With a tuned pipe or a red Jett muffler a Webra 55 will turn An APC 9X10 at just over 16K on 10% MAXIMUM nitro content and that combo is what I run on a Viper and one of my Diamond Dusts. A bit less RPM on 0%. Running more nitro will blow plugs until it looses compression due to a hurt piston or the connecting rod ends are elongated enough to reduce the stroke, not good! 15% nitro also may have contributed to the breakage of two cranks that I know of. Probable detonation.

The Webra 55 is a very powerful, light and reliable engine if broken in correctly, light load prop at a rich two stroke at WOT.

I'd reserve the APC 10X10 for a Jett 90 LX at 16 K or the OS 55 AX and red Jett muffler at about 13.800, in both cases that prop would only unload in a real clean plane.