to cause the scavenging effect.
Yes and no, scavenging is caused by a negative pressure differential in the combustion chamber, backpressure goes against this as it creates a positive pressure differential. It's better to have absolutely no backpressure in any engine, even in N/A engines due to modern engines taking advantage of the benifit of scavenging, turbocharged/supercharged engines are already forced induction engines, so scavenging is a really, *really* bad idea, as the extra boost will leak out the exaust port if u tune it that way. All scavenging does is pull more air in near TDC between the exaust and intake stroke, by leaving both the exaust and intake valves open slightly at the same time. High cam profiles on drag cars take advantage of this affect all the time, but have shitty low-end rpm power due to the side-effect of the exaust gasses being pulled back in near TDC during the intake stroke. What manufacturers did to mitigate this side-effect is introduce tech like VVT (GM's variable valve timing solution) and VTEC (Honda's variable valve hight solution), which allow for smooth low-end torqe and punchier high-end power utilizing only all the benifits of scavenging. So backpressure is really only an limiting factor of your vehicle no matter if its N/A or not. And if u say "what about Nitrous Oxide emmissions, isnt that more dangerous to the environment than loose hydrocarbons?", ill just say there's already a device that mitigates that issue, it's called the EGR valve, get a bigger EGR valve if u dont want backpressure from it.
Backpressure is one of those things where its presence is a negative factor but since improving other factors (exhaust velocity and pulse tuning) tend to increase it, people think its presence is what is making the improvements. The reality is backpressure needs to be minimized while maximizing the other factors. Everything is a compromise. I like to put it this way, "backpressure is a necessary evil that should be minimized".
I was thinking about this, I don't think there's increased back pressure on an exhaust that has great velocity compared to one that has a larger diameter and no scavenging. I think it's just the idea that people got in their head was bigger exhaust automatically equals less back pressure and smaller means more. So in their head they think there's a happy medium because too big made less. But the reason why a properly designed exhaust with great scavenging and high exhaust velocity works better than an oversized exhaust is because it reduces back pressure. Higher velocity air exhibits reduced pressure, not more, that's how scavenging works it creates a greater pressure differential. It's part of Bernoulli's equation. It's the same issue when sizing irrigation pipe. You go too small you start increasing the speed of the fluid, if the fluid starts speeding up too quickly what will happen is that it'll start causing turbulence along the sides of the wall effectively making the diameter of the pipe too small. It's the same issue when old gear heads think a high flow water pump doesn't allow coolant to "cool off" in the radiator because it's moving too fast. In reality what is happening is the volume trying to be moved at a certain speed has to deal with the fluid friction on the walls at a certain speed it create eddies along the walls, which reduces overall flow because it's effectively decreasing the size of the interior diameter of the pipe. Same thing when tuning an exhaust.
YES! Thank you! And scavenging efficiency can be optimized to fit the demands put on the engine by using a tuned header. ANNND if you want your N/A 6.2 to pull a house up hill in overdrive &&& sound good doing it, you will have more back pressure than a racing header will create. THE END
@@jasonreynolds9097 You mean you'll have less back pressure at low RPMs. It's all a game of reducing back pressure at different loads. Since there's not perfect way to reduce back pressure completely at all RPMs builders have to tune for how they want it to best perform per it's intended purpose. If the goal is to improve TQ at at 1500rpms then you'd tune the exhaust so that it flows the best at that RPM, so tubing sizing, runner lengths, etc are best matched to the power output and load characteristics of the engine, resulting at that RPM lower back pressure. But since it's a give and take system at high RPM you might not get as much power because the back pressure will increase. Unless someone has a really ingenious way of controlling exhaust gas pulses in a way that isn't a give and take you'll choose where to reduce back pressure to provide the best power and efficiency anywhere else on the power band will encounter more back pressure and reduce efficiency. In the end it's all about how to reduce back pressure for your intended purpose, it's just we currently have no way of eliminating it from every part of the TQ curve. That's why it's silly to say "we need back pressure".
@@sigmaprojectsGreat explanation, I have studied fluid mechanics and this kind of topics can be hard to grasp from the theory cause they seem very counterintuitive. As an additional fact I would like to add that this sort of phenomena happens due to the influence of “inertial forces" versus “viscous forces" and by changing the diameter of a pipe you are kind of favoring one against the other, but every system has optimal points of work and if you move outside of them, things won't behave as expected.
Erik is getting really good at this, almost like he as taken an acting class or two. It is awesome to see him teaching like this, and following in grandpa's footsteps.
I love how concise and accurate this video is about the back pressure argument when it comes to exhaust scavenging for an NA engine. Hands down the best video I’ve seen so far explaining this engineering concept.
you are Close the word Scavenging is the problem here still missing the point of air to fuel intermixing to attain the most possible work to power from the fuel that is consumed there is a point where back presser is needed and at a point where it becomes too great
Completely agree devyn! Was really well done Clay backpressure isn't needed. If you have freah fuel/air going out the exhaust then it's now about optimizing valve timing for the setup
@mattmorrison6958 if you have fresh fuel and air coming out of the exhaust you have a major issue. The fuel should be converted to carbon. Nox indicates an improper burn.
@@claycoates5056 yeah what you said here completely contradicts everything they showed in this video. back pressure of any kind is bad. scavenging is different and good but only works with a proper set of headers as shown.
It was a great party trick but it wasn’t a demonstration of exhaust scavenging specifically. When one port is open and has positive pressure (the shop blower), the others are closed. That negative pressure at the other ports is lost. On the exhaust side of the engine, there is both flow (from the piston moving the gases out) and continued expansion of hot gases from the combustion process (the continued expansion of these gases is primarily what a turbo uses to spin its turbine). When the exhaust port shuts, that flow stops but the expansion continues. The flow created by the piston is only minor and hence why this demonstration is properly representative of how scavenging works. only good as a party trick. The bulk of the required exhaust flow comes from the continued expansion (positive pressure) of hot gases after they leave the engine, and which continues well after the port is shut (and for several more combustion cycles for each cylinder). That continued expansion presents a problem for other ports when they open and try to flow out their initial gases, they are stuck trying to push against those previously burnt but still expanding gases. Headers address this by separating the cylinders for longer, firstly avoiding the potential conflicts to other ports, and then by being “tuned” length, they can use the properties of the expanding gases (momentum etc) to help create a negative ‘pulse’, which if timed properly, can assist the piston extracting the next lot of gases. This tuning or matching to application is also a reason why people think a little backpressure is needed. Put too big headers or exhaust onto a standard engine, you will lose the effects of headers and will not make more power. People’s minds tend to equate that to “you need less flow (backpressure)”, rather than “you should use the right exhaust for your application”. Hi performance two stroke engines absolutely rely on a tuned exhaust to make power, they use specially designed exhaust systems, with different diameters and changes in diameters to make use of the expanding gases, and to create very strong negative ‘pulses’ back to the cylinder to not just evacuate exhaust but to also help draw in a fresh mixture (two stokes primarily draw in mixture when the piston is going up which is opposite to a conventional four stroke). Also, residual exhaust gases in the combustion chamber can be good - Mazda are successfully using exhaust gases to reduce the effective cylinder volume in their new (and recently patented) supercharged two stroke to allow a much leaner fuel mixture without detonation, effectively achieving reliable diesel type lean burning.
I'm no expert, though I am a mechanic and an enthusiast, and I've been toying with exhaust for 20 years. The first time I put headers and pipes on a small V8 truck was 20 years ago and afterwards it could no longer do a burnout. I was young, confused, and devastated. Even put a smaller tire on it just to see what would happen, still no burnout. However, the truck did have a noticeable increase in acceleration after 3k RPM. A few years later I swapped the 2.5' pipes for 2' pipes and like night and day the truck could burnout forever, not even needing the break peddle. This was the beginning of my interest and research into exhaust. I promise this much, a stock non-turbo daily driver needs a smaller pipe to increase the velocity at the exhaust valve while in low RPMs to maintain cruising power and off idle torque. Also, my fuel milage increased with the smaller pipe. Scavenging and velocity is kind of my thing. lol I'd love to prove it.
@@19Borneo67 Mmhmm and if the dyno were reading idle to 3500 RPM, back pressure and velocity would win against open headers on any N/A engine any day. The "some people" he is referring to that say "you need a little back pressure to make power" are referring to both wide open throttle and part throttle on the street, where people push the skinny pedal.😂😂😂
@@19Borneo67 I really wish I had read your comment, cause I would have just liked it. I mean, the dyno in the video is built to recreate real driving. 🙉🙊
@@19Borneo67 Did you see the dyno in the video? No? Weird, must be because their real-world Dyno that they use for developing these parts isn't in this video. lol
I learned this 40 years ago while studying engineering. I've had so many 'discussions" about back pressure it is NOT funny. A great video that fully describes the matter in ways that would make any engineer proud. Cheers.
The Eyes have it. Science them ex haust savaging skills them ther header jobers have. But please make a video showing us how to choose the proper exhaust tune to reach peak efficiency for both N/A and Turbo engines at different points of engine RPM. Then compare their size and backpressure so we can lay this to rest.
Common misconception about hair dryers and vacuums when you hear the motor spin up faster it is because the load on the fan (impeller) is actually decreased allowing the motor to spin more freely. You can feel the pressure build but opposite of a piston pump which gains resistance as pressure is built.
Yep. I used to design pumps for liquids and it's the same thing where if you close a downstream valve the torque required goes down. Counterintuitive but it's true.
As a retired Master Plumber, I can say with confidence, that any bend in a pipe of any kind reduces flow. Any time gasses or liquids have to merge into itself will create cavitation to some degree. How much does a 90 degree bend reduce air flow? A long radius 90-degree bend (a bend radius of 4.5 inches, or 11.4cm) has a flow restriction equal to about 5 feet (about 1.5 metres) of straight pipe.
@@CL9k24a3Fluid dynamics are true for all fluids. The only thing that makes a difference is the physical properties of the fluid. The same rules always apply.
One of the better videos you guys have done. Animation and production values getting even better. I always tell people you don't want backpressure, but on non-turbo engines it is important to maintain exhaust velocity for scavenging with a proper header.
