I think the prop thing is like stated by many others.....whatever works for you and your engine/plane combo will be what you need. I have two 3.2 Sachs based engines, both older, broke in, and run strong. I have not tached either engine, but here is what I know: one has a C&H electronically advanced ignition, is mounted on a 63" Ultimate bipe weighing around 18lbs (I know, it's too heavy, but it still fly good...just lands a little hot)and swings a Zinger Pro 20X10 and pulls the plane straight up without hesitation. The other engine has a TCSA C&H ignition on an 80" CAP232 around 15lbs, swings a Zinger Pro 22X6 and will NOT pull this plane straight up without running out of steam. I just took this plane in on trade and am sorting out all the things that need to be done to it, as soon as I have the chance, I'll run the 20X10 on it to see how it does. It should be much better than the Ultimate due to lighter weight, but with the 22" prop, this engine is not turning up the rpm's the other engine does, you can just tell by the sound. Both have the timing set at 30 degrees, and have new carb kits, etc. The mufflers are different in that the CAP has two pipes out of the muffler where the Ultimate has three. Everyone tells me I need to run 22" props on these engines, but so far, the 20" have proven to fill the bill. I would suspect a 21" would probably be the best choice, and will probably try one of these as soon as I can get things sorted and order one. Any suggestions on what brand to try in a 21" prop would be appreciated, also where to purchase the props. thanks...PACO