Manifold pressure in a stopped engine is always the same as they ambient pressure outside. So in effect, you're alway carrying a barometer around in the aircraft with you. So if you can't get an altimeter reading and for some bizarre reason don't know the field elevation, the MP needles will tell you and you can set the altimeter to that. They're not very accurate, but will do in a pinch.
Due to reasons beyond my knowledge, and in no small part to Bernoulli, the MP decreases as you start the engine as the flow increases. It's a bit like a wing - the higher speed above it decreases ambient pressure and vice versa.
Last edited by AdamFrisch; 21st Sep 2012 at 22:29.