I have turned near 10,000rpm with an APC16-10 prop with open exhaust. The stock muffler will choke the power equaling a 160FX around 9000rpm. I cut the rotor valve to 184 degrees open without loosing idle quality. I found best performance with 17"-18" props turning just under 9000 with open exhaust. I did not use a tuned pipe on this particular engine but have seen them used on other BGXs and they do work.