Question for the Motor Heads

[MadBlaster]

I read somewere a while back that the constant speed prop is based on oil hydraulics and aerodynamic forces on the prop from airspeed. Basically, the faster you go, the prop wants to run fine pitch from the aero forces, so to counteract that, there's a governor mechanism that runs off oil pressure to make it run course pitch. So you have the two forces constantly balancing to keep the rpms in range so the engine doesn't overspeed. So whether your running fine or course gears, its the same principle. It's a passive control more or less. Not like a variable pitch prop in manual mode ala 109.

Manifold pressure, I'll take a guess as I'm not a pilot either. I think it's just like a car. Natural aspirated, the engine creates vacume on the piston downstroke and sucks in the fuel/air mix. The higher the rmps, the more sucking power and the higher the manifold pressure. In supercharger setup, pump is belted somehow to the drive shaft and forces air/fuel into the manifold via a pump. I guess what the guys are saying up above to explain your observation about manifold pressure going up when prop pitch is changed to 'course' is that at high rpms, output of the supercharger pump somehow hinders the engine vacume (ala "law of diminishing returns"), causing it to be lower than it would be if the engine was naturally aspirated. So, it's a trade off when you use a supercharger. What you lose in power at the high end of the rpm scale you gain on the low end of the scale (i.e., higher manifold pressure at course pitch setting where the load on the prop in static state (no acceleration or deceleration) is highest).