Manifold Absolute Pressure Sensor Information

MAP, or Manifold Absolute Pressure sensors have two responsibilities within the engine management system: to measure barometric pressure at key on, and report engine load to the powertrain control module. The powertrain control module uses the information for proper air/fuel mixture control under all operating loads and altitude conditions.

A common addition to boost horsepower on today's engines is a turbocharger. Turbochargers contain two turbines; one is driven by the engine’s exhaust flow, the other moves ambient air from an external source into the engine’s air intake system. The task of the turbocharger is to force more air into the combustion chamber than the engine can ingest on its own. When you pack additional air into the combustion chamber, the engine will also require more fuel. When you have the combination of more air and fuel in the combustion chamber, the end result is more power. A system such as this allows a vehicle to have a more fuel-efficient engine yet create an abundance of torque when the engine is pressed into a higher RPM range.

Fuel control on modern engines is performed by the computer system. The computer analyzes the sensor inputs and then calculates the amount of fuel that is needed for the engine to maintain an air/fuel ratio of 14.7:1, which is the stoichiometric value, or the most efficient mixture of air and fuel. Because the turbocharger is changing the engine's calculated air intake value by pumping air into the engine, the engine load value must be measured differently. A sensor that is capable of adapting to both positive and negative pressures within an engine is required when utilizing a turbocharger. Manufacturers will use sensors that can read these pressures at a barometric pressure multiplied by 1, 2 or 3.

A 1-BAR MAP sensor reports barometric pressure times 1, which is approximately 14.7 psi or 30in/Hg (inches of mercury). As an example, a typical three-wire MAP sensor has 5 volts and ground to power the circuit. The signal circuit, or return voltage to the computer, is about 4.5 volts at barometric pressure of 14.7 psi; this measurement is taken at key on/engine off. The sensor can withstand about 1.5 BAR of pressure, but the return voltage will peak out at about 4.8 volts. This type of sensor is designed for a normally aspirated engine to read a negative pressure, or vacuum, at a typical barometric pressure.

2 BAR or 3 BAR simply mean two or three times barometric pressure. The sensor will measure each direction, vacuum or pressure.

For reference:
1 BAR is for a normally aspirated engine
2 BAR will handle forced induction up to 14.5 psi of boost
3 BAR will handle forced induction up to 29.0 psi of boost

Return voltage on a 2- or 3-BAR sensor at key on/engine off will be approximately 2.5 volts @ 30in/Hg pressure depending on the sensor calibration. The engine vacuum (or pressure if it's in boost) will send the voltage either above or below 2.5 volts.

A vehicle's computer is calibrated to function at a specific level as programmed by the design engineer. A 2- or 3-BAR MAP sensor placed in a system designed for a 1-BAR sensor will not function correctly; in fact the system will run very rich because of the low return voltage signal. A programmable computer is a must for any tuner attempting to boost an engine's output with the addition of a turbocharger and a 2- or 3-BAR MAP sensor.

Manifold Absolute Pressure Sensors