I think the main reasons people used to think backpressure was needed was because headers do have a power drop if they're run open with no collector extension, and also because if you merely unrestricted a severely restricted exhaust (as was more common then) on an old carbureted engine, it could actually effect the fueling enough that it runs worse. On the topic of manifolds, as a gas guy I'm always shocked how even modern diesel engines are designed as if gases don't need to flow at all in a heavily turbocharged engine. All the same flow and reflected wave dynamics exist when a gas is under pressure. I'm curious how much free horsepower could be had on something like a duramax with a header style manifold over that horrific log.
Headers dont "drop power", they "move" it. The same as any other exhaust setup. This is why Honda uses a Single outlet "header" on their J series v6's that make 250 hp from the factory... They use it to put the power down low for take off and drivability rather than your "wide open free flowing" headers that make all their power at the high end (where you want it for drag racing).
@@D.L.PDevelopments see my comment about honda J series single port "headers". You can also look up actual dyno results with back to back testing to prove this claim. Banks power is WAY late... Decades.
@@crisnmaryfam7344 They use it because it's cheap and easy, period end of story. Long tube headers pickup torque hugely over an otherwise well flowing manifold with no scavenging
This is why I appreciate you folks. Actual research and testing. Perfect example being the 4.0L Jeep "Revolver" header. It WORKS! I love mine. Definitely worth the money. Thank you for great products that actually improve efficiency.
When a hairdryer outlet is blocked the load decreases because it is no longer moving air. That is why the rpm of the electric motor increases. By blocking the outlet you have moved all the way to the left of the pump curve 🤯 A better analogy would be throttling the outlet 😁
@levimayes8488 No. It applies even if you completely block the OUTLET side as well. This is because if no air can go out, then no air can go in. And if it isn't moving any air, then it isn't under any load. Which is why you can hear the motor speed up. It's important to remember that devices like hair dryers also use the air moving through them to cool the motor, so blocking the airflow, whilst reducing the load on the motor, will still cause it to overheat.
That's true of centrifugal blowers and pumps. The opposite is true of positive displacement blowers and pumps. Purposely throttling the exhaust on a diesel engine is one method of increasing engine braking on a diesel engine.
Gale, you are one slick ol’ dude I swear. While making everyone think “wow cool I just got a killer education on back pressure” you smoothly slid those advertisement in for your products, 😂😂….small, quick, but they were there, …nicely done sir 👏 And btw, well done on the exhaust and header, they look top notch! I try to tell my buddies if you ain’t buying Banks your getting second best.
Great video and I'm glad you touched on even, though very briefly, that belt driven boost is the only parasitic loss boost. The shop air in the 4.0 header was amazing. Best demonstration of exhaust scavenging I've ever seen.
My understanding: You can't get scavenging without having an exhaust system, which could be thought of as a restriction, but really you're benefitting from the continuous positive controlled motion of air away from the engine, and any actual backpressure is a side-effect not a thing you specifically target. People that say an oversized exhaust has 'not enough backpressure' should actually be saying it has 'not enough velocity'. Correct me if i'm wrong!
This is exactly why chopping off factory exhausts especially ones with cast manifolds makes engines run like shit and not rev out anymore. My daily drive is a 92 Nissan Bluebird with stock spec rebuilt CA20e, headers running into standard 2" or 2.25'' tube, cat delete, original resonator halfway and rear muffler replaced with 3.5" resonator. The velocity is so good it makes big flames out the back like a race car when shifting over 3000 rpm!
At 3:20 you spin the turbo and it spins quite freely for some time, thus a reduction in back pressure drag along with a decrease in spool up time would make a considerable gain in performance. I've worked with turbos that wouldn't spin at all brand new right out of the box!!
Scavenging is the opposite of backpressure. It's sucking gases from the cylinder by creating a negative pressure wave. I hate when people say you "need" a bit of backpressure. Drives me crazy.
Me too, i actually gave up trying to explain it to people. It’s just to much work getting thru the bullshit sometimes. I’ve had guys get red in the face and all bent outta shape arguing you GOTTA have back pressure 🤬. That back pressure myth is a strong one. Every once in a while someone will have enough knowledge and get it that you dont want/need any back press but most I encounter swear by it!
I had my performance muffler guy eliminate as much back pressure as possible while leaving the cat in place to stay legal. He removed the resonator - never realized my car had one - and he installed much bigger exhaust pipes and freer flowing mufflers. GREATLY IMPROVED PERFORMANCE & the sound is incredible.
I think this misconception comes from info about 2 stroke exhausts, where they don't burn all of their gases so they have to design the exhaust in a way to shoot the gas back into the cylinder. I think this video describes super well the effect this has on many vehicles and i love it!
You are right, two strokes are a completely different design, and need the correct back pressure (pressure waves) to get the full potential of that engine Which is why you see expansion chambers on two stroke dirt bikes. It's a real science to make a two stroke exhaust, because it needs to resonate at the correct frequency to aid cylinder filling at a specific rpm range
@@XJ_doc back in the day when I had a scooter, I googled making exhausts for 2-strokes and I made one out of car exhaust, block of wood and hammer, grinder and a mig and I instantly got another 20km/h!
i saw the title and thought to my self "this vid will be bs". then i seent it was by the banks channels and i knew it would a good video, banks and team always deliver the facts!
Volvo had an interesting Log style exhaust manifold for their 5 cylinder. It had 3 seperate holes for exhaust to exit at the turbo flange and it was set up in such a way that no 2 consecutive firings would use the same hole. That way, none of the pulses colided, although they did have an unequal distance to travel. I'm looking at a 90s t5 manifold right now and cylinder 1 has its own outlet, cylinder 2 and 5 share an outlet, and 3 and 4 share an outlet. With a firing order of 1-2-4-5-3, the manifold almost behaves like headers.
Having driven a '95 Volvo 850 turbo and later a '97 Volvo 850R for around 9 years I do remember the tri-divided exhaust ports on the exhaust manifold. I do think those disappear on the Volvo S60R manifold though and that manifold is more of a shared design...though it's considered and performance "upgrade" on the 850 turbo, R, and GLT. I miss the 850R, but I am liking the '04 Corvette I traded for. Both are fun cars for daily driving enjoyment.
@@mlc7boosted popup headlights FTW! Won't find a volvo with those unless you import a 480. The R manifold is still somewhat tri-divided but cylinders 3, 4, and 5 all share an outlet. It still flows better than the 850 T5 manifold because of its larger diameter but at higher RPMs, a "Japanifold" will flow slightly more than the R.
Outstanding presentation in every respect. I developed technician training courses and instructional materials for most of the Asian car and motorcycle companies based in So Cal for 20 years. This was top level stuff.
Thank God you're doing this video, I've been arguing with people forever about this. At best your reducing overlap any way your doing it your reducing the atmospheres ability to move air into the motor. If you need to shrink primary tubes to the collector then do that but after the collector there should be zero pressure as fast as possible.
Absolutely 100% spot on. I've been building older carbureted V8's, carbureted Inline 4's (motorcycles) and of course the almost indestructible Jeep fuel injected 4.0 HO's for decades. This whole "back pressure is good" myth started in my experience back in the 70's and 80's when guys would bolt on a respectable brand of headers, decent mufflers and loose power on a stock machine... WEll, YEAH! They never jetted the carb up to take advantage of scavenging and the Helmholtz effect of resonant tuning on, of course, carb'd vehicles/bikes. Once tuned in, a genuine (oh damn man!) response was typical. On FI vehicles, the stock computer fuel trim can easily compensate for reduced back pressure especially if you live at high altitude. Case in point.... On my 4.0 I had a Magnaflow cat, generic SS header (AFM which I believe is out of business and had to weld up the the cracks twice) and generic cat back system that always passed emissions until my state changed to California emissions standards. I band clamped in a Cali spec cat, passed with flying colors but lost 2+ mpg and it lost a huge amount of power while merging into traffic (above 3k rpm), it was sad. Just for grins, I got a Banks after cat system and popped my old cat back in and gained 3+ mpg and my 3000+ rpm power was back! On top of that, the Banks sounds so much better at idle and no more drone than the summit cat back turbo muffler. I'd love to try your headers but is there any guarantee in regards to cracking?
Banks is not the first company to make an intercooler style exhaust tip and market it as performance enhancing. I’m not one to question Banks’s claims, but all previous versions have been proven to do nothing. Or at least nothing in the testing environment/setup used in those tests. My question is what testing results does Banks have to prove that their tip makes/liberates power? What exhaust, engine, muffler combination and so on was used to verify the results? What are the actual real world benefits and what vehicle/powertrain/exhaust combination is optimum to see real world results?
There is so much I cant stand about this subject. Especially when people try to "debunk" backpressure. The term was an oversimplified misconception that was likely made up by a sales person. In my opinion the common understanding of "backpressure" in an exhaust system can really only start after the last meaningful point of harmonic tuning in the system its self. These points generally being dictated by the changes in pipe diameter and the distances between them. Examples being, exhaust runner to header/manifold primary, primary to collector, collector to emissions or turbo, and so on. Everything after that "can" only create back pressure if that part of the exhaust is able to present a meaningful impedance to the engine's exhaust flow at or beyond some point in its operating exhaust cfm range. In this circumstance I do agree backpressure can only be a bad thing, but it also generally only becomes significant in max engine mass flow situations that most trucks rarely experience. I'm attempting to speak as accurately as I can, so apologies for the wordiness.
I would have to argue the point of most trucks rarely seeing "max engine mass flow," as you state. Especially when you consider trucks, by their very nature, are designed to pull weight. Whether that be payload, or a trailer. A truck, pulling say a camper, is more likely to see higher throttle loads regularly than you average commuter car. Especially when you consider gas engines, as they usually share very similar engines with smaller passenger cars. For example, my 5.0 F150 will need more torque to accelerate from stop to highway speed at the same rate as a 5.0 equipped Mustang due to it's greater curb weight and higher drag coefficient. Now, consider that same scenario but with a 6k pound trailer in tow. The truck will need roughly double the torque as before to accelerate at that same rate because we've more than doubled it's gross combined weight.
Great vid in lots of ways. It’s worthy to note, however, that the physics at play in boosted apps are differently influential than those in N/A apps. Apples and oranges. To be strictly technical, the best exhaust system in boosted apps is basically as close as possible to no exhaust at all, which is obviously illegal on the road. In N/A, the fluid dynamics in the exhaust influence torque curve and effective rpm range, among other factors.
