I tacked a new box stock Ryobi 31cc string trimmer with straight shaft and got 8100/8200 RPM. Nothing was disassembled or touched not even the carb adjustments. The test was no load except string in the trimmer head.

Eventually every prop imaginable will be tested. I may reassemble the other engine back to box stock and check the RPM. Do you know the RPM of maximum torque? Does the magneto limit the RPM?