My ASP .91 came with a muffler that the magazine review said was the same as for the .75 and the .61. Comment was that the small muffler drove the max HP down but increased the torque at lower speeds so that bigger props were most effective. I run mine on FAI fuel using speed shop methanol and Klotz Lubriplate castor/syn blend. 1700 rpm idle and 8400 rpm high with 14x8 MAS prop. Good transition and smooth running at all speeds. I may try larger diameter props just for Ss and Gs.