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.

Chris,


Extending the header and adjusting the system's length for the lower RPM, is virtually all that needs to be done.

If your Macs pipe is the 'Quiet' type, it is a cone-and-disk design, which has a wider range and is easier to tune.
The muffled pipe is more peaky...

Even the 'Quiet' type will need extending the header by 1.5-3", depending on how much lower the RPM will be.