Hi. I had some experience running one of these in a pylon ship when my f3d engine died. The MAC 8.5 unmuffled pipe worked the best. The sizes around 8.0 to 9.0 inches in diameter worked best. Pitch varied with conditions, but 7-8 was the range. The takeoff run was longer, but the engine unloaded to a little over 25k in the air according to the telemetry unit we were using and the speed was over 225. Happy speeding!