Exhaust Pressure?

[Strelok16]

Right, I need a little help settling an argument i'm having with a friend.

I hope this doesnt make me look stupid... it seems like every time i'm starting to feel like I know a lot about cars, I run into something that reminds me I really dont, lol. well at least it doesnt involve sand so I figure it cant be that bad...
Anyway, he's telling me that having a little pressure in your exhaust is best for performance, and to get the best performance possible, you should look for the right muffler that gives you exactly the amount of pressure your engine needs.

I'm arguing that having as little pressure in your exhaust as possible is best for performace, and so the highest performing exhaust possible would be straight pipes. (obviously not the best for your neighbors though)

But he's saying that some mufflers are actually better than straight pipes.

Now I know that you want some suction in your intake, but what about pressure in your exhaust? Am I totally wrong, or do I get to go to my friend and say "TOLD YOU SO!"?

[fbc]

I'm a little out of my depth here too, but I think what he's referring to is the 'scavenging' effect where the airflow around the exhaust headers helps draw the exhaust air out of the chamber. A pure straight-through system with no scavenging may result in worse air-flow out of the combustion chamber. What that has to do with mufflers I'm not sure - but I'm do know I'm going to learn something from this thread, so yay

[Viper007Bond]

I've always heard that the proper back pressure is better than having no headers/exhaust pipes. This is one the reasons (I think) that headers are engineered so that each cylinder's exhaust is the same length.

Somewhere in here is probably the answer: http://en.wikipedia.org/wiki/Manifold_(automotive)

[the Interceptor]

As far as I know, N/A engines work best with some counterpressure from the exhaust system, while turbos work best with as little countrerpressure as possible. I can't exactly explain why though. I can understand that the work for the turbo gets easier when you decrease the counterpressure, but fot the N/A, I don't know.

[AltimaXX]

God, the backpressure myth makes me want to harm myself. Just this week, my friend went to have an exhaust system put on his car and was harangued by the exhaust tech about "not having enough backpressure" and that making the car run poorly. It's all wrong, but so many people believe it, it's astounding.

I'm terrible at explaining things, but I'll give this a try.

It comes down to the difference between flow, pressure, and pipe diameter. The exhaust is there to carry waste gases away from the cylinders after combustion. You want it to flow those gases quickly and efficiently for the motor to perform at its best. If a pipe is too small, then it will choke down the gases and they won't be able to flow quickly enough. However, bigger is not always better, because if the pipe is too big, then the gases will slow down, which is also a bad thing. Think about it this way. If you blow on one end of a drinking straw, you won't be able to blow very much air out of it, but the air that does get through will be moving very quickly. Your lungs will be straining to push air through the pipe. Now compare that to blowing on a paper towel tube, or worse a tube like you'd mail a poster in. You'll be able to flow all the air you want through the poster tube, with no "backpressure" fighting your lungs, but the air will be moving very slowly through the pipe.

Now, translating that example to your exhaust, if you fit an exhaust that's too small, your motor will have the same problem as your lungs with the drinking straw...it will choke the flow down. Obviously, because a motor is merely a large air pump, the more and the easier that air can flow in and out, the better. Okay, but why isn't the biggest exhaust you can find the best? Because of exhaust velocity, and scavenging. If the air in the exhaust pipe is flowing quickly, it will actually "suck" the air coming behind it from the next cylinder's firing. With multiple cylinders firing repeatedly, you get a continuous flow of air out the pipe, and the "sucking" of the gases in the pipe pulling the gases that are behind them generates what's known as the "scavenging" effect. Scavenging is a very good thing, and helps with power across the board.

The trick to sizing exhaust piping is choosing piping that will allow good velocity at low-mid RPM to increase scavenging, but will also flow enough air at high RPM so as not to choke down top end power. You can bias the piping one way or another, but bigger piping to aid high-RPM power will negatively effect low end, and vice versa. It becomes one of the tradeoffs you have to make when tuning a car...just like cam, header, and intake manifold design.

Sorry for writing a book..I hope that makes sense. Feel free to ask if you have any questions.

[the Interceptor]

Yes, I have a question: isn't designing an exhaust system with the right size to maintain airspeed while not choking the engine the same thing as designing an exhaust system to have a specific amount of counterpressure? As I understand it it's the same idea, just seen in another way. You design the system to have a specific amount of air pressure. That pressure speeds up the exhaust gases and creates the "scavenging" ... right?

[Labcoatguy]

Remember that the exhaust flow from an individual cylinder is not a steady flow, but a series of pulses in time with the ignition of fuel from each cylinder. The point of scavenging is to bend the manifold or header tubes such that when they meet, the pulses don't overlap and "bump" into each other, but enter the end pipe in an alternating fashion such that when one pulse is traveling through, the next closest one is delayed enough to enter the end pipe right after that first one goes into it. There are some diagrams and animations online that display this much more clearly, but I can't recall off the top of my head where they are.

