He needs to get the experience first hand. I dont know how the TH75 will respond to a pipe, most sport engines are not problematic but some are. My OS61s and 108 are sweethearts with a Macs muffled pipe with no trouble throttling in a hover. I set the pipe/prop to surge at 70% throttle on the ground which is about the same in hover. In forward flight it surges at 50% throttle and is a little rich at this point. If I lean up the mid range for best level flight its too lean for hover. I do know that if you prop the engine very far below its intended rpm range fuel draw will suffer horribly. Keep all this in mind when changing/matching props and pipe lengths.