Manifold Pressure
John Deakin has an interesting article on how air gets into the engine and why “Manifold Pressure” is really “Suction Pressure”. One takeaway from the article is that your MP gauge may not be calibrated accurately. To find out what it really says you need to make some photos and do some arithmetic.
This will show on the MP gauge as 29.92 inches at sea level on a standard day. I know, it’s hard to read it that accurately on the usual instruments, but you should see it very close to 29.9, and that’s “close enough.” If the sea-level airport has a big high-pressure area located over it with a local station pressure of 31.10, for example, then your gauge should show 31.1 inches of manifold pressure. If the airport is located at some higher elevation, the MP gauge will show an inch less for each thousand feet above sea level.
If you turn the knob until the altimeter reads 0 feet, the pressure in the Kollsman window will reflect the local pressure and will come close to your manifold pressure gauge.
In this case the manifold pressure gauge reads about 28.75″ when it should read 29.92″. It’s reads low by about 1.2″. So if I want to set the MP to the top of the green, I should set it to 26″ instead of 27.2.