Hi Erik, I happened across your channel and found it to be extremely informative and interesting. I found myself binge watching a lot of them. It is extremely apparent that Gale is a genius to say the least. However, I have one small problem, and that is I have absolutely no idea what language you and Gale speak (that’s code for a lot of the technicalities go over my head). Hello from Australia. Keep the informative information coming.
Thank you for sharing this, I learned a lot! I have a 2000 jeep xj and will for sure be looking into a bank full exhaust system for it after watching this video!
Coming from the 2-stroke world, I find that most car people are at a real caveman level regarding exhaust and scavenging. I really like how Banks consistently delivers good, accurate information.
Lmao believe it or not the 2.0 after market headers is literally just a 90° elbow pipe because the head design although the 2.0 suffers from a small turbo
Got into this video with questionmarks. But as you stated. Backpressure is often mistaken for exhaust scavaging. And yes. Good video, nicely made, good sequence. And hella informative
I had open pipes exhaust with my 2002 Harley 88tc, I put the original pipe tips back on that greatly reduces noise, and I added 10 hp easily by using restrictive factory pipe tips.
Awesome, as an engineer defending supercharging at times, I have always tried to stress that turbo charging has a dynamic parasitic loss issue too. Talking to turbo fanboys is hard work though 🤣😉 I think you are the first ever channel I have ever seen make the link about parasitic loss and turbo charging, well done 😊👌 I guess I should not be surprised, but it definitely is refreshing to hear it said in the mainstream auto engineering space 😊
I bet the "engine needs back pressure" tale is a result of removing the exhaust restrictions causes the engine to be less powerful, but the less back pressure caused the intake side to become lean.
In my experience, when you remove the exhaust system, there is a small power increase: BUT, there is a 2 or 3 times more increase in exhaust noise. This makes you think that the engine is much more powerful, but when you drive the car you don't get the performance that the increase in noise suggests. (I spent 23 years in Ford R&E Engine Lab testing engines)
Yep. If top fuel dragster builders could get 5 more HP by using longer headers, they would certainly use them. They use just enough length to deflect the flames away from the car.
Yeah. Would have been a good segue into how the exhaust scavenging he did demonstrate also helps the intake stroke get a better charge into the cylinder. I suppose, though, given how every effect is linked to others, if you're only gonna talk for 10-12 minutes, there's just some things you're gonna have to leave out. Otherwise your audience is gonna feel like they just got a sip of water from a firehose.
Interesting video, nice demo on the scavenging affect and lack off. Ties in with stuff I’d been taught in the 00s about turbo exhaust system “ the best post turbo exhaust is no exhaust “ Not always practical lol 👍🏻
The very first time I heard of back pressure and the explanation it came with, I said to myself, "That doesn't make sense at all." Now, scavenging, that made sense. Backpressure always translated to resistance in my mind. Thanks to the Banks team and this video, it turns out I was right. I just couldn't put it into words. Thanks for proving I wasn't tripping about it all these years. 😅
Humans exhaust and intake are the same orifice. Not a good analogy. Maybe more like farting a marathon through a straw. Which I think I can do, personally.
Now you are incorrect about the hair dryer fan increasing load as you block the air outlet. You're not a master electrician so i wouldn't expect you to know this though. It's not a positive displacement pump, so as the restriction in flow increases you move less fluid thus less work on the motor. This is particularly helpful for constant pressure fluid pump systems. Just keep in mind it's an example that is not correct for this comparison. Engines are positive displacement pumps and as a result any increase in exhaust pressure or decrease in intake pressure will increase the power requirement for a given output
It's not about back pressure, but about exhaust gas velocity. A neck in the pipe will increase velocity in that region, and this increased velocity can be used to improve scavenging. This has been proven in drag strips. But a neck will create backpressure, which people assume is what creates power when it's just a side-effect of the resistance to flow. It's all about flow and velocity. It's like a triangle of flow, velocity and pressure.
Well said and explained, I have been saying for as long as I can remember, turbos are not free power, as everyone says, belt driven blowers get their power from the intake stroke, turbos get power from the exhaust stroke. With the like of this, you are getting to free power, thanks to Gale and his genius
Yeah, I say that turbos get their power to drive the compressor at minimum cost to the engine. Mechanically driven compressors take their power directly from the shaft that actually drives the car.
@@JNorth87tough video to watch? Banks knows more about airflow, pressure and anything to do with feeding engines air than anyone you know and who taught them. These guys know more about this stuff than we ever will. The example of the hair dryer was to explain it simply to morons
Excellent presentation! Depends - in theory it's bad because the piston has to push against the pressure to expel it and in a perfect world there would always be a vacuum in the exhaust. In practice it can't be achieved, although Detroit diesels, and some other large 2-stroke diesel engines, used exhaust scavenging pumps - that were re-purposed by "hot rodders" to became the foundations of the x-71 series superchargers! Best one can do is try and control the pressures to best achieve exhaust scavenging and initial intake flow - this is done with tailoring the pipe diameters for velocity, and lengths so place the low(er) pressure waves in the exhaust pipe to be low at the best time for cylinder filling and net torque. I do wonder if some work on the turbine housings, to improve the transition from the turbine outlet's inlet through the housing to the outlet's outlet, and the pipe's transition from that flange to the full diameter, wouldn't show some significant improvements?
Detroit blowers weren't scavenging pumps, they were on the airbox side of the engine and forced air into the engine to deal with the fact detroit 2 strokes can't self pressurise. Those blowers were the source for hotrodders.
that is incorrect using the hair dryer as an analogue ... when you block the air flow the fan/pump stops moving air which is the definition of work. moving fluid requires work. the fan is spinning but not moving anything... it is doing no work and current on the hair dryer would decrease...thus power goes down. put it on a watt meter... the blades have no differential pressure across them to resist flow. Part of the reason is a hair dryer is not a positive displacement pump it's a centrifugal pump.
This comes up so often, people absolutely adamant that ‘you always need back pressure, it gives you more torque’ then they get really upset when they can’t explain it 😅 Being a truck mechanic, I remember Gardner diesels having issues meeting minimum horsepower requirements as truck weights increased. Their quick fix was a tuned exhaust, Gardner called it ‘negative pressure’ for a loaded 32 ton truck , it used about a five inch diameter pipe that with matching stack. It used a flapper to stop rain getting in, but the pressure pulses were so strong that at three quarters throttle it still sucked shut, made a proper din 😂 drove the driver mad
Where does the mythological backpressure myth come from. I hear a lot of people debunking the myth. But I've never seen anyone promote it. The myth itself seems to be a myth.
There are plenty of situations where velocity is reduced and it has negative impact on cylinder evacuation and cylinder filling. He explained it in the video.
I had a pick up with a 402 BB . It had been sitting for a few years . I pulled it and installed new rings n bearings . It already had an aluminum intake and a big holley but no headers and a Crane cam . My budget didn't allow headers at the time . A buddy owed me and he worked at a muffler shop. He used 2 inch pipe out of the manifolds into turbo mufflers and 2 1/2 in pipe out of those and dumped just in front of the rear axle . The truck ran great. Before we did that I talked with 2 other shops and they wouldn't do what we did, and said the truck wouldn't run right due to no back pressure . Never really understood the backpressure thing tho .
Creating a negative back pressure increases power to a point but once you start over scavenging you're just wasting fuel. Back pressure can be a good thing though, as it can help with low end (low rpm) performance. At low rpm the valves are open longer so there's more time for the A/F mixture to get into (and escape from) the cylinders. You need a little bit of back pressure to stop from over scavenging. Another helpful note is HP vs Torque. How they relate to each other and how back pressure effects them both.
I’m 53. My dad graduated from high school in 1958 and was the most amazing mechanic I’ve ever known. He taught me that back pressure was necessary. It’s so weird, I wonder where the myth came from.
I'm 51. My dad said the same thing. Bring a mechanic doesn't mean you understand fluid dynamics. People confuse a loss in velocity as a need for back pressure
Very important information to clarify is that pressure does affect the power curve. The more free flowing the exhaust, the lower the rpm range for peak power, the more restricted pressure makes your torque curb higher in the range. Awesome video, especially the scavenging part, but I believe a turbo diesel is the wrong example. Diesel already has a very low and narrow RPM range, practically non existent. Then you pair that with a turbo for diesel engines that has to produce peak flow at very low and narrow rpm ranges. Its like a quarter or less of most gasoline engines.
Your example of the hair dryer is wrong. Motor RPM actually increases due to less load , because a smaller column of air is being moved. You are comparing two different types of pump, Positive displacement versus Dynamic.
This was a great video I used to believe the back pressure myth I know other people that still do! This is a great video to show why its always bad because they may not believe me when i tell them but they cannot argue with Banks!
You need to manage exhaust. In an ideal world you have none, the challenge is that just like you said you can create intakes and exhaust that improve volumetric efficiency above 100% through careful engineering of certain harmonics. Whether it benefits depends on the exact engine, induction, and exhaust impedances
Been more of a fan each video I see but now I like it so much I'm going to ask for motorcycle products. Ya boy wants a top tier exhaust from the geniuses you are. Unbelievably brilliant and extremely easy to understand content. Never stop. But also please start motorcycles. Thank you for the education time and time again.
Where i come from the issue of backpressure was with 2-stroke engines. It should be tuned to the motor, if too low you waste fuel by letting the the air-fuel mixture going unburnt into the exaust or if too high you loose power.
So what I have always heard was that on a NA vehicle the lowest possible back pressure was always best for maximum horsepower, but would cause low end torque to suffer. That isnt the case with a turbo, because the back pressure reduction spools up the turbo faster, making good torque. That doesnt happen without boost.
The reason for low end tq to be lost an engine with an exhaust designed for higher RPMs is because they're designing it for the air flow amount. At low RPM you're moving less air, so an appropriately sized smaller exhaust will allow for better scavenging due to optimal exhaust velocity. That's why there's so many devices on an engine to help adjust the size the intake and exhaust systems among other aspects like runner lengths and duration. For a turbo charged engine there's going to be differing sizes of various components based on how the engine can pump out exhaust gases. Just like how if you over size the turbo for an engine and it takes forever for it to spool or vice versa, too small of a turbo and it spools up fast giving you instant power, but completely choked up top, same issues.
@@sigmaprojects Sounds accurate. Also sounds like it proves this entire video to be false on everything other than turbocharged vehicles. Sooooo you DO need some back pressure on most other vehicles.
