Manifold Pressure
I have looked at your profile , as you fly a H 269 helicopter this reply is specific for that helicopter.
In the case of manifold pressure, this is a misconception. As there is no turbo on the H 269 there is NEVER pressure in the inlet manifold, only vacuum. The word manifold , as applied to the H269 refers to the quadruple pipe connecting the fuel distibutor to the individual cylinders. The MAP (vacuum) is taken from a port midway between the fuel distributor and the inlet valves of the engine. When the engine is stopped the MAP will read atmospheric pressure,( eg 29.92 or 1013mb). When the engine is running, regardless of power being used , there will always be vacuum in the manifold. The engine was designed to produce 225 hp( reduced to 190hp ) . To do this this it requires a certain WEIGHT of air. When the engine is idling rotors engauged, the MAP will read approxiametley 10 -11 inches .At this moment the butterfly in the venturi will be open a small amount, the vacuum in the manifold will be quite high. When you are hovering using , lets say 24 inches the butterfly will be open MORE and vacum will be LOWER, however because of reverse gearing in the MAP gauge you are now able to use the gauge as an indicator to power being used.
ENGINE TEMPERATURE/POWER CHART.
You will be aware of the above chart on the heli panel.
As the engine requires a given weight of air to produce the design power the volume of air required will vary from day to day with temperature /height being flown.
In simple terms the chart says , on a colder day(more dense) maximum design power is reached at say 24.1inches, but on a warmer day the engine needs a greater volume ( less dense) say 26.0 inches - but still the same weight, therefore the butterfly will be open MORE on warmer day to allow the greater flow of air required.
The MAP has nothing to do with cooling of the engine after flight.