Buzz Put whatever prop you like on your 51 and 90 but you will miss the boat.The ST 90 and 51 will shine if you keep WOT at 12,000 RPM. You will have full use and proportional control of your throttle. By over proping you will lose throttle control at about 70% open. Move it farther open and you can't tell the difference. Read Bax he likes RPM.