Let me amend my answer to be "same value if the engine is not operating,
the gague is correctly calibrated and you are AT SEA LEVEL".

Remember that local altimeter settings are corrected for sea level when
you are at a locale that is at elevation. 29.92" setting in Denver is a
"corrected reading" for what the pressure would be if you were at sea
level. So, ballpark.. the value is 1" per thousand (without digging out
reference material) pressure change/altitude. Denver on a standard day
(setting 29.92") should have a manifold pressure gauge reading (5000
ft/5" less) just under 25" when the engine is off.

Sorry for my initial error.
I'm sure if there are others, they will be quickly pointed out.
Dave

Dave S wrote:

Should be the same value if the engine is not operating and the gauge is
correctly calibrated. I would suspect the gauge is out of calibration.

Dave

Andrew Gideon wrote:

Did I misunderstand my reading on the subject, or should the Manifold
Pressure on an engine that's off be the local atmospheric pressure?

I've just noticed that this is not the case in my little R182.

- Andrew