In my k20a2 ep3 I used this resonator to replace the cat ahead of my Nvidia n1 exhaust. The previous setup was choked to a 2" pipe after the cat/into the catback (want right? Pay $$$ or diy..) which certainly affected top end pull (still upgraded from stock) and presumably made it a bit quieter... Yet with this resonator the drone is dramatically reduced, approx equivalent to having the silencer in place. When opened up its clearly breathing better and a bit louder - more throaty, zero rasp. Stock headers for now. Totally happy with the new setup, louder when I want it and quieter when I want it. Only required an o2 extender to kill the cel... No more emissions for this year. Excellent quality product!