At what point should manifold vacuum be at maximum in a gasoline fuel engine?

• A. Idle speed
• B. Cruising speed
• C. Wide open throttle
• D. Full load acceleration

Answer: (B)

Manifold vacuum is the dip in pressure inside the intake manifold as the engine draws air through the throttle. It reaches its highest value when the throttle is nearly closed, which happens at idle. At idle, only a small amount of air is needed, and the throttle plate restricts the flow, creating a strong vacuum in the manifold—the pressure drops well below atmospheric pressure.

When the vehicle is cruising, the throttle is partially open to maintain speed, so more air is drawn in and the manifold pressure rises toward atmospheric pressure, reducing the vacuum from its idle level. At wide-open throttle or during full load acceleration, the throttle is fully open, manifold pressure is near atmospheric, and the vacuum is much lower.

So, the maximum manifold vacuum occurs at idle speed.