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This is baffling me. Does anybody with experience with this type of indicator have any idea what the blue range of this gauge means?

Weird blue range

It's mounted in a mock-up panel. I know nothing about where it originally came from, though it seems pretty clear that it's from a turbocharged piston twin.

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It's a manifold pressure limit with altitude. It relates to a limitation on the differential between ambient and total manifold pressure at different altitudes with turbocharged or supercharged engines; a limit on the amount of actual boost with altitude in other words, not the total pressure.

At 30000 ft, standard atmospheric pressure is only just under 9 inches. 20 inches on the gauge is 11 inches of boost above atmospheric.

At 20000 ft, atmospheric pressure is just under 14 inches, and you're limited to 35 inches, which is about 21 inches of boost.

Between 16000 ft and sea level, with atmospheric pressure between roughly 16 and 30 inches, the limitation is the red line of 40, which is anywhere from 10 to 24 inches of boost above atmospheric.

The increasing limitation in maximum boost with altitude is usually related to the reduced cylinder cooling ability of the thinning air with altitude, and the potential for detonation.

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The blue scale indicates the maximum manifold pressure (MP) at a given altitude.

  • from sea level (SL) to 16000 ft: the maximum MP is 40 inHg (inches of mercury).

  • from 16000 ft to 30000 ft, the maximum MP goes down to 20 inHg.

The altitudes are barometric altitudes.

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  • $\begingroup$ Do you have a reference such as a POH that describes this? What type have you seen this in? $\endgroup$
    – Pugz
    Mar 13, 2019 at 13:59
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    $\begingroup$ @Pugz, no, unfortunately not, but it is written on the gauge itself. $\endgroup$
    – bogl
    Mar 13, 2019 at 14:01
  • $\begingroup$ So this is indicating the maximum possible given existing atmosphere at a given altitude, not any sort of limit, but just for pilot reference to to compare to the altimeter? $\endgroup$
    – Pugz
    Mar 13, 2019 at 14:09

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