@@boost331 I'm sorry if I didn't make it clear. You never want back pressure. I said the reason for loss of low end power with a larger diameter exhaust is because the exhaust velocity will be slower at those RPMs. Slow moving air equals higher pressure, but it's also about how much volume of air you need to move. So at low RPM not a whole lot of air is moving, so if you want to cause scavenging effect you need a little smaller exhaust to improve velocity which will reduce reducing back pressure. But if you're trying to make more power at higher RPMs when the engine is moving a lot more air you'll need an appropriately sized larger exhaust to improve exhaust velocity for the volume of air you're trying to move to keep up the scavenging. Reducing back pressure. In every situation for your desired results you always want to reduce back pressure. It's just about sizing the exhaust to be the most optimal. Tuning runner lengths and such to capitalize on the pressure waves is also reducing back pressure. It's just not easy to make a one size fits all exhaust for every single situation. Modern engines have some ways to do with it, like variable valve time and duration. Think like a big cam engine, where at low RPM it sounds like it's going to die. The valves are opening up super wide and because at low RPM it's not needing all that lift the opening being too large slows the velocity and doesn't help in reducing back pressure, but when you rev it up and you need to move a lot more air the size is appropriate for not being restrictive and the exhaust can move better. For an every day car auto manufacturers will design the engine to have the least back pressure at lower RPMs since they know for those commuters they're not revving them up, they're just getting them from point a, to point b. Basically in a perfect world engineers would design a sophisticated exhaust system that could change in size infinitely to maximize all the benefits of every size and shape to reduce back pressure at any RPM and load, but such a design doesn't exist so they try to just design for the least amount of back pressure at the point where they think the driver will use the car the most.
Some motorcycle exhausts use butterfly valves to add backpressure at lower revs, but that is to compensate for the high valve overlap needed for higher power at near redline (typically well over 10,000 rpm). It is largely done for emissions and noise, but I have seen arguments that if the resonance is right, it can help lower-end torque. Unfortunately, the hair dryer analogy is off. Many centrifugal pumps or fans use less power when flow is restricted. That is why you hear the motor speed up rather than bog down. In many applications, restricting the output also reduces cooling air, so the motor will burn out even though it is at a lower load. Plastic fans or pump elements can also melt from lack of throughput (I found this out the hard way).
I don't think the butterfly valve is to add back pressure, but to improve gas velocity at low RPMs. By doing so it also improves MPG/Emissions. So it can be fore both. Many older cars would do this on the intake side too.
@@sigmaprojects It depends on the application. There are also some that are largely for noise mitigation. On engines that are tuned for peak power at very high rpms, the valve overlap is so great that unburned fuel/air mixture gets through the exhaust valve at lower engine speeds. In those cases, the increased back pressure reduces velocity going out of the exhaust valve and reduces scavenging. The first butterfly in an exhaust system that I recall was around 1980 on the two-stroke Yamaha RD400 and it was there largely to reduce emissions during deceleration. It predated their YPVS system.
@@jfess1911 I agree there are many reasons, but a butterfly valve reducing the size of the exhaust at low RPM will increase velocity, not reduce it. At low RPM larger exhausts will not be good at scavenging and you'll get a build up of exhaust gas pulse waves increasing back pressure. This is a common practice on intake runners too.
@@sigmaprojects Again, it depends on application. A typical butterfly valve only increases the velocity at the tips of the plates while the velocity is slow on either side and more turbulent downstream. It acts as a variable restriction. This is how gasoline engines have been throttled for decades. This is how I have mostly seen butterfly valves used. In an exhaust system, this setup typically just adds backpressure and a surface that can reflect pulses, but that second property is often not utilized. On the other hand, It is possible to design a system with multiple runners where the valves shut off some of them, increasing flow on the others. There are also more sophisticated systems, more frequently used for intakes, with multiple runners of varying lengths which are opened or closed by butterfly valves, depending upon operating conditions.
@@jfess1911 are you saying in the motorcycle system with the butterfly valve in the exhaust system it can operate at part close? Usually they're either open or closed to divert exhaust gases. What bike application are you thinking of?
I would be a bless to have gale make the video about how exhaust scavnging ties back to exhaust back pressure man, hope you guys at banks can make it one of these days. All the engineering about exhaust has it complexity y precision and it's so useful to learn it for any aplication! Hope to see that video soon💪
Where this myth came from is rooted in the seventies. When auto makers first started down the path of cleaning up emissions, they leaned out the carbs to lower hydrocarbons. I actually saw one vehicle that cracked the exhaust manifold, causing the engine to stumble. When the manifold was replaced , the engine returned to running normally. The normal back pressure was needed to maintain proper fuel mixture. When the built in back pressure bled out the crack , the mixture went too lean and the engine ran ragged. This was one reason for those looking for more power, would jet up the carbs along with better exhaust systems.
I think the whole back pressure argument came from the race cam in a street car crowd believing they fond a way to regain some of the cylinder pressure they were losing through their wide open cam timing. The fact is thinking you can gain power by leaving unburnable gas in the combustion chamber is ridiculous.
2014 Camaro SS 6.2L I ran Flo-Master Outlaw mufflers, X-Pipe and BBK tuned short headers and got about 26 mpg out of the car on the highway. From a stop I could push past the traction control and chip tires. Now comes the BBK full headers 1 5/8" with only two high flo cats. 19.5 mpg and no low end push. Now when I'm cruising at 80 mph... I can get to 136 mph real quick though.
I have a BRZ with the Edelbrock supercharger (with a well developed tune by DDPR), with the stock small header and exhaust I (closely) tracked fuel consumption for 9000 miles of mixed driving so the MPG figures were pretty refined. Then, even though the lore says that exhaust diameter doesn't matter in forced induction I decided to put in a cat-back system to change the sound. So stock was 2.0" OD (actually a bit under that as its something metric) and I went with a 2-3/4" and was pleased with the sound but noticed that there might have been an improvement in gas mileage.... And the following months proved it. I WAS GETTING at or just under 1.25 more MPG. Now that doesn't sound like lot but when your buying 93 octane gas that adds up over the months. And there was a fractional improvement the next dyno run for baseline before we installed larger injectors and I went alcohol.
For those of us that are using non turbo engines There is calculators available online to find the proper size, however you gonna have to find the best design for your situation to minimize the losses from bends.
![Felix "Unfair" | [Stray Kids : SKZ-PLAYER]](http://i.ytimg.com/vi/Oswujxm2Ag0/mqdefault.jpg)
Backpressure is bad, but exhaust velocity is good for an N/A engine. An exhaust that is "too large" will reduce exhaust gas velocity AND back pressure. The loss of back pressure isn't the issue, but the loss in exhaust gas velocity IS a problem, because it reduces scavenging. This is only applicable to non-turbo engines.
This is very true. So many times you hear of people who put a high flow exhaust on their car, then complain of loss of low rpm torque.
It also very much applies to turbo engines. It's harder to target an appropriate runner length and diameter and the effect isn't as dramatic but you can definitely modify your VE at various RPMs both on and off boost with a properly designed turbo header. The improved VE off boost can actually help low rpm spool time as well as provide a bit more part throttle response without hindering higher flow performance (within reason).
Agreed 100 %. Just change your small cam 283 exhaust from 2 to 2-1/2, you will notice right away lost low rpm torque. Big pipe only benefits high rpm power.
I could be wrong, but if you reduce the exhaust gas velocity and there's no scavenging there should be technically more back pressure than an exhaust that has higher flow exhaust and scavenging. Since you need a pressure differential to cause the scavenging effect.
Yes and no, scavenging is caused by a negative pressure differential in the combustion chamber, backpressure goes against this as it creates a positive pressure differential. It's better to have absolutely no backpressure in any engine, even in N/A engines due to modern engines taking advantage of the benifit of scavenging, turbocharged/supercharged engines are already forced induction engines, so scavenging is a really, *really* bad idea, as the extra boost will leak out the exaust port if u tune it that way. All scavenging does is pull more air in near TDC between the exaust and intake stroke, by leaving both the exaust and intake valves open slightly at the same time. High cam profiles on drag cars take advantage of this affect all the time, but have shitty low-end rpm power due to the side-effect of the exaust gasses being pulled back in near TDC during the intake stroke. What manufacturers did to mitigate this side-effect is introduce tech like VVT (GM's variable valve timing solution) and VTEC (Honda's variable valve hight solution), which allow for smooth low-end torqe and punchier high-end power utilizing only all the benifits of scavenging. So backpressure is really only an limiting factor of your vehicle no matter if its N/A or not. And if u say "what about Nitrous Oxide emmissions, isnt that more dangerous to the environment than loose hydrocarbons?", ill just say there's already a device that mitigates that issue, it's called the EGR valve, get a bigger EGR valve if u dont want backpressure from it.
That scavenging demo with the header and sticky notes was simple and beautiful. Thanks for continuing to battle misinformation!
That particular header is the Revolver. For 4.0L Jeeps.
Yeah that was really neat to see
Backpressure is one of those things where its presence is a negative factor but since improving other factors (exhaust velocity and pulse tuning) tend to increase it, people think its presence is what is making the improvements. The reality is backpressure needs to be minimized while maximizing the other factors. Everything is a compromise.
I like to put it this way, "backpressure is a necessary evil that should be minimized".
This is correct
I was thinking about this, I don't think there's increased back pressure on an exhaust that has great velocity compared to one that has a larger diameter and no scavenging. I think it's just the idea that people got in their head was bigger exhaust automatically equals less back pressure and smaller means more. So in their head they think there's a happy medium because too big made less. But the reason why a properly designed exhaust with great scavenging and high exhaust velocity works better than an oversized exhaust is because it reduces back pressure. Higher velocity air exhibits reduced pressure, not more, that's how scavenging works it creates a greater pressure differential. It's part of Bernoulli's equation. It's the same issue when sizing irrigation pipe. You go too small you start increasing the speed of the fluid, if the fluid starts speeding up too quickly what will happen is that it'll start causing turbulence along the sides of the wall effectively making the diameter of the pipe too small. It's the same issue when old gear heads think a high flow water pump doesn't allow coolant to "cool off" in the radiator because it's moving too fast. In reality what is happening is the volume trying to be moved at a certain speed has to deal with the fluid friction on the walls at a certain speed it create eddies along the walls, which reduces overall flow because it's effectively decreasing the size of the interior diameter of the pipe. Same thing when tuning an exhaust.
