I'm shure there is a differance, how much Idon't either. I'm shure that would depend on the HP level. My super and excursion were close at one time. The super has a 3.5 and the x a 4". As the HP went up the X even weighing more started pulling away. The 5" systems have a 4" down pipe. I don't know if diesel and gas are the same being that to big hurts performance.