Would installing a better exhaust with larger piping help with MPG at all on a 2.2L saturn ion? I have a 2.5" Cat-back right now, but the cat pipe is 1 7/8". If I change the cat and downpipe to 2.5" would it help me at all?
Stay with the 1 7/8". Maybe get some straight through mufflers and a high flow cat but don't go to 2.5 all the way or you'll have almost nothing as far as EGV is concerned and your low-end torque and fuel economy will suffer.
In fact, if you went to a 2" cat back with straight through exhaust you'd most likely do even better. Personally, I'd put the manifold, then the cat immediately after that and run 2" halfway back to a 2x24" glasspack and then drop the size to 1 3/4" to another 12-18" glasspack right before the exhaust exits. I'd also have the exhaust follow the driveshaft tunnel to keep it straight as possible(assuming RWD of course).
the generic answer is that it will help, but if you are just changing the pipe diameter and not the cat, it may not do a lot because the cat may be the bigger restriction. I know when I did duals on my truck, they said that the only back pressure I had was from my cats. also realize that you need some backpressure for your torque.
knew a guy that cut off his muffler entirely on an old escort to make it sound cool. he didn't have the money for the fart cans that the ricers use. he lost so much power. he had to rev it to 5k to get anything out of it.
also you need to look at if it is worth it. if you only see a few tenths of an MPG from doing it. your time may be better spent in other areas.
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