Benched it for two tanks and flew it with 12x6 apc prop, 15%coolpower and OS F plug. The power was just marginal and I hate that. What's the right rpm for this engines with a APC 12x6? Thanks.

If you are forced to run 15% nitro fuel, go down in prop size to an 11x6 or 11x7. The engine is happiest between 11000 and 12000 rpm, so stay in this range.

TRY SOME 30% COOL POWER HELI FUEL IF YOU CAN GET IT.