Actually, you don't need to be over 6000 rpm. If you create high enough HP #'s, you can turn an "AA" impeller and move more water than the "A" you are probably turning now. You can put a 10000rpm engine in it, but if doesn't have the HP and TQ to move water, you won't be able to do jack. The HP and torque numbers are more beneficial than the overall RPM #'s.