why does manifold pressure increase when rpm is decreased
 

Why does manifold pressure increase when rpm is decreased?

Manifold pressure increases with reducing RPM because it is increasing toward ambient air pressure.

When running, a simple piston engine (not super/turbo charged) always has a manifold pressure of less than ambient air pressure. When stopped the manifold pressure is the same as ambient pressure.

The carburettor manifold is a venturi (waisted tube) which has the effect of increasing the air velocity through it whilst reducing its pressure.

This reduction in air pressure draws fuel from the tank through a metering valve.

As a piston engine slows down so does the air travelling through the manifold and thus manifold pressure increases.

It is mainly because of the throttle restricting flow into the manifold. An engine under load can have a high manifold pressure and yet low revs and vice versa, so the relationship of rpm to MAP is not a direct one.

There's no need to go into great depth here.

Think of the engine as an air pump. The faster it rotates, the more air it can remove from the inlet manifold, reducing the MAP. If the engine slows down, air can get in quicker than it gets sucked out, causing the MAP to increase.

I should apologise for my earlier poor answer due to not reading the question properly. If you do not move the throttle and the RPM drops for some reason, yes the MAP will increase. Shytorque's post explains it simply. It is seen in a mag check or when exercising the prop.

More generally, the MAP does not necessarily increase with reduction in RPM. It could increase or decrease, depending on throttle position and load as well as RPM.

When you close the throttle the MAP drops and vice versa. The manifold pressure will in turn affect the pressure in the cylinder which is effectively your torque. A key concept is that torque x RPM is proportional to power. Hence the power setting table of an aircraft with CS prop gives different combinations of MAP and RPM resulting in the same power - some with higher revs and lower MAPs than others. On the other hand, with a fixed pitch prop if you close the throttle the MAP and RPM will both reduce. Because that also reduces the power, it is still consistent with all of the above. (Power is set by throttle on a fixed prop whereas power is set by throttle and RPM control on a CSU).

MAP is related to engine load demand. When the intake is throttled it creates a restriction of the airflow to the engine, and a reduced MAP. The most efficient operating condition for a recip engine is normally at WOT. Which would also mean high MAP.

How does one calculate where your MAP must be at a certain Alt/Elevation during a max power take-off? For instance: we know that the manifold must be +-22MAP at an elevation of 5500amsl, but how do we KNOW that 22MAP is the golden number at 5500elev? How can I calculate what my MAP will be (for example) at an elevation of 8500' for take-off? I want to ignore the rule-of-thumb and rather use a graph of some sort! :-) Hope someone can help me!