Originally Posted by spdandpwr

yea, they say that you should keep stock cats because you loose boost when you make the exhaust more free flowing on the s/c kit.

I think the problem though is with the cams on the car. Overlap of the cams profiles is the important part. You dont want overlap on a f/i car. (overlap is the length of time both the intake and exhaust valves are open at the same time.) All internal combustion motors rely on cylinder pressure. On f/i cars need only one set of valves open at a time to reduce boost loss out of the exhaust.

What's happening with the Z is you're losing boost with unrestrictive exhausts because of the overlap in the cams. A custom cam would allow you to run more boost, a free-er flowing exhaust and make wayyyy more power.