I will try again. The gauges do the same thing. They measure the pressure in the inlet manifold and convey the result to the cockpit in either lbs per square inch or inches of mercury. With everything stationary both instruments will read the ambient pressure.
The checks for both are the same. You wind your altimeter to Zero and if the subscale is lower than standard 1013.2 then the boost or MAP gauge will read either either below Zero or below 29.92 ins. With an inches subscale they will read the same.
The ratio of boost to MAP is 2.036, or 2 for short; so for every pound of boost pushed into the engine a MAP gauge would increase by 2 ins and when the engine goes into suction mode, vice versa.
An old check on the Hercules used to be a climb at Zero boost and see what height the supercharger coudn't maintain boost any more.
Last edited by Fareastdriver; 20th Feb 2012 at 18:45.