- I dunno nuthin' about them four strokes; if they truly go by the "less power than a 2-stroke" rule then a 40FS may not be adequate -- so if you're going to stick with four-bangers, do whatever a four-banger expert tells you to do
A 40 4-stroke is plenty of power for a Banshee. The "60% rule" is about right (or optimistic) if you look at the peak HP, but you can't use anything like the full power of a 46LA in a stunt plane, unless you can find a prop with 2" of pitch. You *can* use the full power of a 40 4-stroke, there's no real issue with flogging it within an inch of its life, since it's not going to over rev or get wild.
You could probably do OK with a 26 Surpass. It would still be better than a Fox/McCoy 35, but you definitely will have to flog it.
The only real question is how to run it, either the conventional 4.5" pitch revs (which liberates more power) or the Igor Panckenko/Bob Reeves way (which generates much less power but has more responsiveness, and is certainly much easier on the engine).
I would expect this combination to be a very good performer, if the wing is moved forward sufficiently.
Brett