After running almost a gallon through this engine, I am now getting 13000 rpm with a 12X6APC prop, about 4 clicks off peak, and still getting a nice smoke trail. Powermaster 15% fuel, and 2 shims in the head. I haven't tached the 13X6 lately, but honestly I like the performance of the 12X6 over the 13X6, though I suppose the extra 2000rpm means the engine won't last as long?? Very powerful engine, excellent transistion, rock solid idle at 2500rpm, and my shuts off, at least for now. Deffinately not bad for $100. I have it in a 7 lb. plane, and unlimited vertical is a forgone conclusion!