Im wondering what diameter my stock muffler is. Im goin to make the stock exaust system all 2.5" and im just wonderin if i can keep the stock muff on. But only if it is 2.5". And ive heard that the stock exaust is 2.5 all the way through except over the rear axel. If i change that part and make it 2.5 will there be about the same performance as if i went out and bought a 2.5 cat back system?

 

any 2.5 will free up the flow better than stock. I think i have seen people keep the stock muffler, or 2.2s putting the SC one on their own, but imo, you would be better off if you are changing all the piping to just get a new muffler along with your setup. If you have a SS/SC, then you are going to have to cut the muffler off anyway to get the piping out from under the car.