After you use a tach for a while you soon realize that all engines are not created equal. You also learn what rpms you need with different size props. The TT 42GP engine will give you 12000 rpm with a APC 10-5 prop which will give you a decent takeoff speed. There was a new guy with a bad engine on his trainer. With a 10" prop on his plane we could only get 10,500 out of the engine. It was barely enough to get the plane off the ground on our grass field.[8D]