[AltimaXX]

Originally Posted by Labcoatguy

Remember that the exhaust flow from an individual cylinder is not a steady flow, but a series of pulses in time with the ignition of fuel from each cylinder. The point of scavenging is to bend the manifold or header tubes such that when they meet, the pulses don't overlap and "bump" into each other, but enter the end pipe in an alternating fashion such that when one pulse is traveling through, the next closest one is delayed enough to enter the end pipe right after that first one goes into it. There are some diagrams and animations online that display this much more clearly, but I can't recall off the top of my head where they are.

Right...which is part of where header design (4-1 vs. 4-2-1 "Tri-Y" style, collector size, primary size and length) come into play...that's how you can tune the exhaust pulses to work with one another.

Interceptor-I suppose in a way you're right. The problem with looking at it that way is that it's often misconstrued into the myth that you want to choke down your exhaust. Instead of jumping on the "bigger is better" bandwagon, people do the opposite.

Really, I'm not sure if I worded my OP right. The goal is to find a piping diameter that has no backpressure, but is not so big that it kills exhaust velocity. People see that a motor makes less power with a massive pipe, and credit that loss to "not having enough backpressure" when really backpressure wasn't the problem at all. It isn't the pressure that creates the high exhaust velocity, rather it's having piping that's just the right size.

[AltimaXX]

Also, to the O.P. You're right in your statement that straight pipes will be best for power, usually. As long as those pipes are sized right, then straight pipes will make more power than any muffler that adds pressure. Mufflers also tend to kill off the exhaust gas velocity, which as I've stated before is bad.
You're also right about wanting suction on the intake side, and interestingly, some of the principles I've tried to explain here apply to intake piping as well. If your intake piping is too big, you'll lose intake velocity, and if it's too small, you'll choke the motor down and it won't be able to breathe.
The adage "Bigger is always better" is far from true...

[Steve Levin]

As others have said, you want to align the exhaust pulses so that they not only don't hit each other and cause a dynamically high pressure blockage, but you actually want to shape it so there is a vacuum/low pressure just when a particular cylinder's exhaust valves open to suck the exhaust charge out (and help pull in the fresh fuel/air charge).

But that all happens pretty high up in the header and collector. Past that, opening up the exhaust will have little effect other than to maybe give you a couple of extra horsepower and a lot more noise

Steve

[epp_b]

Totally off-topic, but...why is "Steve"'s name in pink?

[AltimaXX]

Originally Posted by Steve Levin

Past that, opening up the exhaust will have little effect other than to maybe give you a couple of extra horsepower and a lot more noise

Steve

I'm a teenage guy and I drive a Honda...as you can imagine...my car is pretty loud...

[fbc]

Originally Posted by epp_b

Totally off-topic, but...why is "Steve"'s name in pink?

Pink? It's blue...

[Ottobon]

Interceptor has the right sort of idea, but it has almost nothing to do with pressure and everything to do with resonance.

So far as having too little pressure? thats not the problem! Well too big of a main pipe (or too short of exhaust) will raise the area of prime power to unusably high RPMS (think F1 car territory.) while simultaneously killing the low-end You can easily tell if the main pipe is too big for the primaries once you start hearing the classic Honda (i have a tiny penis) Fartcan sound.

/Rant
No offense Maxima, i don't know what your cars like, just going off the classic amateur tuners. Yes i did tried the whole 2.5 inch exhaust thing when i had a MR2 and once i did it i was so embarrased that i completely rebuilt the exhaust in record time. I just get miffed because i4s in general sound so much better when the main pipe doesn't drawf the primaries. Nobody uses uncorked 2 inch exhausts on 2.0 liters, but they should, it can perform just as well and it will sound 100x better.
/End Rant


Back to the subject though, from what racers have told me most of the time they see gains in mid-range power WITH a good muffler, i believe it works similarly to how expansion chambers help 2-strokes, either that or its like adding a longer exhaust, which many cars could actually use. Anyways, the midrange is important in cases were you need power all over the place, like on a autocross course, but in a F1 car or a all out drag race straight pipes, even zoomies can be preferable because they will give more peak power when tuned properly, just that it will only be availible at a very limited range in the RPM band, so for a street car the disadvantages out-way the advantages.


One neat little tidbit though: It seems in all my research that a engine can be tuned to sound in many a good number of ways and it will still make similair power, so long as all the other elements are tuned to work together. The one exception being the unequal length subaru boxer primaries, which have practically no benefit to performance whatsoever (despite this people still seem to make it work though)

[AltimaXX]

No offense taken, I make fun of the way Hondas tend to sound (as well as the average "tuner's" tendency to fit the biggest exhaust they can find) more than just about anybody. (I hate it) My car is fairly loud, but just because I like it that way, not because I'm running an ungodly large exhaust.

[Strelok16]

Thanks for the replies everyone, I think ive learned some interesting stuff from this thread, and maybe even usefiul too, since I'll probably be doing my own exhaust on my car. Well if I can get my hands on a welder that is, but thats a different story...

So it looks like my friend and I were both a little bit right... but since it isn't actual pressure in your exhaust you want... I was more right

[epp_b]

Pink? It's blue...

Oh, well, I'm more colourblind than a dog, so...

[Viper007Bond]

It's the VIP color -- light blue.