A MAP sensor (manifold absolute pressure) is one of the sensors used in an internal combustion engine's electronic control system. The manifold absolute pressure measurement is critical to an engine's electronic control unit (ECU) in order to calculate fuel and spark requirements. Not all engines use the speed-density airflow determination method, so not all engines need a MAP sensor. A MAP sensor is almost exclusively used in conjunction with fuel injection.
Manifold absolute pressure is an accurate indicator of the mass of fuel an engine requires, vacuum is only a differential to some other pressure. Carburetors relied significantly on vacuum, and although they were precise, they were not accurate fuel metering systems, and therefore unable to satisfy modern automobile emissions control legislation. Accurate and precise air mass measurement is required to meter fuel mass in a stoichiometric manner to achieve acceptable emission control under all operating conditions. A MAP sensor is a component used in the speed-density method of accurately determining the engine's air mass flow rate.
The MAP sensor measures the "absolute pressure" (not manifold vacuum) in the engine's intake manifold. The mass of air entering the engine is directly proportional to its density. Density is directly proportional to the air's absolute pressure, and inversely proportional to the air's absolute temperature.
(Engine Mass Airflow Rate) ≈ (Air Density) × RPM
- For a constant temperature
(Engine Mass Airflow Rate) ≈ MAP × RPM
- Example: An engine that is operating at wide-open throttle on top of a very high mountain has a MAP of about 15" Hg (inches of mercury - approximately equal to the barometer). The same engine at sea level achieves 15" of MAP at approximately half-throttle. The engine requires the same mass of fuel in both instances because the same mass of air is entering the engine at both altitudes. In this comparison, the absolute air pressures are the same, the volume of the intake manifold is the same, and provided the temperature is the same - the densities are the same.
Obviously the engine will develop more output at sea level if the throttle is opened all the way because the MAP will rise to equal the sea level barometer's higher absolute pressure, increasing the density, which increases the mass, and consequently more fuel can be burned.
The MAP sensor can also be used to measure the barometric absolute pressure (BAP sensor) when the engine is not turning, i.e., when the intake manifold is not being evacuated by the engine's pumping action.
When the engine is running, the difference between the BAP and the MAP is the engine's manifold vacuum. As atmospheric pressure decreases with increasing altitude, so must vacuum to maintain the same MAP.
BAP = MAP + Vacuum
(See Earth's atmosphere.)