From what I know MAP is a way of measuring engine power with variable pitch propeller. Greater throttle setting - greater power - greater MAP on the gauge. It happens because MAP is in fact vacum pressure so opening the throttle gives greater fuel and air flow into the manifold, explained here: Why does manifold pressure increase with power?

But I also read somwehere that when we reduce rpm without touching the throttle, the MAP will increase, according to this topic:What happens to manifold pressure reading following a reduction in both throttle and RPM?

Reducing throttle decreases manifold pressure. Reducing rpm reduces suction, so the MAP should then increase?

The question is: why does reducing rpm increase MAP?

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    $\begingroup$ You need to actually ask a question. You have simply referenced two other questions with answers, but haven't sufficiently described your confusion with them. $\endgroup$ Mar 14 '20 at 18:05
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    $\begingroup$ @Konrad. Sorry they closed this question. If it helps, try to imagine the (intake) manifold as a room between 2 doors with a giant fan pulling air through door 2. The fan is your pistons trying to pull air into their cylinders. If you partially close door 1 by reducing throttle while the fan is running at the same speed, the pressure in the room drops. If you slow your fan down (reduce RPM) the pressure in the room goes up. $\endgroup$ Mar 14 '20 at 22:40
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    $\begingroup$ Try this Flying Magazine reference, it may help. $\endgroup$ Mar 15 '20 at 0:46
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    $\begingroup$ @Jan Hudec that may be the source of the confusion, because if the airspeed is high enough, reducing pitch may increase rpm by "windmilling" the prop, even without adding throttle. Increased rpm does not necessarily mean more throttle. One is power, the other is load. $\endgroup$ Mar 16 '20 at 14:17
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    $\begingroup$ @RobertDiGiovanni, we are talking about constant-speed propellers, so increasing RPM means advancing the prop lever, which commands the governor to find a pitch for the higher RPM. If the throttle stays put, the MP will drop as the engine is trying to pull more volume through the same opening. I am not sure what power will do though, because the throttle drag increases and the propeller drag increases, but the engine will still be pulling in and burning a bit more fuel. I am guessing it should increase just a little bit. $\endgroup$
    – Jan Hudec
    Mar 16 '20 at 16:53

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.


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