ORIGINAL: NikolayTT
I would try the engines used in the RC-cars, some reach up to 40,000 rpm just like that. Some fellows may say that those are small displacements, but that is on my opinion good if we consider the Diameter of the Propeller and the Linear speed of its tips. Even with 6 inch propeller 30,000 rpm gets the tip beyond the Ultra-Sound speed and to ... dangrous desruction. So I would ask the Question: - Yes, Fast Engine, but with what kind of Propeller ? - Well, mabe Impeller then but that is 'a bit' strange use of combustion engine, isn't it...
Car engines turn high RPM because they're lightly loaded. Put any sort of meaningful prop on them and they'll not keep up with a proper airplane engine. If you want a fast, small engine get a Fora. They're designed for F2D and turn about 29-30K with a 7" prop - you'll need to get one of the fiberglass toothpicks they sell with the engines, or use, say, a MA Scimitar 6.5x3.5.
A somewhat bigger engine? Hunt e-bay for Nelson engines, you'll probably find someone's old Q500 engine for sale, that'd be fast... probably way too fast..
Oh wait - what's this - you want a SPORT RC engine that actually has a throttle?? In that case, go with the majority here and give Dub Jett a call...
Iskandar