I think its unlimited with just about anything over a 91 four stroke. I had the 150 on hand already, and took it out of a 2m pattern plane to get the engine running right. I figured I'd do all my tinkering on the engine with it mounted to a plane that floated, that I was familiar with the flight envelope of. I've been having so much fun with it, I dont wanna take the engine off and put it back on the 11 lb plane. LOLOL

I think the saito 100,120 or the ys 110 are ideal for this plane. the 150 isnt much heavier than the 120. If I didnt have it already, I would have a 100 or 120 on it.