YES! Thank you! And scavenging efficiency can be optimized to fit the demands put on the engine by using a tuned header. ANNND if you want your N/A 6.2 to pull a house up hill in overdrive &&& sound good doing it, you will have more back pressure than a racing header will create. THE END
@@jasonreynolds9097 You mean you'll have less back pressure at low RPMs. It's all a game of reducing back pressure at different loads. Since there's not perfect way to reduce back pressure completely at all RPMs builders have to tune for how they want it to best perform per it's intended purpose. If the goal is to improve TQ at at 1500rpms then you'd tune the exhaust so that it flows the best at that RPM, so tubing sizing, runner lengths, etc are best matched to the power output and load characteristics of the engine, resulting at that RPM lower back pressure. But since it's a give and take system at high RPM you might not get as much power because the back pressure will increase. Unless someone has a really ingenious way of controlling exhaust gas pulses in a way that isn't a give and take you'll choose where to reduce back pressure to provide the best power and efficiency anywhere else on the power band will encounter more back pressure and reduce efficiency.
In the end it's all about how to reduce back pressure for your intended purpose, it's just we currently have no way of eliminating it from every part of the TQ curve. That's why it's silly to say "we need back pressure".
@@sigmaprojectsGreat explanation, I have studied fluid mechanics and this kind of topics can be hard to grasp from the theory cause they seem very counterintuitive.
As an additional fact I would like to add that this sort of phenomena happens due to the influence of “inertial forces" versus “viscous forces" and by changing the diameter of a pipe you are kind of favoring one against the other, but every system has optimal points of work and if you move outside of them, things won't behave as expected.
Erik is getting really good at this, almost like he as taken an acting class or two. It is awesome to see him teaching like this, and following in grandpa's footsteps.
Is Gale Erik's grandpa?
you can’t tell?
Yes
So he's not a DEI hire?
@@JohnSmith-xu7ev shit man... that had me laugh good.
I love how concise and accurate this video is about the back pressure argument when it comes to exhaust scavenging for an NA engine.
Hands down the best video I’ve seen so far explaining this engineering concept.
you are Close the word Scavenging is the problem here still missing the point of air to fuel intermixing to attain the most possible work to power from the fuel that is consumed
there is a point where back presser is needed and at a point where it becomes too great
Completely agree devyn! Was really well done
Clay backpressure isn't needed. If you have freah fuel/air going out the exhaust then it's now about optimizing valve timing for the setup
@mattmorrison6958 if you have fresh fuel and air coming out of the exhaust you have a major issue. The fuel should be converted to carbon. Nox indicates an improper burn.
@@claycoates5056 yeah what you said here completely contradicts everything they showed in this video. back pressure of any kind is bad. scavenging is different and good but only works with a proper set of headers as shown.
That demonstration of scavenging with post it notes was the best thing way to show it ever. Love it
It was a great party trick but it wasn’t a demonstration of exhaust scavenging specifically.
When one port is open and has positive pressure (the shop blower), the others are closed. That negative pressure at the other ports is lost.
On the exhaust side of the engine, there is both flow (from the piston moving the gases out) and continued expansion of hot gases from the combustion process (the continued expansion of these gases is primarily what a turbo uses to spin its turbine).
When the exhaust port shuts, that flow stops but the expansion continues. The flow created by the piston is only minor and hence why this demonstration is properly representative of how scavenging works. only good as a party trick.
The bulk of the required exhaust flow comes from the continued expansion (positive pressure) of hot gases after they leave the engine, and which continues well after the port is shut (and for several more combustion cycles for each cylinder).
That continued expansion presents a problem for other ports when they open and try to flow out their initial gases, they are stuck trying to push against those previously burnt but still expanding gases.
Headers address this by separating the cylinders for longer, firstly avoiding the potential conflicts to other ports, and then by being “tuned” length, they can use the properties of the expanding gases (momentum etc) to help create a negative ‘pulse’, which if timed properly, can assist the piston extracting the next lot of gases.
This tuning or matching to application is also a reason why people think a little backpressure is needed. Put too big headers or exhaust onto a standard engine, you will lose the effects of headers and will not make more power. People’s minds tend to equate that to “you need less flow (backpressure)”, rather than “you should use the right exhaust for your application”.
Hi performance two stroke engines absolutely rely on a tuned exhaust to make power, they use specially designed exhaust systems, with different diameters and changes in diameters to make use of the expanding gases, and to create very strong negative ‘pulses’ back to the cylinder to not just evacuate exhaust but to also help draw in a fresh mixture (two stokes primarily draw in mixture when the piston is going up which is opposite to a conventional four stroke).
Also, residual exhaust gases in the combustion chamber can be good - Mazda are successfully using exhaust gases to reduce the effective cylinder volume in their new (and recently patented) supercharged two stroke to allow a much leaner fuel mixture without detonation, effectively achieving reliable diesel type lean burning.
I'm no expert, though I am a mechanic and an enthusiast, and I've been toying with exhaust for 20 years. The first time I put headers and pipes on a small V8 truck was 20 years ago and afterwards it could no longer do a burnout. I was young, confused, and devastated. Even put a smaller tire on it just to see what would happen, still no burnout. However, the truck did have a noticeable increase in acceleration after 3k RPM. A few years later I swapped the 2.5' pipes for 2' pipes and like night and day the truck could burnout forever, not even needing the break peddle. This was the beginning of my interest and research into exhaust. I promise this much, a stock non-turbo daily driver needs a smaller pipe to increase the velocity at the exhaust valve while in low RPMs to maintain cruising power and off idle torque. Also, my fuel milage increased with the smaller pipe. Scavenging and velocity is kind of my thing. lol I'd love to prove it.
These guys are talking about wide open throttle readings on a dyno. Has very little to do with real world driving.
@@19Borneo67 Mmhmm and if the dyno were reading idle to 3500 RPM, back pressure and velocity would win against open headers on any N/A engine any day. The "some people" he is referring to that say "you need a little back pressure to make power" are referring to both wide open throttle and part throttle on the street, where people push the skinny pedal.😂😂😂
@@19Borneo67 I really wish I had read your comment, cause I would have just liked it. I mean, the dyno in the video is built to recreate real driving. 🙉🙊
@@19Borneo67 Did you see the dyno in the video? No? Weird, must be because their real-world Dyno that they use for developing these parts isn't in this video. lol
break peddle? Ahhh... brake pedal.
I learned this 40 years ago while studying engineering. I've had so many 'discussions" about back pressure it is NOT funny. A great video that fully describes the matter in ways that would make any engineer proud. Cheers.
Please do a video on the science of exhaust headers!
Seconded. I have much to argue/teach/scoff at on the subject!
The Eyes have it. Science them ex haust savaging skills them ther header jobers have. But please make a video showing us how to choose the proper exhaust tune to reach peak efficiency for both N/A and Turbo engines at different points of engine RPM. Then compare their size and backpressure so we can lay this to rest.
@@jasonreynolds9097it is engine specific... Nothing to lay to rest there. What works on ls not gonna work on k... I have tested that my self
Common misconception about hair dryers and vacuums when you hear the motor spin up faster it is because the load on the fan (impeller) is actually decreased allowing the motor to spin more freely. You can feel the pressure build but opposite of a piston pump which gains resistance as pressure is built.
Came here to say this.
Yep. I used to design pumps for liquids and it's the same thing where if you close a downstream valve the torque required goes down. Counterintuitive but it's true.
Interesting i wouldnt have thought about that with a liquid pump.
@@MrPdiggity Centrifugal pump yes, piston pump no.
When a vacuum cleaner gets clogged and spins faster and someone remarks that "it's straining" my head nearly explodes.
As a retired Master Plumber, I can say with confidence, that any bend in a pipe of any kind reduces flow. Any time gasses or liquids have to merge into itself will create cavitation to some degree. How much does a 90 degree bend reduce air flow?
A long radius 90-degree bend (a bend radius of 4.5 inches, or 11.4cm) has a flow restriction equal to about 5 feet (about 1.5 metres) of straight pipe.
Doesn't happen that way on exhaus gasses.
@@CL9k24a3Fluid dynamics are true for all fluids. The only thing that makes a difference is the physical properties of the fluid. The same rules always apply.
One of the better videos you guys have done. Animation and production values getting even better. I always tell people you don't want backpressure, but on non-turbo engines it is important to maintain exhaust velocity for scavenging with a proper header.
I think the main reasons people used to think backpressure was needed was because headers do have a power drop if they're run open with no collector extension, and also because if you merely unrestricted a severely restricted exhaust (as was more common then) on an old carbureted engine, it could actually effect the fueling enough that it runs worse. On the topic of manifolds, as a gas guy I'm always shocked how even modern diesel engines are designed as if gases don't need to flow at all in a heavily turbocharged engine. All the same flow and reflected wave dynamics exist when a gas is under pressure. I'm curious how much free horsepower could be had on something like a duramax with a header style manifold over that horrific log.
Is that why drag cars that are chasing every last HP run like that? Makes no sense
Headers dont "drop power", they "move" it. The same as any other exhaust setup. This is why Honda uses a Single outlet "header" on their J series v6's that make 250 hp from the factory... They use it to put the power down low for take off and drivability rather than your "wide open free flowing" headers that make all their power at the high end (where you want it for drag racing).
@@D.L.PDevelopments see my comment about honda J series single port "headers". You can also look up actual dyno results with back to back testing to prove this claim. Banks power is WAY late... Decades.
@@D.L.PDevelopments Like what? Open headers? I think you'll find most NA drag cars now have either collector extensions or a full exhaust.
@@crisnmaryfam7344 They use it because it's cheap and easy, period end of story. Long tube headers pickup torque hugely over an otherwise well flowing manifold with no scavenging
This is why I appreciate you folks. Actual research and testing. Perfect example being the 4.0L Jeep "Revolver" header. It WORKS! I love mine. Definitely worth the money. Thank you for great products that actually improve efficiency.
When a hairdryer outlet is blocked the load decreases because it is no longer moving air. That is why the rpm of the electric motor increases. By blocking the outlet you have moved all the way to the left of the pump curve 🤯
A better analogy would be throttling the outlet 😁
I was about to make a long and technical post about how the hair dryer has no load. You beat me to it lol
@levimayes8488 No. It applies even if you completely block the OUTLET side as well. This is because if no air can go out, then no air can go in. And if it isn't moving any air, then it isn't under any load. Which is why you can hear the motor speed up. It's important to remember that devices like hair dryers also use the air moving through them to cool the motor, so blocking the airflow, whilst reducing the load on the motor, will still cause it to overheat.
