Kev the MP is not an indication of power being transmitted to the prop.
MP indicates how much air is available to be mixed with the fuel and the ratio of fuel to air is what gives you the power to do work....like spin the prop.
MP is actually an indication of suction. Sitting on the ground with the engine shut down you will see the MP gauge reading 29.5 inches at sea level in ISA conditions. Engine running at idle might be 10 inches which is lots less air pressure because the throttle butterfly is almost blocking the air intake up stream of the venturi which is where MP is measured creating a partial vacuum in the intake manifold. Mixture in this case would be full rich so fuel being delivered to the combustion chamber is dependant only on the engine driven pump which is a function of prop RPM. Not much fuel and not much air means not much power.
Another example is climbing up to say 5000' where you will be indicating, with throttle wide open, about 25 inches and setting 2000 rpm. Then pull the mixture to idle cutoff. The MP won't change and if you lower the nose to maintain speed the RPM won't change but is the engine producing any power?
In your example the MP is unchanged but the RPM is reduced. Reducing the RPM means the engine driven fuel pump turns slower delivering less fuel...less fuel/same air means less power developed by the engine.
The only control you have in the cockpit that is capable of adjusting power from 0-100% is the mixture control.
Last edited by Chimbu chuckles; 3rd Jul 2006 at 13:01.