Intake vacuum exists in the intake manifold as a result of the pumping action of the engine's pistons and the restriction created by the throttle valve.
Were it not for the throttle choking off the flow of air into the engine, there would be little if any vacuum in the intake manifold (like a diesel).
The downside of intake vacuum is that it creates pumping losses and reduces engine efficiency.
Were it not for the throttle choking off the flow of air into the engine, there would be little if any vacuum in the intake manifold (like a diesel).
The downside of intake vacuum is that it creates pumping losses and reduces engine efficiency.