NA 2.2 Exhaust size, What's the best size? Options

[Bigmeanbulldog55]

I know the topic has been discussed a few times, but I'm wanting to put it to a pole. I'd like to get a good gain out of my new headers, and I think a nice sized exhaust would help that. It's a 2.2 n/a with little mods. I'm thinking 2.25 will give me the best gains, but I'm not positive enough to just do it. Thanks for the input.

[Galcobar]

Cars do not need backpressure -- it's a myth.

Big pipes have lower initial backpressure, but quickly create turbulence (a form of backpressure) because they let the gas expand, cool and slow down. Exhaust therefore cannot exit the engine as quickly and efficiently, and energy that should be going to turning the crank is instead being used to push exhaust gases out.

Backpressure is an inevitable consequence of not having a variable exhaust size. The harder the engine is working, the more exhaust it needs to get rid of, and the ideal situation would be for the exhaust pipes to grow as the engine pumps out more fumes.

Since we don't have that technology, what we do is pick a compromise size. It's not as efficient at low revs as a small pipe, because the exhaust gases have room to expand, cool and slowdown, reducing the scavenging effect and forcing the engine to expend energy on moving what is now heavier gas. At high revs it's not as efficient as a larger pipe, because the gas bottlenecks. Either way, you get backpressure. The compromise aims to minimize that backpressure over the band, and preferably have the most efficient exhaust extraction (high velocity, lowest backpressure) at the RPM point where the engine spends the most time.

Really big pipes are good only if you're producing enough exhaust to be efficiently using them. This is where FI and NO2 come in, since they effectively simulate greater displacement and therefore a bigger engine.