Originally Posted by NickTurnon

If you want to stay quiet & pass emissions then you will want to keep your stock cats.
So that means no high flow cats, no test pipes, etc..

You didn't mention the type of car that you have, but im going to assume base or sport 370z.

Intakes yield good gains. K&N dropins + MAF Tubes = 16whp compared to Stillen g3s 18whp

QUIETER EXHAUSTS with gains:
Fast Intentions 18" Resonator
Stilen Catback
CNT Catback
Nismo Exhaust

Do some searching on here buddy...