Originally Posted by tjlazer

From the dyno results I have seen, it's not 40HP with HFCs. It's 8-10 (with testpipes), and around 4-5 for HFC's. The Cat back can net upto 20HP, and another 17 or so from a CAI.

So with ricer math (which seems to work on the Z34) you should see around 25HP from just a catback and HFC's, and around 30 with catback and no cats at all.