Sounds to me like the 90 size pipe just happened to be the right length on the .61 for the high pitch prop. Then it dug in and scooped some air on the downhill runs. If the pipe was shorter, then it might work better on the .60 size props. It is usually a good idea to check rpm with no pipe and then with the pipe to see if the pipe is helping, or is just a bad muffler which could be holding you back, as it seemed to do on the 11-7 .61. Normally it will pick up a thousand rpm or whatever when moving, and can then hit the pipe-which will then give many more rpms in turn.
Last edited by aspeed; 09-21-2017 at 04:58 AM.