I have used the black carbon APC 4.75x4.75 would be worth trying.. But the engine has a rpm wall so to speak.
During testing with a APC 5x3 to see what it can put out 23,210 static clearly a low enough load to let it run the rpm up.
I think the engine has the potential to hit 100 mph with the APC 5.25x6.25 if not then 4.75x5.5 will be another worthy prop to let it breath a bit cooler. 5x5 yet another have to keep the pitch up above a 4.
Some estimates:
6.25P if at 20k unloaded 118mph
5.5P / 21.5k / 112 mph
4.75P / 23.2k / 105 mph
Another thing you could do if you don't mind extra noise is run it with just a header pipe still can tap for pressure.
The muffler is rather restrictive the header will let it unload further yet still dulls the sound and diverts the goo in one steady direction. Shown with a TD .09 spinner, 4mm round hex head and a APC 5x3
