I am running the OS .50SX with APC 12x6 getting 11,000rpm with an Ultrathrust pipe and 15% nitro. I plan on moving to 11" props and testing this weekend to raise the rpm for racing....I am using a 3.5" spinner though which may not allow me to drop diameter and still fly the .60 size plane this motor is in. The Hanager 9 P51.
Enjoy,
Jim