I agree that the AeroTurbine muffler is a nice mod for the ISC engine. We installed the 4040 at JB Equipment in the Hemet, Ca. area. I am not so sure that a bigger (5050 muffler) is better in this case. Turbo spools up much quicker, noticeable power increase in the lower gears when accelerating. Much cheaper than stock OEM type muffler and should last the life of the MH. Will know more about mileage improvement after we get a few more miles, but I sense looking at the 'silverleaf' that it is better.