[Al Lewis]

It's a rough estimate but that's pretty much how it works. Normally a 50cc engine will operate from 6600 to 7000 depending on the engine. Also, if you have an engine that will operate a good prop, 23 X 8 @7000 rpm you wouldn't want to put something on it to over load it just for the sake of getting the rpms down. Normally the benchmark prop is the one you want to stay close to. That way the manufacturer has done all of the work for you. If they call for a 23 X 8 for a benchmark you wouldn't want to run a 24 X 10 and probably not a 22 X 8. If, on the other hand, you are running the benchmark prop and not getting the advertised numbers you may want to drop down a size to try and get your rpms at their benchmark. Once in a while you'll get a company that will list a benchmark prop with a given rpm that is simply not attainable just to sell the engine. Then when you don't get the same numbers they swear it's because of your mix, or it's not broke in yet, or they used a different prop. Always an excuse. The benchmark listing for the MT-57 is JXF 24X8 - 7000 RPM @ 5000Ft Elevation. The engine should be able to reproduce those numbers with that prop within a certain number of rpms, say 100 either way.