Manifold Pressure

[doright]

In the priority ranking of bugs and desired upgrades this one ranks pretty low. The manifold pressure gauge drops to the lowest point when the engine is not rotating, it should go (up or down) to atmospheric pressure: 29.92inHg Std Atm @sea level.

You can make the case that sitting on the runway with engine off there is no power to the gauge, but many aircraft the gauge is an aneroid bellow with no electrical connection. A dead rotating engine (not producing power) would be less then atmospheric unless a supercharger is spinning.

Being of the P38 crowd I've had ample opportunity to observe this gauge when rtb'n with one engine out (feathered).

[smoe]

I have no idea if I'm right here, but the indicated manifold pressure may be relative (difference between inside manifold vs. outside air, and not absolute) so a dead engine probably should indicate zero?

[doright]

I have no idea if I'm right here, but the indicated manifold pressure may be relative (difference between inside manifold vs. outside air, and not absolute) so a dead engine probably should indicate zero?

Generally nope (as in all things aircraft related the only absolute is that they rarely interact well with the ground), otherwise you would have very negative numbers with throttle pulled back and positive numbers if supercharged. Or vice versa if those damn tiny tubes got reversed when hooking them up.