We use to race .25 OS fx engines around here locally on a Q500 airframe. The airframe you use will be a determining factor in the correct prop, but we found the APC 8x6 and APC 7.8x6 to be two of the best props for this engine. Look to peak the engine out close to 16,000 rpms.

Definately take the baffle out, and enlarge the muffler exhaust hole. This will help.