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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?

Robert DiGiovanni
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Konrad
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  • I really don't understand your question @Konrad. – GdD Mar 14 '20 at 17:26
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    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. – Michael Hall Mar 14 '20 at 18:05
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    @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. – Robert DiGiovanni Mar 14 '20 at 22:40
  • Thanks, that is very good explanation. So, on the other hand if we do not touch throttle and increase rpm we will see a drop in map and decrease in power? – Konrad Mar 14 '20 at 23:16
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    Try this Flying Magazine reference, it may help. – Robert DiGiovanni Mar 15 '20 at 00:46
  • Thanks, this article is great and explain everything. – Konrad Mar 15 '20 at 15:14
  • @Konrad, Power is only proportional to MP at constant RPM. At higher RPM the same MP corresponds to higher power. – Jan Hudec Mar 15 '20 at 20:44
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    @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. – Robert DiGiovanni Mar 16 '20 at 14:17
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    @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. – Jan Hudec Mar 16 '20 at 16:53
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    A little bit here on how the "governor" helps out. – Robert DiGiovanni Mar 16 '20 at 17:54
  • I've got few more questions: if we move prop lever we're changine rpm and then pitch is changed or by moving prop lever we change blade angle and then rpm adjust to it. And on the tachometer of airplane with variable prop we have engine rpm or prop rpm, because I heard lot of versions. – Konrad Mar 17 '20 at 12:45
  • And one more, according to the answer that I pasted below (I found it somewhere it the internet) everything there seems to be logic BUT if we maintain power setting and decrease rpm the pitch angle should increase. So constant-speed prop shouldn’t generate same amount of power because of blade angle change? I understand that growth on MAP gauge reading while decreasing rpm but I don’t got why power is decreasing, we have constant speed prop, so shouldn’t it reamain constant while changing rpm? – Konrad Mar 17 '20 at 13:49

1 Answers1

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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.

Konrad
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