Effect of prop diameter on 912 series engines

Your engine was never really designed to run in the 4000's rpm range and prop pitch isn't just about speed. Plus owners around the world many times get to use an in flight adjustable prop and take advantage of different pitch settings for climb and cruise, but others can't like here in the USA. We usually just get to use a ground adjustable prop unless it's in a certified plane. The engine can run at 5500 rpm all day long and up to 5800 for 5 minutes. If you over pitch a prop and still only fly at 4900 rpm then the engine is still under more stress trying to turn that over pitched prop. Plus you lose climb, cruise, fuel economy and have higher engine temps.

You want to unload the stress on the engine, but yet have a good performing prop with good flying results. For the ground adjustable guys this spot is right about 5600 - 5650 at WOT in level flight at your AVERAGE altitude. This setting will help un-stress the engine and give you a well balanced performing prop. You don't have to fly fast and it will make a difference in climb.