They are not the most powerful engine but they will work. Years ago a friend had a McCoy 29 redhead (front rotor stunt) on an A/B Spacer, which I believe was about 565 sq. in., and it went surprisingly well. I tached a similar McCoy 29 — again, if I remember rightly — at about 14,000 rpm with a 10/4 Grish nylon prop. In my experience in 30 years of power flying having a well trimmed airplane is a lot more important than having the hottest equipment!