Which factor directly influences manifold pressure?

Manifold pressure is primarily influenced by the throttle opening in an engine's induction system. The throttle controls the amount of air-fuel mixture that enters the engine's cylinders. When the throttle is opened wider, more air enters the manifold, which increases the manifold pressure, assuming that the engine's speed and conditions are stable. Conversely, if the throttle is closed, the manifold pressure decreases due to the restriction on airflow.

While ambient temperature, fuel type, and air density do play roles in engine performance and efficiency, they do not directly control manifold pressure in the same way that the throttle opening does. Ambient temperature affects air density, which in turn can influence engine performance, but the immediate factor altering manifold pressure is the position of the throttle. Thus, understanding the relationship between throttle position and manifold pressure is crucial for managing an engine’s power output effectively.