Came here to say this as well.
That's true of centrifugal blowers and pumps. The opposite is true of positive displacement blowers and pumps. Purposely throttling the exhaust on a diesel engine is one method of increasing engine braking on a diesel engine.
There was another comment spelling this out earlier but it seems to have disappeared... 🤔🙄
Gale, you are one slick ol’ dude I swear.
While making everyone think “wow cool I just got a killer education on back pressure” you smoothly slid those advertisement in for your products, 😂😂….small, quick, but they were there, …nicely done sir 👏
And btw, well done on the exhaust and header, they look top notch! I try to tell my buddies if you ain’t buying Banks your getting second best.
Great video and I'm glad you touched on even, though very briefly, that belt driven boost is the only parasitic loss boost. The shop air in the 4.0 header was amazing. Best demonstration of exhaust scavenging I've ever seen.
OMG! That has to be the best explanation I have ever heard! Yes, yes, yes! Exhaust manifolds are the worst! Great job.😊
My understanding: You can't get scavenging without having an exhaust system, which could be thought of as a restriction, but really you're benefitting from the continuous positive controlled motion of air away from the engine, and any actual backpressure is a side-effect not a thing you specifically target.
People that say an oversized exhaust has 'not enough backpressure' should actually be saying it has 'not enough velocity'.
Correct me if i'm wrong!
This is exactly why chopping off factory exhausts especially ones with cast manifolds makes engines run like shit and not rev out anymore. My daily drive is a 92 Nissan Bluebird with stock spec rebuilt CA20e, headers running into standard 2" or 2.25'' tube, cat delete, original resonator halfway and rear muffler replaced with 3.5" resonator. The velocity is so good it makes big flames out the back like a race car when shifting over 3000 rpm!
At 3:20 you spin the turbo and it spins quite freely for some time, thus a reduction in back pressure drag along with a decrease in spool up time would make a considerable gain in performance. I've worked with turbos that wouldn't spin at all brand new right out of the box!!
Scavenging is the opposite of backpressure. It's sucking gases from the cylinder by creating a negative pressure wave. I hate when people say you "need" a bit of backpressure. Drives me crazy.
This^
Yes, we need an exhaust system, which is a bit of back pressure
Me too, i actually gave up trying to explain it to people. It’s just to much work getting thru the bullshit sometimes. I’ve had guys get red in the face and all bent outta shape arguing you GOTTA have back pressure 🤬. That back pressure myth is a strong one. Every once in a while someone will have enough knowledge and get it that you dont want/need any back press but most I encounter swear by it!
@@JohnSmith-xu7ev 🤦
@@JohnSmith-xu7ev any back pressure from an exhaust is due to some undesirable trait usually. Especially for modern cars.
It's amazing that there are people still regurgitating the back pressure mantra. That myth needs to die.
I had my performance muffler guy eliminate as much back pressure as possible while leaving the cat in place to stay legal. He removed the resonator - never realized my car had one - and he installed much bigger exhaust pipes and freer flowing mufflers. GREATLY IMPROVED PERFORMANCE & the sound is incredible.
I think this misconception comes from info about 2 stroke exhausts, where they don't burn all of their gases so they have to design the exhaust in a way to shoot the gas back into the cylinder. I think this video describes super well the effect this has on many vehicles and i love it!
You are right, two strokes are a completely different design, and need the correct back pressure (pressure waves) to get the full potential of that engine
Which is why you see expansion chambers on two stroke dirt bikes.
It's a real science to make a two stroke exhaust, because it needs to resonate at the correct frequency to aid cylinder filling at a specific rpm range
@@XJ_doc back in the day when I had a scooter, I googled making exhausts for 2-strokes and I made one out of car exhaust, block of wood and hammer, grinder and a mig and I instantly got another 20km/h!
i saw the title and thought to my self "this vid will be bs". then i seent it was by the banks channels and i knew it would a good video, banks and team always deliver the facts!
Volvo had an interesting Log style exhaust manifold for their 5 cylinder. It had 3 seperate holes for exhaust to exit at the turbo flange and it was set up in such a way that no 2 consecutive firings would use the same hole. That way, none of the pulses colided, although they did have an unequal distance to travel. I'm looking at a 90s t5 manifold right now and cylinder 1 has its own outlet, cylinder 2 and 5 share an outlet, and 3 and 4 share an outlet. With a firing order of 1-2-4-5-3, the manifold almost behaves like headers.
Did you know banks made the first Volvo turbo for production cars?
Having driven a '95 Volvo 850 turbo and later a '97 Volvo 850R for around 9 years I do remember the tri-divided exhaust ports on the exhaust manifold. I do think those disappear on the Volvo S60R manifold though and that manifold is more of a shared design...though it's considered and performance "upgrade" on the 850 turbo, R, and GLT. I miss the 850R, but I am liking the '04 Corvette I traded for. Both are fun cars for daily driving enjoyment.
@@mlc7boosted popup headlights FTW! Won't find a volvo with those unless you import a 480. The R manifold is still somewhat tri-divided but cylinders 3, 4, and 5 all share an outlet. It still flows better than the 850 T5 manifold because of its larger diameter but at higher RPMs, a "Japanifold" will flow slightly more than the R.
Outstanding presentation in every respect. I developed technician training courses and instructional materials for most of the Asian car and motorcycle companies based in So Cal for 20 years. This was top level stuff.
Amazing video! Please do that header explanation one too!!
Thank God you're doing this video, I've been arguing with people forever about this. At best your reducing overlap any way your doing it your reducing the atmospheres ability to move air into the motor. If you need to shrink primary tubes to the collector then do that but after the collector there should be zero pressure as fast as possible.
the animator for the animation at 1:10 apparently doesn’t understand how piston rods work. haha.
Absolutely 100% spot on. I've been building older carbureted V8's, carbureted Inline 4's (motorcycles) and of course the almost indestructible Jeep fuel injected 4.0 HO's for decades. This whole "back pressure is good" myth started in my experience back in the 70's and 80's when guys would bolt on a respectable brand of headers, decent mufflers and loose power on a stock machine... WEll, YEAH! They never jetted the carb up to take advantage of scavenging and the Helmholtz effect of resonant tuning on, of course, carb'd vehicles/bikes. Once tuned in, a genuine (oh damn man!) response was typical. On FI vehicles, the stock computer fuel trim can easily compensate for reduced back pressure especially if you live at high altitude. Case in point.... On my 4.0 I had a Magnaflow cat, generic SS header (AFM which I believe is out of business and had to weld up the the cracks twice) and generic cat back system that always passed emissions until my state changed to California emissions standards. I band clamped in a Cali spec cat, passed with flying colors but lost 2+ mpg and it lost a huge amount of power while merging into traffic (above 3k rpm), it was sad. Just for grins, I got a Banks after cat system and popped my old cat back in and gained 3+ mpg and my 3000+ rpm power was back! On top of that, the Banks sounds so much better at idle and no more drone than the summit cat back turbo muffler. I'd love to try your headers but is there any guarantee in regards to cracking?
Banks is not the first company to make an intercooler style exhaust tip and market it as performance enhancing. I’m not one to question Banks’s claims, but all previous versions have been proven to do nothing. Or at least nothing in the testing environment/setup used in those tests. My question is what testing results does Banks have to prove that their tip makes/liberates power? What exhaust, engine, muffler combination and so on was used to verify the results? What are the actual real world benefits and what vehicle/powertrain/exhaust combination is optimum to see real world results?
Sweet, complete confirmation that deleting the emissions on a diesel it the way to go lol. Sold!
There is so much I cant stand about this subject. Especially when people try to "debunk" backpressure. The term was an oversimplified misconception that was likely made up by a sales person. In my opinion the common understanding of "backpressure" in an exhaust system can really only start after the last meaningful point of harmonic tuning in the system its self. These points generally being dictated by the changes in pipe diameter and the distances between them. Examples being, exhaust runner to header/manifold primary, primary to collector, collector to emissions or turbo, and so on. Everything after that "can" only create back pressure if that part of the exhaust is able to present a meaningful impedance to the engine's exhaust flow at or beyond some point in its operating exhaust cfm range. In this circumstance I do agree backpressure can only be a bad thing, but it also generally only becomes significant in max engine mass flow situations that most trucks rarely experience. I'm attempting to speak as accurately as I can, so apologies for the wordiness.
I would have to argue the point of most trucks rarely seeing "max engine mass flow," as you state. Especially when you consider trucks, by their very nature, are designed to pull weight. Whether that be payload, or a trailer. A truck, pulling say a camper, is more likely to see higher throttle loads regularly than you average commuter car. Especially when you consider gas engines, as they usually share very similar engines with smaller passenger cars. For example, my 5.0 F150 will need more torque to accelerate from stop to highway speed at the same rate as a 5.0 equipped Mustang due to it's greater curb weight and higher drag coefficient. Now, consider that same scenario but with a 6k pound trailer in tow. The truck will need roughly double the torque as before to accelerate at that same rate because we've more than doubled it's gross combined weight.
I knew it!!! I been reducing back pressure on most of my vehicles by simply cutting off the exhaust. Poor man's reduction of back pressure...
Great vid in lots of ways. It’s worthy to note, however, that the physics at play in boosted apps are differently influential than those in N/A apps. Apples and oranges.
To be strictly technical, the best exhaust system in boosted apps is basically as close as possible to no exhaust at all, which is obviously illegal on the road.
In N/A, the fluid dynamics in the exhaust influence torque curve and effective rpm range, among other factors.
Hi Erik, I happened across your channel and found it to be extremely informative and interesting. I found myself binge watching a lot of them. It is extremely apparent that Gale is a genius to say the least. However, I have one small problem, and that is I have absolutely no idea what language you and Gale speak (that’s code for a lot of the technicalities go over my head). Hello from Australia. Keep the informative information coming.
Thank you for sharing this, I learned a lot! I have a 2000 jeep xj and will for sure be looking into a bank full exhaust system for it after watching this video!
Back pressure is bad according to banks, run no exhaust
Coming from the 2-stroke world, I find that most car people are at a real caveman level regarding exhaust and scavenging. I really like how Banks consistently delivers good, accurate information.
