well...i would think that if you reduce the size of the pipe coming out of the mufflers, you would gain in torque. Also the 40 series flows have a nice low tone, but you will have some cab resonance at certain rpms, but normal driving rpms. You can dynomat the cab to cut down on the resonance, and that will kick down that issue about 60%. Flows are nice and gibson are nice also. both have good low tones, and i have run both on my truck. I am running 3" piping all the way to the mufflers then 2.5 out of each muffler. Not a lot of backpressure, and it still has balls.