Velocity stack

A velocity stack is a generally cylindrical tube with a radiused inlet end device which is added onto the air entry location or locations of an engines intake system, carburetor or fuel injection. It can be attached to an airbox inlet or to each cylinder in an IR (individual runner). Used with or without an "airbox". It is designed to:

1. Allow smooth and even entry of air into the intake duct with the flow streams boundary layer adhering to the pipe walls.

2. Modify the dynamic tuning range of the intake tract by functioning as a small reverse megaphone which can extend the duration of pulses within the tract.

3. Alter the dynamic tuning speed (dynamic engine compression) if the overall length is changed thereby changing the tuned length to a lower or higher rpm.

Modified engines often have the original air box and associated ducting removed and velocity stacks are installed as aerodynamic replacements.

Modern fuel injection systems with a single air inlet typically incorporate some sort of tapered entrance, nullifying what gains might be had from a true velocity stack. However in professional motor racing where even fractions of a percent improvement count, this small improvement is welcome. The length of the stack is known to have a direct effect on a particular engine rpm range. As a result it’s not unusual to see a race engine with differing lengths of stacks to fine tune each cylinder. “Racy” in appearance, velocity stacks do not usually have a major impact on airflow or performance unless the stock horn is of an inferior design.

Beyond air flow improvement a velocity stack may be useful in containing "stand off" which may result from removing the original ducting. "Stand off" is fuel which is spit back out of the intake by pulsing action in the intake system. A velocity stack gives the engine a chance to re-inhale that fuel which can be helpful.

The acceleration of air flow into a duct is inherently a highly efficient process and the difference between even the crudest radius inlet, and the most aerodynamic shape possible is slight, amounting to no more than a few percent. The flow coefficient of a perfect entry would be 1 while the flow coefficient for a sharp edged entry would be .9. A difference of only 10% from the worst possible entrance to the best possible entrance. This does not mean that engine airflow would increase 10% as the inlet end is usually not the smallest part of the system. The actual overall gain would be much less.

One of the current issues (2008) affecting velocity stack design on IR (individual runner) engines is that what an engine wants, to improve power, is may be different in radius and taper from what radius and taper flows best on an air flow bench but these effects are close to unmeasurable.