Manifold Pressure issue

[P-38L]

Helo TD

I just noticed some problems in the manifold pressure gauge of some airplanes like the P-38 Lightning. When the engine is off the Manifold Pressure gauge shows exactly the outside pressure, but the problems appears when the engines are working at 0% of thrust, the needles should show no more than 12 or 15 inches of mercury, instead of that the needles show 30 inches of mercury.

You can compare that with the B-25 Mitchel that shows at 0% of thrust 10 or 11 inches of mercury.

I hope this issue can be solved soon.

Thank you very much.