Velocity stack

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A velocity stack is a trumpet shaped device which is added on to the air entry of an engines fuel system, carburetor or fuel injection. It is designed to:

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

2. Slightly 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 if it includes extra length thereby changing the tuned length to a lower rpm.

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

Although “racy” in appearance, velocity stacks do not usually have a major impact on airflow or performance. Most fuel systems already incorporate some kind of tapered entrance, nullifying what gains might be had from a velocity stack.

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.