my exhaust is 3" from turbo back. Hi-flow cat, the cat back has no resonator. It's not overly loud idling but become really droney between 2K-3Krpm which is where you do majority of your driving (if it's a street car). Very annoying. The EPA test is done at about 4,500rpm from there it's not that loud - I think about the same as the engine. What exhaust do you currently run?