What Is a Plenum on an Intake Manifold?

The intake manifold’s primary function is to transfer the air and fuel combustion mixture to the intake ports contained in each cylinder head. This distribution optimizes engine performance and efficiency. The intake manifold must contain higher pressure than the outlet, which is provided by the cylinders during the intake stroke. This higher pressure is produced by an air enclosure, or chamber, called the plenum.

Intake manifolds contain runners, or tubes that extend to the cylinder head intake ports from the plenum. The runners take up a smaller section of the plenum surface than the inlet, thereby aerodynamically supplying air to the plenum.

Intake manifold runners take advantage of the Helmholtz Resonance phenomenon, which results in air resonance in a cavity, such as the plenum. When a valve closes, air outside of the valve compresses against it, created a pocket of high pressure. This pressure equalizes with lower pressure in the plenum, which creates cycles, or pulses, of oscillation. This enables the plenum to operate at a high volumetric efficiency when equalizing air flow to the engine cylinders.