This engine has very peculiar power & torque curves. There is a sharp increase in power output right at ~ 12,500 rpm (like "comming on the pipe") & it is important to prop it correctly to take advantage of that sharp jump. An APC 9-6 will quickly let it get up there & wind out to its happy place @ ~ 16,000. A 9.5-7 (if you can find one), or 10-6, or 10-7 also work very well. Don't be afraid to wind it out, this engine still makes huge power as high as 20,000 rpm!!!!. Conversely it can lugg really brutal props if it has to -- it is capable of spinning an APC 11-7 @ ~ 12,700 static --- but that is really giving away power potential. For good vertical, it will turn an APC 11-4 @ ~ 15,000+.