I don't mean to throw cold water on the gain of HP through the exhaust advocates but.... The function of the catalytic converter depends on some back pressure upstream. Thus here in CA all that is legal is a cat back modification to keep the air clean for our kids. If HP gain is from better scavenging of exhaust gases from the engine that's not possible with the cat in place. All that can be accomplished is better sound, unless you begin with the cat and that's illegal here in CA. I don't know the smog laws in other states but even so the minimal gain doesn't offset the total cost. It seems there are better ways to increase HP with that money.

I found a noticeable difference with the larger throttle body and Needswings CAI. The midrange RPM torque comes alive. That was about the same cost as a professionally installed cat back system for just a louder exhaust. Give me power over noise everytime!!

Just Sayin'
Xfirepop