ORIGINAL: Sport_Pilot
...Actually, that was only true of the engines since the early '80s. Before that the stunt engines used mostly low nitro and the high performance engines were set up for the 25% nitro Missile Mist. Many of the early K&B engines were set up for 20-25% nitro also, though they ran nice on 10%, not worth a darn on 0%.
Actually, Hugh, the K&B 6.5 engine (tested in 1977) got top marks from Peter Chinn, in MAN magazine; throwing everyone off the podium and getting the highest RPM on a 10x6 prop... On just 5% nitro.