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Manifold Pressure

Asked by: 1131 views Aircraft Systems

When performing ground run up check, generally speaking what should the manifold pressure gauge read during full power and idle? I assume at full throttle the MP gauge should read current atmospheric pressure. Let me know any thoughts. POH does not have any good guidance.



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2 Answers

  1. Best Answer

    Russ Roslewski on Feb 11, 2016

    When the engine is off, the MP gauge reads approximately atmospheric pressure. But this is “actual” atmospheric pressure, not the “sea level equivalent pressure” you get from the ASOS and therefore set into your Kollsman window on your altimeter.

    Standard pressure at sea level is 29.92 – call it 30 for easy numbers. At sea level, your MP gauge should read about this with your engine off. If, however, you are at an airport at 1000 feet MSL, it will read about 29 – one inch per thousand feet difference. On the ground at Denver, about 25 inches.

    At full power, the manifold pressure should be close to this engine-off value, but not exactly – there are still some restrictions in the throttle body that cause a little bit of a venturi effect and bring the pressure down a little. But it should be close to this value.

    At idle, I really don’t know. Many MP gauges are only marked down to 10″ or something, and the needle goes lower than that.

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  2. Dan Chitty on Feb 11, 2016

    Thanks for the feedback.

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