I went with a OS 91FX with a tuned pipe on my Shoestring. And BTW this motor has been nothing but reliable and good to me. I didnt have to cut the cowl out for the engine at all it fits perfectly in there. Now the tuned pipe was another story... I had to run it out the side and mount it to the landing gear. That part was a little tricky. I would really like to run it under the belly of the plane for better areodynamics, but with the belly i just dont know how im gonna manage that. But man this thing REALLY gets down!! Im running a 12X8 APC prop and spinning 13,200RPM on the ground. I havent done much pipe tunning, so i expect i could get some more RPM out of it by shortening the pipe a tad. I make sure not to make any fast hard banking turns while at full speed, due to the scare of folding the wing up This plane is my favorite plane in my fleet. It basically needs no runway and vertical is unreal! It will go up and out of your site if u let it. It never stops![>:] Deffinatly the fastest 1/4 scale plane at my field