Manifold absolute pressure is defined as?

• A. The atmospheric pressure at sea level
• B. The pressure in the exhaust manifold
• C. The pressure of the cabin air
• D. The absolute pressure of the fuel/air mixture within the intake manifold

Answer: (D)

Manifold Absolute Pressure is the pressure inside the intake manifold where the air-fuel mixture is drawn into the cylinders, measured as an absolute pressure relative to a perfect vacuum. This pressure reflects how dense the incoming air is and is used by the engine computer to calculate fuel metering and performance. It isn’t the atmospheric pressure at sea level, nor the pressure in the exhaust manifold, nor cabin air pressure. In normal operation the reading changes with throttle position: it drops when the throttle is closed (creating a vacuum) and rises toward ambient as the throttle opens; in turbocharged engines it can exceed ambient pressure due to boost, but it remains the absolute pressure in the intake manifold.