Manifold Absolute Pressure sensor. Measures pressure in the intake manifold. Used by ECUs to calculate boost levels and fueling.
MAP sensor (Manifold Absolute Pressure sensor) is an engine sensor that measures the absolute pressure (pressure relative to a vacuum) in the intake manifold. At idle or light load, the manifold is in vacuum (typically -10 to -20 inHg below atmospheric pressure). At wide-open throttle on a turbocharged engine, the manifold can be at 25-30 PSI above atmospheric or higher. The MAP sensor reading tells the ECU how much air is in the manifold for fuel calculations.
MAP sensors are essential for forced induction tuning. Boost is essentially a positive MAP reading; the boost gauge is typically a MAP sensor display. The ECU uses MAP readings to determine fuel injection quantities and ignition timing. On naturally aspirated engines, MAP also indicates engine load (vacuum levels correlate with throttle position and load).
Aftermarket MAP sensors are common upgrades for boosted engines. Stock MAP sensors typically have limited range (often 1-2 bar boost capability). Aftermarket sensors with higher range (3-4 bar boost or more) are required for high-boost applications. The sensor itself is small and inexpensive; the major cost is typically wiring and engine management integration. Common manufacturers include AEM, Bosch, and various OE suppliers.