I just put on pacesetter headers and a 2.5 inch pipe to the cat, another 2.5" pipe to a muffler and then a 2.25" tailpipe. It made a bit of a difference but should be more helpful as a modify the vehicle more later. (such as a performance cylinder head and perhaps a larger throttle body). I have heard that opening the exhaust has more effect on higher RPMs but doesnt do much for low RPMs. Thats about all I know on this subject.