Now make some tuned manifolds for a ecoboost. Great video . Thank you
Want
They would cost as much or more than the FullRace manifolds I bet.
Lmao believe it or not the 2.0 after market headers is literally just a 90° elbow pipe because the head design although the 2.0 suffers from a small turbo
Got into this video with questionmarks. But as you stated. Backpressure is often mistaken for exhaust scavaging. And yes.
Good video, nicely made, good sequence. And hella informative
I had open pipes exhaust with my 2002 Harley 88tc, I put the original pipe tips back on that greatly reduces noise, and I added 10 hp easily by using restrictive factory pipe tips.
Awesome, as an engineer defending supercharging at times, I have always tried to stress that turbo charging has a dynamic parasitic loss issue too.
Talking to turbo fanboys is hard work though 🤣😉
I think you are the first ever channel I have ever seen make the link about parasitic loss and turbo charging, well done 😊👌 I guess I should not be surprised, but it definitely is refreshing to hear it said in the mainstream auto engineering space 😊
Best video I have ever seen explaining the difference between backpressure and scavenging!
I bet the "engine needs back pressure" tale is a result of removing the exhaust restrictions causes the engine to be less powerful, but the less back pressure caused the intake side to become lean.
In my experience, when you remove the exhaust system, there is a small power increase: BUT, there is a 2 or 3 times more increase in exhaust noise. This makes you think that the engine is much more powerful, but when you drive the car you don't get the performance that the increase in noise suggests. (I spent 23 years in Ford R&E Engine Lab testing engines)
Yep. If top fuel dragster builders could get 5 more HP by using longer headers, they would certainly use them. They use just enough length to deflect the flames away from the car.
@@budgie98 my experience removing exhausts off standard engines is they run like a bag of shit and won't rev out!
@@Bob_Adkinstop fuel dragsters run at high rpm with a blower so there is no need for exhaust scavenging!
the scavenge demo with the sticky notes and air was excellent! awesome video!
Strange - I missed the part where he explained the principles of valve overlap... Good ad for exhaust systems that use turbos.
Yeah. Would have been a good segue into how the exhaust scavenging he did demonstrate also helps the intake stroke get a better charge into the cylinder.
I suppose, though, given how every effect is linked to others, if you're only gonna talk for 10-12 minutes, there's just some things you're gonna have to leave out. Otherwise your audience is gonna feel like they just got a sip of water from a firehose.
Interesting video, nice demo on the scavenging affect and lack off.
Ties in with stuff I’d been taught in the 00s about turbo exhaust system
“ the best post turbo exhaust is no exhaust “
Not always practical lol 👍🏻
Me simple man. Me hate back pressure.
i would absolutely love watching Gale do a video explaining how tube size/length/style effects backpressure and how that translates out to power band
Backpressure is essential for two-stroke engines though.
Maybe that is where the 'myth' comes from.
Its funny for me to hear Erik referring to his grandfather as Gale. Love your products and the vids keep it up!
I’ve been arguing against this bogus claim for so god damn long, it’s ridiculous.
So you dont run exhaust headers?
@@JohnSmith-xu7ev having scavenging from proper headers and mass flow is different to having back pressure
The very first time I heard of back pressure and the explanation it came with, I said to myself, "That doesn't make sense at all." Now, scavenging, that made sense. Backpressure always translated to resistance in my mind. Thanks to the Banks team and this video, it turns out I was right. I just couldn't put it into words. Thanks for proving I wasn't tripping about it all these years. 😅
Run a marathon breathing through a straw , hows that backpressure working ya now bud??😂😂
Its fine for toyota and honda
@@JohnSmith-xu7evDamn you’re stupid 😂
Humans exhaust and intake are the same orifice. Not a good analogy. Maybe more like farting a marathon through a straw. Which I think I can do, personally.
Try farting with hemorrhoids lol.
You described an intake restriction. Thanks for sharing though
As a kid I was told headers were always better. Didn’t know exactly why but as grew older I found out. Great video! 👍🏼👍🏼
Now you are incorrect about the hair dryer fan increasing load as you block the air outlet. You're not a master electrician so i wouldn't expect you to know this though.
It's not a positive displacement pump, so as the restriction in flow increases you move less fluid thus less work on the motor. This is particularly helpful for constant pressure fluid pump systems. Just keep in mind it's an example that is not correct for this comparison.
Engines are positive displacement pumps and as a result any increase in exhaust pressure or decrease in intake pressure will increase the power requirement for a given output
It's not about back pressure, but about exhaust gas velocity.
A neck in the pipe will increase velocity in that region, and this increased velocity can be used to improve scavenging. This has been proven in drag strips.
But a neck will create backpressure, which people assume is what creates power when it's just a side-effect of the resistance to flow.
It's all about flow and velocity. It's like a triangle of flow, velocity and pressure.
I cannot wait to send this to my friend who has argued back pressure with me for years now...
@creamwobbly he said "I can't believe I'm actually admitting this, but you were right. That means I need a hood stack for the cummins"
Same here bro!! lol already sent
Well said and explained, I have been saying for as long as I can remember, turbos are not free power, as everyone says, belt driven blowers get their power from the intake stroke, turbos get power from the exhaust stroke.
With the like of this, you are getting to free power, thanks to Gale and his genius
Yeah, I say that turbos get their power to drive the compressor at minimum cost to the engine. Mechanically driven compressors take their power directly from the shaft that actually drives the car.
@@andyharman3022 lol ok
The motor in your hairdryer speeds up because it is no longer moving air. It overheats because it lacks the positive airflow to cool the motor.
Came here to say this. Man this is a tough video to watch.
@@JNorth87tough video to watch? Banks knows more about airflow, pressure and anything to do with feeding engines air than anyone you know and who taught them. These guys know more about this stuff than we ever will. The example of the hair dryer was to explain it simply to morons
@@dougsavery5442 Yeah tough video to watch if someone is incorrectly using an analogy.
@@dougsavery5442 Example was wrong. Period.
@@dougsavery5442 Borla would disagree with you.
Excellent presentation!
Depends - in theory it's bad because the piston has to push against the pressure to expel it and in a perfect world there would always be a vacuum in the exhaust.
In practice it can't be achieved, although Detroit diesels, and some other large 2-stroke diesel engines, used exhaust scavenging pumps - that were re-purposed by "hot rodders" to became the foundations of the x-71 series superchargers!
Best one can do is try and control the pressures to best achieve exhaust scavenging and initial intake flow - this is done with tailoring the pipe diameters for velocity, and lengths so place the low(er) pressure waves in the exhaust pipe to be low at the best time for cylinder filling and net torque.
I do wonder if some work on the turbine housings, to improve the transition from the turbine outlet's inlet through the housing to the outlet's outlet, and the pipe's transition from that flange to the full diameter, wouldn't show some significant improvements?
Detroit blowers weren't scavenging pumps, they were on the airbox side of the engine and forced air into the engine to deal with the fact detroit 2 strokes can't self pressurise. Those blowers were the source for hotrodders.
that is incorrect using the hair dryer as an analogue ... when you block the air flow the fan/pump stops moving air which is the definition of work. moving fluid requires work. the fan is spinning but not moving anything... it is doing no work and current on the hair dryer would decrease...thus power goes down. put it on a watt meter... the blades have no differential pressure across them to resist flow. Part of the reason is a hair dryer is not a positive displacement pump it's a centrifugal pump.
Yes, please. An exhaust header video would be great! I eagerly await it's release.
Put the blow dryer on an ammeter and see what happens when you cover it.
My vote is less amperage.
@@huckbeduck, because less back pressure (in that case)
Intake or exhaust side?
This comes up so often, people absolutely adamant that ‘you always need back pressure, it gives you more torque’ then they get really upset when they can’t explain it 😅
Being a truck mechanic, I remember Gardner diesels having issues meeting minimum horsepower requirements as truck weights increased. Their quick fix was a tuned exhaust, Gardner called it ‘negative pressure’ for a loaded 32 ton truck , it used about a five inch diameter pipe that with matching stack.
It used a flapper to stop rain getting in, but the pressure pulses were so strong that at three quarters throttle it still sucked shut, made a proper din 😂 drove the driver mad
Where does the mythological backpressure myth come from. I hear a lot of people debunking the myth. But I've never seen anyone promote it. The myth itself seems to be a myth.
There are plenty of situations where velocity is reduced and it has negative impact on cylinder evacuation and cylinder filling.
He explained it in the video.
@@Christdeliverme I think you responded to the wrong comment
I had a pick up with a 402 BB . It had been sitting for a few years . I pulled it and installed new rings n bearings . It already had an aluminum intake and a big holley but no headers and a Crane cam . My budget didn't allow headers at the time . A buddy owed me and he worked at a muffler shop. He used 2 inch pipe out of the manifolds into turbo mufflers and 2 1/2 in pipe out of those and dumped just in front of the rear axle . The truck ran great. Before we did that I talked with 2 other shops and they wouldn't do what we did, and said the truck wouldn't run right due to no back pressure . Never really understood the backpressure thing tho .
Some people are wrong.🤣🤣🤣
Creating a negative back pressure increases power to a point but once you start over scavenging you're just wasting fuel.
Back pressure can be a good thing though, as it can help with low end (low rpm) performance. At low rpm the valves are open longer so there's more time for the A/F mixture to get into (and escape from) the cylinders. You need a little bit of back pressure to stop from over scavenging.
Another helpful note is HP vs Torque. How they relate to each other and how back pressure effects them both.
So this was a commercial to advertise a banks exhaust. Got it. 😂
I already have it. It's pretty tit
Yup, im keeping stock, my trucks going to dissolve from rust anyway
Well…. Yeah. Are you surprised?
I’m 53. My dad graduated from high school in 1958 and was the most amazing mechanic I’ve ever known. He taught me that back pressure was necessary. It’s so weird, I wonder where the myth came from.
As an engineer and an auto tech, it is true. The guy contradicts himself in the video
@@JohnSmith-xu7evany back pressure is bad.
I'm 51. My dad said the same thing. Bring a mechanic doesn't mean you understand fluid dynamics. People confuse a loss in velocity as a need for back pressure
@@randyschmidt19
Im both, and you do need back pressure. Tell me why you shouldn't put headers on a car
@@randyschmidt19
Then honda civic should have 6 inch exhaust like my diesel truck
I got into a argument with two guys at work about back pressure I'm glad to know i was right they were wrong
Borla would disagree with you.
