DDTECH

People who use that, never see below 7k anyway..

mhax

Some new cars/motos will not use a washer but a butterfly...why...because race car! But hey...they probably just did that because they are stupid and that they thought it would be cool (sarcasm)! CAD simulation and advanced sciences are behind the design of cars/motos, backpressure, scavenging effect, are part of it. If you only think high rpm, fine, put a big pipe. I'm not thinking like that, i prefer having a constant increase of power as a function of rpm.

Why area under power curve? Because race car!

mhax

And that's also why that kind of people think back pressure is a myth! But even without understanding anything about it, they are right, for high rpm at least!

DDTECH

Ok..

I work on porsche's / audi's everyday.. They have the little "exhaust flaps" you are describing. The 996's,997's, 911,s4,s5's,R8's all have this. It makes the tone quieter and its towards the back side of the exhaust.. like 5 inches from the tips.. its controlled by vacuum...@ wot, they open all the way.. That statement holds about as much water as a a paper bag. Nobody's denying that a smaller pipe will speed up exhaust air, but its only going to help so much.. thats why people FLARE to bigger pipes.


Alot of people actually flare to 3'' from a 2.5'' collector, then at the axle back, flare to 3.5''..i'll let that sit for a second.

gstrudler

Mhax you seem to be using exhaust velocity, scavenging and backpressure interchangeably, and they are not. I think everyone here agrees that exhaust velocity is a good thing, and a smaller pipe provides better velocity at low rpm and a bigger pipe at higher rpm. Higher velocity in turn provides for better scavenging of the cylinders due to the pressure differences.

Backpressure (as I understand it) happens when the velocity needed to move a particular volume of the air through the exhaust pipe exceeds the physical velocity possible, and there exists a pressure buildup since not all the air can fit through. I can't think of a situation where this would be beneficial. The ideal pipe size would allow for maximum velocity with no backpressure; the problem obviously being that this changes for a given rpm, throttle angle, and engine characteristics.

mhax

I'm using the words interchangeably because most people think it is just about one thing when it is not. I was trying to make a simple explanation of the principles behind exhaust physic, but probably my english is not good enough to make me easily understandable. Since it would be very hard to have an infinite variable diameter exhaust, we use variable back pressure in a constant diameter exhaust to optimize torquer/power for all rpm. I won't change my mind about that, since i learned that this is how it works.

q16racer

id suggest you test your learning in the field other than relying solely on what your engineering books tell you.

sounds like a good senior year project.

mhax

Engineering books are the same used in the industry. And i'm happy to see the book must be right since more and more exhaust system now comes with variable back pressure exhaust. Some people seem to understand the same thing that i do...Wow bmw and ferrari engineers do agree with me, you should call them and tell them they are wrong!

DDTECH

look up backpressure in the SAE books.. if you find it, lemme know.

Again, this exhaust "flap" system you're talking about is to hinder noise..not create this back flow..

I'm done. Build a b20vtec and run a 2'' exhaust on it. Make tons of POWA ok boi.

mhax

I can look at volume/velocity/pressure/scavenging, everything is there.
No it's not only for noise, but to make torque/power at low rpm.

If i ever build a b20vtec and want to have torque/power at low rpm and nothing in higher rpm, i'll run a 2" exhaust for sure.

96dxB16

Maybe if you want to make more power below 3k rpm?

I only made gains everywhere going from 2.25" to 3" with a new header as well.


I agree Volume, velocity, pressure and scavenging is very important, this is up to header design. After your collector you are done.

Riake

Your fluids book is correct, why wouldnt it be?

In essence that valve is altering back pressure. That is not the purpose of it though; its purpose is to alter velocity. As you said, because variable diameters are not possible for a given space, there must be another way.

Obviously the fluid has a different need at every single rpm point, but backpressure is never one of those needs.

mhax

Indeed, the purpose is to alter velocity. Backpressure is used to alter velocity. But i'll have to agree in a certain way that you don't want backpressure when the velocity of the fluid is already high.

gstrudler

So was this stock header & 2.25" piping to new header & 3" piping? If so it's a little harder to know what was making the differences. Also, tuned on each setup?

96dxB16

Old header was a JDM DC 4-1 (with the 2.5" collector) vs. an RMF narrow copy. Both setups were tuned.

Agreed, I don't know which parts made more or less power where. But I didn't loose any power anywhere as people thought I would with a 3" exhaust and "whatever" header.

xzodiackillerx

you can call them up and ask them how adding "backpressure" helps add torque to an exhaust that is sized right..

tianporn

Not really a fair test because you changed your header as well. Also look the the differences in A/F. In some areas there is up to 1 A/F point difference which will have make a difference to power. You need to at least tune them to similar A/F ratios.

92TypeR

You know Engineers at both Maranello and Munich? I've met an Engineer from BMW Oxnard, but as of 2007 they were only doing North American emissions testing/vehicle testing at that location... Maybe the corporate direction there changed?

If you could provide their contact information, it would be worth the effort to have someone active in the industry provide their input on this subject.

What Engineering book do you use for exhaust manifold design? Winterbone/Pearson? Dalton? AG Bell? I spent much of my time in undergraduate doing manifold design, and I can't for the life of me remember anywhere in these texts where restricting flow was conveyed as a positive.

The valves located downstream in the exhaust of modern cars (mainly near the muffler section) are for noise suppression. A simple Google search will show you many Corvette owners who install a switch to keep the valves open show marginal power gains where the valves were previously closed.


I think you have a general misunderstanding of the principals of exhaust scavenging and manifold design. The two main goals of producing an 'ideal' manifold is A) to time the negative pressure wave from a paired exhaust runner to the E.V.O. event at an RPM range of your choosing, and B) remove as much restriction from the flow of gas as possible.

You seem to be confusing gas flow with pressure waves. You need to treat both as two separate characteristics of the manifold. The pressure waves move much quicker than the exhaust gasses, and are a result of the E.V.O.. When two pipes merge, the positive pressure wave caused from the E.V.O. event of one cylinder will then cause a negative pressure wave to run back up the paired runner. If you time this right, the negative pressure wave will reach the exhaust valve right as the valve begins to open on the exhaust stroke.

Your manifold design will pair runners so that the pressure waves create power where you want it. There should be no interference to the exiting of exhaust gasses.

Everyone in industry I've ever spoken to, and all the texts I've read, have reinforced the idea that everything after the final merge (last opportunity for pressure waves to affect scavenging), is for emissions and noise suppression/exhaust gas routing.

Cheers

DDTECH

checkmate.

Hdeucedeuce

Is it a turbo?? Lol. U should never go 3" on naturally aspirated engines. As stated before by experienced tuners like g16racer it's one word: backpressure.

DDTECH

wtf .. We've been discussion why going "3'' is better..

reread the entire thread...

adosdrummer

ah. if your loseing low end from a bigger exaust its because of your cam..

its flowing better and taking the clean air out the exaust.

GhostAccord

Needing Back pressure is complete BS. Why do people still say this crap? Move out of the carb'd forums and into the light of PGM-FI.

Going larger isn't always better if in the end you are loosing overall flow velocity. If you have a properly tuned header and, more importantly, a tuned merge collector. Installing a 3" pipe usually wouldn't be an issue on an N/A build. However, if you just slap a 3" piece of pipe onto a poorly designed header it could make you loose power. Velocity/flow tuning is key! Keeping exhaust gasses hot and fast is the key. A larger pipe diameter, gives a larger cooling area, that in turn will cool the exhaust gases faster. It also gives the exhaust gas molecules more room to randomly bounce around. vs going in a straight line. These two factors will slow gas speeds that could actually become a restriction to flow and performance.