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Understanding Manifold Pressure.

Manifold pressure, which we use to help set power when flying an airplane with a constant-speed prop, actually should be called manifold vacuum, because that's really what's being measured: how much air is being sucked into the engine's cylinders.

When an engine is not running, its manifold pressure is the same as ambient pressure: 29.92 in. Hg at sea level on a standard day. Once the engine is running at idle, manifold pressure might be down around 10-12 in. Hg. At wide-open throttle at sea level, however, that same normally aspirated engine's manifold pressure gauge should show close to ambient pressure, allowing for inefficiencies.

When we retard the throttle, we close a plate in the induction system, restricting the flow of air into the cylinders. This reduces power since there's less air available for combustion, reflected by a lower manifold pressure reading. If we had an airplane engine turning a fixed-pitch propeller and equipped with both a tachometer and a manifold pressure gauge, each instrument's values would fall when reducing power from full throttle.

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Title Annotation:AIRCRAFT SYSTEM
Publication:Aviation Safety
Date:Jun 1, 2019
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