Actually a rotor and a constant-speed prop act the same (as far as MP is concerned).
But back to the R22. The MP gauge is measuring the absolute pressure inside the intake manifold. When you do your carb heat check, you are at 70% RPM (if memory serves) and flat pitch. In this instance, the engine is trying to draw air across a partially closed throttle plate, pulling down the MP. A small reduction in RPM (due to full carb heat) will cause a noticable rise in MP. The same thing happens during the mag checks. The rise is not an indication of changed air density, it is an indication of reduced engine RPM
Now about that pesky MP rise when you go from a high to low altitude. Here's something that's initially hard to wrap your mind around - for a given MP setting, the engine makes more power the higher you go. Remember, the engine not only has to pull the air in, it has to push it back out too. The lower the air density, the less back pressure, so you get more power for the same MP. That's why you have decreasing MP limits right up to the "Full Throttle" point. You can reverse-engineer this - at 6,000', the engine will make 121 HP with a lower MP than when making 121 HP at sea level.
Last edited by Flingwing207; 25th March 2006 at 03:54.