Velocity stacks

Air moving rapidly over a Velocity stack, creates a vacuum  inside the opening when the air is at a perpendicular angle to the length of the tube. 

Turbulence

At high speed, the rushing air tends to create a partial vacuum inside the tube.  The condition is counterproductive to air flow.  The faster you go, the worst it is.  Vacuum created by the engine is trying to coax air into the cylinders and the high-speed air flowing over the open end of the stack is pulling it out.

Reversion creates other problems.  Reversion refers to reversed airflow, or in simpler terms, it’s when air in the intake runner reverses direction for a split second.  The condition is caused when a burst of pressure escapes into the intake runner from the cylinder during valve overlap.

Reversion creates resonance shock waves inside the tubes, which exit the open end of the tube at various rates depending on engine speed.  It has also been proven that these shock waves interfere with each other when the stacks are in close proximity.

By putting a plenum over the velocity stacks, air entering the "filled space," is slowed, smoothed, and straightened.  The plenum then becomes an endless source of calm, clean air.  Shock waves dissipate within the confines of the plenum without interfering with the shock waves emitted from an adjacent stack.