I think I have an 18 x 8 Top Flite or Zinger prop around here some where. When I get time, I'll get the Tiger Moth out and change the 18 x 6 on it for an 18 x 8 and see what it turns. Does 8500 on the Top Flite Power Point 18 x 6. I was pleasantly surprised that it ran that well, and checked it several times with my TNC tach. I know the TNC is accurate, as I checked it against a calibrated full size aircraft tach and it was within 1 (one) rpm of being in perfect agreement.

AV8TOR