I have a OS 46 FX on a WM P-51. The baffle has been removed from the muffler. I use OS-8 glow plugs. I am currently using Wildcat with 10% Nitro and 18% synthetic oil.

Which prop should I use if I want to go fast?

I've tried a APC 12x6 (too slow), APC 11x7 (acceptable), APC 10x7 (a little faster). I am currently using a APC 10x8 which gets me good speed at 11,500 RPM. I have a APC 10x9 waiting to be tried.

If my overall goal is speed, is there anything wrong with going with a small diameter to get more RPM and high pitch to get more speed?

I know I can get the RPM higher than 12,000 RPM if I lean it out, but I don't know what limit I should set for max RPM. The guys at my field use the OS 46 FX on various planes and normally use props with larger diameter and limit their WOT RPM to 11,000 RPM. Although I want speed, I don't want my OS 46 FX to live a long life.

What is the practical maximum RPM for this engine?

Thanks,
Juice