Manifold Pressure Gauge

The MAP gauge indicates the absolute pressure of the fuel/ air mixture between the throttle and the inlet valve and has a sealed capsule and a capsule that is exposed to manifold pressure. The MAP gauge is calibrated in inches of mercury where 29.92" Hg is the standard pressure at mean sea level and for an ascent, every 1000 ft the MAP drops by 1 inch.

In the case of a normally aspirated engine then the highest pressure is at sea level with the throttle fully open. In this condition then the indicated MAP will be less than ambient due to losses within the induction system. On a turbocharged engine then MAP above ambient is achieved.

If moisture forms in the MAP gauge line then it could result in erratic operation of the gauge, therefore a means to drain the moisture is required. This takes the form of a purge valve situated near the instrument inside the cockpit and with the engine running it is opened forcing air at atmospheric pressure through the instrument drain line into the engine thereby removing any moisture.

For a multi-engine aircraft then there is a purge valve for each engine. The purge valve(s) is operated during engine run up checks prior to take off. On engines fitted with a constant speed propeller it is used as an indication of power.