Now I have a few gallong through this engine I am really winding her out in the air. I have checked the rpms and it looks like the 12X6 on the ground is spinning at 11,200 but in the air it is another story all together.. I am now flying wide open for much of the flight and I often check the temp of the engine on landing by feeling of the engien at different points and to be honest the little 51 isn't even breating hard to spin this big ole prop at these speeds. The only thing that feels warm is the head and it isn't very hot. The Ultra Sport 40 is deffenately getting a workout. Still haven't mounted a 13X6 but will soon.