Hi Homepage,
You're right, an engine does compress, however in normally-aspirated engines that only happens in the cylinders once the inlet valves are closed during the compression stroke.
Manifold pressure is measured in the engine inlet manifold after the throttle butterfly valve but before the inlet valve, so never gets to measure the compression in the cylinders.
The pressure drop that you see after starting is due to the throttle valve acting as a choke when partially closed. Chokes cause a pressure drop when the air is moving (ie. engine running) but don't when it isn't moving (ie. engine stopped). This is because when the air is stationary, the pressure can equalise around the throttle valve, however if the air is moving in one direction there is a pressure drop caused by the restriction and turbulence behind the valve which can't be relieved (because the air is flowing one way).
So, the manifold pressure you see when the engine is running is roughly proportional to the size of the opening at the butterfly valve and therefore the throttle position.
Hope this explains it.
cheers!
foggy.
<stuff below added later>
What I should have added is that the reason the air is moving past the throttle valve is because the cylinder is sucking it in during the induction stroke, as Tinstaafl correctly says.
Tinstaafl - looks like we posted together!
[This message has been edited by foghorn (edited 04 April 2001).]