You are wrong
@@mustangecoboosthpp3869 the company that makes and sells parts that create back pressure tell you you need back pressure?
Very important information to clarify is that pressure does affect the power curve. The more free flowing the exhaust, the lower the rpm range for peak power, the more restricted pressure makes your torque curb higher in the range. Awesome video, especially the scavenging part, but I believe a turbo diesel is the wrong example. Diesel already has a very low and narrow RPM range, practically non existent. Then you pair that with a turbo for diesel engines that has to produce peak flow at very low and narrow rpm ranges. Its like a quarter or less of most gasoline engines.
Your example of the hair dryer is wrong. Motor RPM actually increases due to less load , because a smaller column of air is being moved. You are comparing two different types of pump, Positive displacement versus Dynamic.
Has nothing to do with type of pump. The example was only meant to be a simple demonstration of how back pressure increases load.
The comparison was not about a motor vs an engine, it was simply to show backpressure or resistance at its most basic level.
I was thinking this too. Good intention but poor execution
@@lucasd8917
Cars dont run with 100% blockage, they ran with less than 5% back pressure
He is just a keyboard warrior trying to know it all.
I think it was a good example
This was a great video I used to believe the back pressure myth I know other people that still do! This is a great video to show why its always bad because they may not believe me when i tell them but they cannot argue with Banks!
So...let's get rid of CARB and the EPA - this equals no back pressure.
Nice trick. On the headers you have the air gun down inside the header manifold. On the log manifold you had the tip outside the manifold.
You need to manage exhaust. In an ideal world you have none, the challenge is that just like you said you can create intakes and exhaust that improve volumetric efficiency above 100% through careful engineering of certain harmonics. Whether it benefits depends on the exact engine, induction, and exhaust impedances
You explained that so well breaking down each step. It seems so obvious but you can see how people got sucked into believing it.
I'd love to grab some banks stuff for my little hyandai, but it seems to be the one model you don't do much for. Great video lots of information.
Been more of a fan each video I see but now I like it so much I'm going to ask for motorcycle products. Ya boy wants a top tier exhaust from the geniuses you are. Unbelievably brilliant and extremely easy to understand content. Never stop. But also please start motorcycles. Thank you for the education time and time again.
Where i come from the issue of backpressure was with 2-stroke engines. It should be tuned to the motor, if too low you waste fuel by letting the the air-fuel mixture going unburnt into the exaust or if too high you loose power.
Been trying to explain this to people for 30 years. But the internet is smart.
I love it, right out the gate!
Negative back pressure, that's what's up!
Please make the header video!
So what I have always heard was that on a NA vehicle the lowest possible back pressure was always best for maximum horsepower, but would cause low end torque to suffer. That isnt the case with a turbo, because the back pressure reduction spools up the turbo faster, making good torque. That doesnt happen without boost.
The reason for low end tq to be lost an engine with an exhaust designed for higher RPMs is because they're designing it for the air flow amount. At low RPM you're moving less air, so an appropriately sized smaller exhaust will allow for better scavenging due to optimal exhaust velocity. That's why there's so many devices on an engine to help adjust the size the intake and exhaust systems among other aspects like runner lengths and duration. For a turbo charged engine there's going to be differing sizes of various components based on how the engine can pump out exhaust gases. Just like how if you over size the turbo for an engine and it takes forever for it to spool or vice versa, too small of a turbo and it spools up fast giving you instant power, but completely choked up top, same issues.
@@sigmaprojects Sounds accurate. Also sounds like it proves this entire video to be false on everything other than turbocharged vehicles. Sooooo you DO need some back pressure on most other vehicles.
@@boost331 I'm sorry if I didn't make it clear. You never want back pressure. I said the reason for loss of low end power with a larger diameter exhaust is because the exhaust velocity will be slower at those RPMs. Slow moving air equals higher pressure, but it's also about how much volume of air you need to move. So at low RPM not a whole lot of air is moving, so if you want to cause scavenging effect you need a little smaller exhaust to improve velocity which will reduce reducing back pressure. But if you're trying to make more power at higher RPMs when the engine is moving a lot more air you'll need an appropriately sized larger exhaust to improve exhaust velocity for the volume of air you're trying to move to keep up the scavenging. Reducing back pressure. In every situation for your desired results you always want to reduce back pressure. It's just about sizing the exhaust to be the most optimal. Tuning runner lengths and such to capitalize on the pressure waves is also reducing back pressure. It's just not easy to make a one size fits all exhaust for every single situation. Modern engines have some ways to do with it, like variable valve time and duration. Think like a big cam engine, where at low RPM it sounds like it's going to die. The valves are opening up super wide and because at low RPM it's not needing all that lift the opening being too large slows the velocity and doesn't help in reducing back pressure, but when you rev it up and you need to move a lot more air the size is appropriate for not being restrictive and the exhaust can move better. For an every day car auto manufacturers will design the engine to have the least back pressure at lower RPMs since they know for those commuters they're not revving them up, they're just getting them from point a, to point b.
Basically in a perfect world engineers would design a sophisticated exhaust system that could change in size infinitely to maximize all the benefits of every size and shape to reduce back pressure at any RPM and load, but such a design doesn't exist so they try to just design for the least amount of back pressure at the point where they think the driver will use the car the most.
Excellent explanation, you videos are a fantastic learning tool for my young students !
Some motorcycle exhausts use butterfly valves to add backpressure at lower revs, but that is to compensate for the high valve overlap needed for higher power at near redline (typically well over 10,000 rpm). It is largely done for emissions and noise, but I have seen arguments that if the resonance is right, it can help lower-end torque.
Unfortunately, the hair dryer analogy is off. Many centrifugal pumps or fans use less power when flow is restricted. That is why you hear the motor speed up rather than bog down. In many applications, restricting the output also reduces cooling air, so the motor will burn out even though it is at a lower load. Plastic fans or pump elements can also melt from lack of throughput (I found this out the hard way).
I don't think the butterfly valve is to add back pressure, but to improve gas velocity at low RPMs. By doing so it also improves MPG/Emissions. So it can be fore both. Many older cars would do this on the intake side too.
@@sigmaprojects It depends on the application. There are also some that are largely for noise mitigation. On engines that are tuned for peak power at very high rpms, the valve overlap is so great that unburned fuel/air mixture gets through the exhaust valve at lower engine speeds. In those cases, the increased back pressure reduces velocity going out of the exhaust valve and reduces scavenging.
The first butterfly in an exhaust system that I recall was around 1980 on the two-stroke Yamaha RD400 and it was there largely to reduce emissions during deceleration.
It predated their YPVS system.
@@jfess1911 I agree there are many reasons, but a butterfly valve reducing the size of the exhaust at low RPM will increase velocity, not reduce it. At low RPM larger exhausts will not be good at scavenging and you'll get a build up of exhaust gas pulse waves increasing back pressure. This is a common practice on intake runners too.
@@sigmaprojects Again, it depends on application. A typical butterfly valve only increases the velocity at the tips of the plates while the velocity is slow on either side and more turbulent downstream. It acts as a variable restriction. This is how gasoline engines have been throttled for decades. This is how I have mostly seen butterfly valves used.
In an exhaust system, this setup typically just adds backpressure and a surface that can reflect pulses, but that second property is often not utilized.
On the other hand, It is possible to design a system with multiple runners where the valves shut off some of them, increasing flow on the others. There are also more sophisticated systems, more frequently used for intakes, with multiple runners of varying lengths which are opened or closed by butterfly valves, depending upon operating conditions.
@@jfess1911 are you saying in the motorcycle system with the butterfly valve in the exhaust system it can operate at part close? Usually they're either open or closed to divert exhaust gases. What bike application are you thinking of?
Just got the torque tubes for my 00 xj, super happy with the way it looks!
My wife said this morning, "i need back pressure,for my gas control",as i waffed the duvet, i'm writing this from the comfort of the sofa.
I would be a bless to have gale make the video about how exhaust scavnging ties back to exhaust back pressure man, hope you guys at banks can make it one of these days. All the engineering about exhaust has it complexity y precision and it's so useful to learn it for any aplication! Hope to see that video soon💪
Where this myth came from is rooted in the seventies. When auto makers first started down the path of cleaning up emissions, they leaned out the carbs to lower hydrocarbons. I actually saw one vehicle that cracked the exhaust manifold, causing the engine to stumble. When the manifold was replaced , the engine returned to running normally. The normal back pressure was needed to maintain proper fuel mixture. When the built in back pressure bled out the crack , the mixture went too lean and the engine ran ragged. This was one reason for those looking for more power, would jet up the carbs along with better exhaust systems.
I think the whole back pressure argument came from the race cam in a street car crowd believing they fond a way to regain some of the cylinder pressure they were losing through their wide open cam timing. The fact is thinking you can gain power by leaving unburnable gas in the combustion chamber is ridiculous.
2014 Camaro SS 6.2L
I ran Flo-Master Outlaw mufflers, X-Pipe and BBK tuned short headers and got about 26 mpg out of the car on the highway. From a stop I could push past the traction control and chip tires.
Now comes the BBK full headers 1 5/8" with only two high flo cats. 19.5 mpg and no low end push. Now when I'm cruising at 80 mph... I can get to 136 mph real quick though.
Yes please do a header video. I also learned that glass packs have back pressure? I never would have thought that.
Looking forward to the science behind the design and considerations for a tuned header 👌
I have a BRZ with the Edelbrock supercharger (with a well developed tune by DDPR), with the stock small header and exhaust I (closely) tracked fuel consumption for 9000 miles of mixed driving so the MPG figures were pretty refined. Then, even though the lore says that exhaust diameter doesn't matter in forced induction I decided to put in a cat-back system to change the sound. So stock was 2.0" OD (actually a bit under that as its something metric) and I went with a 2-3/4" and was pleased with the sound but noticed that there might have been an improvement in gas mileage.... And the following months proved it. I WAS GETTING at or just under 1.25 more MPG. Now that doesn't sound like lot but when your buying 93 octane gas that adds up over the months. And there was a fractional improvement the next dyno run for baseline before we installed larger injectors and I went alcohol.
For those of us that are using non turbo engines There is calculators available online to find the proper size, however you gonna have to find the best design for your situation to minimize the losses from bends.
I remember trying to teach Honda owners this back in the early 2000's. Man that was hard work.
You guys are awesome!!!! Thank you so much for informing us on this matter!