Hi Dave
What is the proper operating range of the Ultrathrust muffler for the OS91FX? The info sheet that came with the muffler only goes up to 60 size.
Here's my results for 2 props, 15% fuel, OS#8 and OS F plugs, all on the same day.

13/10 APC - OS muffler 10200 - Ultrathrust 10800
13/8 Master 3 blade - OS muffler 10400 - Ultrathrust 11600

These are peak ground rpms, not flight ready rpms.
I was having a little problem getting a clean needle setting with the ultra thrust but by the end of the session I had dertermined that I had a high speed needle problem and that clouds the results. I found that some times just touching the side of the needle would cause the rpm to jump up 300rpm.
I was surprised at the difference in gain between the 2 props and that's why the question. On the 13/10 near peak rpm the engine sounded like an engine just about to get on the pipe but not quite there. The 3 blade was clean from 11100 to 11600 but if fell below 11100 it would drop to about 10500. It did not seem to want to run between10500 and 11100. Again this is all clouded by the needle being little messed up

Denis