I've done several of each.... I found the 30cc to be quite a bit more powerful. They are harder to find, but worth it to me...

The 25cc engine is not bad though. Look up the thread "Non machine shop Homey" or something like that. Without too much work I got one to turn an APC 16 x 8 prop around 8700. You have to pin the ring and take the bridge out of the exhaust port to really get anywhere with the 25cc. With these size props, a few hundred rpms is really noticeable in performance, and it's a decent horsepower gain.

They are good engines for the price, which is often "give away". Another thing that is cool is that you know you can rev the hell out of them, because the boat guys run them over 10,000 rpms without significant reliability problems. I would take one over a Ryobi any day. The Ryobi is good for scale airplanes that you don't want the engine parts sticking out of the cowl however, like my big Tiger Moth.... .

AV8TOR