Originally Posted by capt. skidmark
i have had a question from a student why exactly the manifold pressure increases when the prop lever is pulled backwards and vice versa
A good question, which has not been matched by good answers!
With the engine running, the manifold pressure gauge is actually a vacuum gauge. It is measuring the suction created by the pistons on their induction strokes drawing in air against a partially-closed butterfly valve.
With the throttle full open, the butterfly valve offers little resistance and there is almost no suction ... the manifold pressure gauge then reads only slightly less than the surrounding air pressure (this small difference being accounted for by filter and other intake losses).
With the thottle fully closed, the butterfly valve is also almost fully closed, the pistons are sucking against its resistance and cannot move much air ... so the manifold pressure drops to a substantially lower value than the outside air.
Now, increase the RPM against a partially closed throttle and the pistons (the suction pump, if you like) are pumping much faster, creating more suction ... so the manifold pressure drops.
Correspondingly, reduce the RPM with a partially closed throttle and the pistons pump less hard and create less suction, the pressure drop from the outside air reduces ... and so the manifold pressure thereby increases.
Does that help?