The instructions for the ENYA Ultra 11 CXD recommend that users select a 9x4 or 9x5 prop for R/C use.
Good thrust, with low noise.
I have no doubt this engine would be happy spinning a smaller prop at higher RPM's, but I am little over screaming small displacement two strokes.
That said, FlyingKiwi metioned to me yesterday that a Rothwell R 320 will spin an APC 7x7 at 18,000 RPM plus! ... not too shabby for a non schneurle Olly Major Replica, if that's what moves you
