RE: Info on .91FX
We don't have an RPM/torque number for the O.S. Max .91 FX engine. It hasn't been released by O.S., if they have it.
We normally prop the .91 FX with an APC 15 x 8 and turn it in the high 8,000's. This is in the range that O.S. has recommended. This works mainly to keep noise down. You can go to a 13 x 8 prop, or even a 12 x 9 or so. The engine will turn up much higher, and also make a lot more noise. Propping the engine to run in the 11,000's won't hurt it, but you'll need to play with different propellers and see what kind of performance your engine/airframe combination gives you.