I had to jump in here, you are probably running that for RPMs, which is OK but i don't beleive the engine was intended for it. Developed for Sport flying, 3D and basic flying. I Was out flying both the 46AX and the 55AX on identical planes. I always favored the 12.25x3.75 props on fun fly's, 4*40's using the 46AX-Perfect. The 55AX also fly's well with this prop (screams) but is happiest with a 13x4. Lower RPMs, monster torque and good spool up so i may try something larger. The engine just seems to want to lug the bigger props to run good, cool and happy. Just my opinion of coarse. Plane below is around 4lbs and is amazing with the 55. I have another one with the 46.