Thats what I read in several sites, 6500 to 7500, and from my tests, their right, at least with a 15 lb biplane anyway. When it was proped to turn 8200, it took forever to get rolling, and sounded like it was spinning it's guts out in the air. It pulls so much harder now, and the flywheel affect of the larger prop ( 18" ) helps it to idle real slow. I love that !