RE: ASP/MAGNUM FS400AR Rebuild/Overhaul
There is a balance to be had between the load on the engine from a large prop, and the acceleration loads from high rpm. Given that most model 4 strokes can tolerate well over 10000rpm there should be no problem at the top end of the engine. the limiting factor will be the big end.Given that the stated max rpm (on the ground) is 8000rpm we can assume the engine is stressed for 9000 in the air. With mine doing about 7400 on the ground I am sure its over 8000 in the air when it unloads. As such I wouldn't worry. As an idea though my instruction manual even says an 18x8 can be used. that's a tiny prop for this engine and I think it would go mad.
And while I understand the desire to run a large prop for that radial sound this engine is not designed for that. The Evolution radials are designed with this more in mind and have much longer strokes and lower operating rpm.
It should be noted that props do vary massively and a 21 inch from one brand may be as fast as the 20 inch from another. the rpm is the key more than the actual prop size.