Engine vacuum
Engine vacuum is the difference in air pressure between the inside of the intake manifold and the outside air. The pressure difference creates suction and helps draw air into the engine.
Engine vacuum is a necessary condition for the operation of a gasoline engine. (Note: a diesel engine works differently.) This helps control the engine RPM. Without proper vacuum, the engine will not start. A vacuum leak can also cause high idle and runaway revs.
How is it measured?
Vacuum is measured in inches of mercury (Hg).
A difference in air pressure of 1 psi moves the mercury column by 2.036254 inches. If your vacuum gauge reads 20 in. Hg, the manifold pressure is about 10 psi below atmospheric pressure.
What creates a vacuum?
The motor creates a vacuum under the following conditions:
- The piston moves down on the intake stroke.
- The intake valve is open.
- The outlet valve is closed.
- The throttle blades are closed (or slightly open).
How does this affect performance?
Vacuum is actually a measure of engine inefficiency. A higher vacuum indicates that the engine is working harder to breathe. This limits performance. However, it is necessary. Restricting air flow and creating a vacuum helps slow the engine and limit revs.
The vacuum created by the piston draws air from the outside, through the inlet, into the cylinder. Throttle blades create restrictions. This reduces the amount of air/fuel going into the cylinder. If there is less air/fuel, the engine cannot rev as fast.
As the throttle blades open, the restrictions and vacuum are reduced. This lets more air/fuel into the cylinder and revs increase. When the throttle is open, airflow is maximized and vacuum is minimized.
When you let off the throttle and the blades click, the rpm is still high. This creates a spike in the vacuum. The restriction creates an air/fuel shortage in the cylinder. The result is a drop in RPM and vehicle deceleration.
The table below shows the vacuum ranges for most engines.
| Engine function | Vacuum range |
| Startup | 3-5 inches Hg |
| Idling | 14-20 inches Hg |
| Driving | 10-15 inches Hg |
| Slowdown | 21-25 inches Hg |
Other vehicle systems also use engine vacuum. This may include:
Brake boosters
High voltage start
Crankcase ventilation
Etc.
The data shown here, especially the complete database of car spare parts, may not be copied. It is strictly prohibited to duplicate the data and database and distribute the same, and/or instruct third parties to engage in such activities, without prior consent from TecAlliance. Any use of content in a manner not expressly authorized constitutes copyright infringement and violators will be prosecuted.