I have three of them, all with APC 10x6 props. Two use a macs one piece muffler. Those two turn 14,500 on 10%, and run a really long time on 8 oz of fuel. One's in a Shrike, the other in a Mikulasko Arrow. Performance on both is astonishing. The third one is on a trainer and I haven't tached it. I have found through experience that the macs mufflers take about 500 rpm off the top end. The OS Ugly Box is pretty efficient, but I don't like it because of it weighs so much and makes balancing the airplanes more difficult.