RE: O.S. GF40 4st gas
The muffled GT33 will turn the same props as the 4 stroker 300-400 rpm more, conservatively.
The piped GT33 will turn much more prop to the numbers I quoted above. Actually, Avgas produces 300-400 rpm less power than premium auto gas will all else equal. In other words, the same 20.5x10 prop will turn around 7800 on the piped GT33. The 33 is a stump puller and has absolutely great mid range response; the best of any 30-40cc class engine I've played with (but I haven't played with them all, so YMMV....SAP180, ZDZ40RE, Mintor 38, PTE36R and DLE35RA)