Manifold pressure vs power vs rpm
Assume cruising at 4,000 ft, throttled to about 20 inches MP and 2,000 rpm. Now reducing the rpm to 1,200 without changing anything else will cause the MP to rise sharply. The ambient pressure hasn’t changed, the throttle plate hasn’t changed. The only thing that has changed is the speed at which the pistons are pumping the air. Since they are moving much more slowly at the lower rpm, they are not sucking nearly as hard – not creating as much of a vacuum – so the MP goes up, towards ambient pressure. The pistons are sucking far less air, the speed of the air going through the intake is less and fuel flow is less. This means there is less power being developed, in spite of a much higher MP.