the same density or mass of air flowing through a smaller duct has higher velocity, therefore lower pressure. The atmosphere then pushes fuel into the venturi/throttle body through the jet into this low area zone - The spray gun principle. The manifold pressure shows the position of the throttle plate and how its governing air flow into engine, and therefore the power the engine will produce, the pressure differential being equal to the square of the airflow. this vacuum is completely separate to the venturi itself, which sees a pressure differential between its narrowest point and the surrounding larger duct (whether this is referenced to atmospheric pressure or not) simply due to air flowing through it having the higher velocity. The highest manifold vacuum is actually when compression braking. especially on these fancy new efi systems with intake air control and fuel cut on decel. and the engine is spinning much faster than at idle.The relationship between manifold pressure (MP) and propeller RPM is a complex one. The different combinations of manifold pressure and propeller RPM are what allow us to set accurate power settings and obtain the performance outlined within the PIlot’s Operating Handbook. Why does lower propeller RPM cause higher manifold pressure? Reducing an aircraft’s propeller RPM causes a higher manifold pressure because as the RPMs decrease, the engine slows, creating less suction during the intake stroke which reduces the vacuum in the intake manifold, allowing the air pressure in the manifold to rise towards atmospheric pressure. There is a bit more to this story, though. Perhaps the best way to better understand this manifold pressure and prop RPM relationship is to take a closer look at what manifold pressure really is. Manifold pressure is a measure of the absolute pressure of the fuel/air mixture within the intake manifold of the aircraft engine.
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