[K_Freddie]

Manifold pressure (MP) is essentially the amount of usefull power being produced by an engine. This pressure decreases with altitude (less oxygen), and increases with a turbo-charger.

Say at a certain altitude you have X MP at Y RPM.
As you go higher the RPM at MP both drop, but can be compensated for

1) by increasing the throttle (and fuel consumption) - this will keep your speed.
2) reducing Prop Pitch (PP) - but this will slow you down (less bite from the blades).

After a certain altitude, even with a turb/super-charger there will only be enough oxygen in the engine to burn so much fuel. Anything extra is wasted and you see a 'waste-fuel' trail behind you. You must reduce your fuel mixture (FM)% above this height.

Now this has the effect reducing MP (RPM), and PP changes have little positive effect.

Going to a lower altitude has the opposite effects as above.
Essentially at every altitude level, there is an optimum, MP, RPM, PP, and FM setting, for fuel efficiency, speed, etc...

Prop pitch adjusts the amount of thrust from the blades, but has the effect of loading the engine. The engine must then have the power (MP) to handle the load or the RPMs will drop (and MP), and compensated for by throttle or reduced PP. You tweak your fuel mixture to max your MP.