The inlet size wil have no effect on the hp, or noise unlesss you increase the tubing in frnt of the muffler. The outlet is a different story. That will effect the sound level, hp, and tq. You can go to big and you will lose low end tq. But you an not really hurt the engine. You need an offset inlet to line up with the stock pipe. If you use a centered it will hit the frame and or the drive shaft.