Exhaust sizes + backpressure + stuff

[Ako]

Ok, this is more me trying to get an answer with some real thought behind it than anything.

Ok, one thing I see written on here is that you don't go over 2.25" for a 1600cc exhaust. Its something I see everywhere, but the only reason for it is "backpressure is good, you lose torque by going too big"

But... Hang on. My poor wee brain can't make sense out of that - isn't the least restriction best?

How does backpressure HELP make power?

The theory for a turbo car is: bigger is better. Makes sense. Cut the exhaust off my city turbo and it flew, and no way in hell it had LOST any low end torque. Ditto with the VR4 - but, still, thats a turbocharged car, so things work differently.

Back to the n/a side of things... How does a smaller exhaust help? Why would 2" be called restrictive, but 3" be called overkill - is it just noise concerns? Even if you lose some low end, surely your not aiming for low end torque in a 1600cc engine anyway?

Sorry if it sounds dumb, just this has been bugging me for a while and noone can give me a definate answer!

[Quint]

Go to 'how things work' i think that explains it well, Failing that, do a search, i'm sure you'll find a link that explains it.

Exhuasts are fricken complicated, trying to explain it hurts the brain, especially after work

[vhpacer]

I know it sounds dumb, but I got told if the exhaust is to big, the gases can turn around and go back up, therefore reducing flow.

[dodgy]

Its quite simple once ya figure it all out.
as the exhaust gas flows out the combustion chamber and thru the manifold, it created a light vacuum behind it. so when the piston goes into its next stroke the vacuum helps suck out any left over gas.
so the faster the gas flows out the exhaust the more vacuum you can get. this is why you dont go straight to a massive pipe from the port.
now just imagine this happening 4 times in sequence (for 4cylinder car).
imagine what the gas flow is doing after the collector, you will get gasflow-static-gasflow-static-gasflow-static-gasflow as it fires thru the cycle.
now, the trick is to get the gasflows all to murge with each other smoothly so they all assist each other. but theres alot more in that!

[mr30%jr]

zausts use pulses to draw the gasses out quicker..... bigger pipe equals slower zaust gass speeds meaning gives it some to expand meaning dumb......

go have a good read on some REAL sites.....

zaust shouldnt really be anny bigger than one of you header pipes untill it gets to your res or mufler

think of headers like zips not funnels

[Ako]

I follow now.

Thought maybe this would make a nice thing for the FAQ bit, considering how often you see the question "what size exhaust should my mr2/trueno/supra" have.

[Malcolm]

this is taken from MR2oc.com, posted by frios:

Actually, An engine doesn't benefit from backpressure at all. People tend to confuse exhaust charge flow velocity with what most people think backpressure is.

Backpressure is a condition where exhausts gasses or residual thereof are "opposing" the free flow of other incoming exhaust pulses in your exhaust system. Where would you find instances of backpressure? An example would be in an exhaust manifold where an "inefficient" turbo turbine is mounted to an "efficient" engine. In this scenario you can imagine exhaust pulses stacking up on each other as they try to get pass an inefficient turbine(think cattle cramming to go through a single open gate here). As each pulse leaves the exhaust port, they would keep slamming one on top of each other opposing the next one. Such a condition would create high pressure, which studying physics would tell, would create un-needed heat.

An efficient exhaust system actually needs to have as little backpressure as possible for any given Combination. Combination is the keyword here since one engine build or set-up can actually require a different diameter piping. If your exhaust system is too small, it becomes restrictive just like that small turbine I described earlier. If you go too large (this is the part where most get confused) the velocity of the exhaust pulses slows down because of the now larger diameter(remember we're talking about pressure differences, expanding gasses, and so on) so the next pulse comes around and you guessed it, just like the restrictive turbine, they begin to slow down and stack into each other at slow velocities which again opposes free flow.

Remember backpressure will hurt your engine. Good charge velocity is what you need by carefully choosing the right diameter piping for your given combination. The reason someone would think the 4AGE would need backpressure (again confused) is because it really doesn't start making power until higher up in the powerband. At low rpms, the large piping is actually creating backpressure(slower pulses). A correctly sized pipe will help keep velocity at the lower rpms where most of us drive.

AFAIK it is possible to have problems with "over scavanging", where the scavanging effect is so efficient at certain revs that it draws some of the intake charge out with the exhaust

[Caveman]

The best exhaust is headers only

From the headers back, if the pipe is too large, the gas will expand, loose energy and slow down. The more dense, slower air will essentially cause restriction.

If the pipe is too small you will limit the flow through the pipe simply because there isn't enough room to flow all the gas at once. This effect will only really be felt at high rpm.

The idea is to get the size exact for the amount of gas you want to flow.

Tuning headers is a crazy science which I have little knowledge of

[vhpacer]

They do some crazy shit with special crayons, a line is drawn along the exhaust, and were it stops melting is the optimum length.