I have about 10 tanks through my OS91FX now and the prop has been the APC 12x14 from the first start. Muffler is the stock and it turns that prop +9000rpm. I am gonna tach it later again when a have leaned the high speed needle more. I use this engine on a speed plane of 1200mm (47inches) wingspann and it goes pretty fast end very quiet. This engine runs very nice and reliable and had only one tank on the ground and the other nine in the air I use OS8 plug and 10% nitro.