With the blue muffler it will turn an APC 11X6 @15400 or MAS 11X7 (S-2 series only) @ 15100 rich'nd for flight. I did notice that I had used a 1/8" spacer so the muffler doesn't touch the fuselage. Did this help or hurt me? I don't think it makes that much difference, but I believe in full disclosure. Note an APC 11X7 is too much prop, yet the MAS just blasts off as well as it unloads and holds rpm in hard turns. The engine is a sport beast. This engine was broken-in wide open throttle, rich two stroke, Powermaster 15%.