I run a 3-1/2" system, X-pipe, 3-1/2" ultra-flows. I set it up like that when I built my 528 Hemi and it made more hp without the pipes on. By the time I built my 605 I had a dyno and tested the motor with and without the exhaust system in place. The 3-1//2" system cost 117hp and 97ftlbs on my 605. I can live with that though as hooking that much power on the street isn't going to happen anyway.Dave