Vacuum (Engine Manifold)

The difference in air pressure between the engine’s intake manifold and Earth’s atmosphere. A pressure differential is created by piston induction strokes and the pressure drop through the intake manifold’s throttle valve. Engine power is controlled by throttling the air flow in the intake manifold, which in turn reduces the air density and manifold pressure. When there is light engine load, the throttle valve will be fully or nearly closed, which will result in a higher manifold vacuum and a lower air density. Conversely, when the engine is fully loaded, the throttle will be completely open, which will result in a lower manifold vacuum, i.e. higher absolute pressure, and a higher inlet air density.