does anyone have experience with changing the piping size from header to cat, and cat to muffler?
if i changed my piping to 2 1/2" , how much whp would i gain?
has anyone done this and then taken there ride to a dyno?
i dont want to spend money on exhaust and i have a hook up on piping...
i'm running a AEM CAI with a DC ceramic header, and a skunk2 intake manifold, the exhaust part, is a must for me, but all i want to change is piping, i'd like to leave my ride as sleeper as possible...

 

400whp it helps when you say what car you have

 

2000 Civic Si, it says it right below
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You'd probably gain ~5whp. You start to see larger gains if you're turbo'd or sc'd since there is a lot more exhaust gas and a lot more back pressure is created.

If you have a larger-than-stock header though, its highly recommended since the stock exhaust piping and it's non-mandrel bends cause a bottle neck.