Velocity stack
A velocity stack is a trumpet-shaped device which is added on to the air entry of an engine's fuel system, carburetor or fuel injection. It is designed to:
- Allow smooth and even entry of air into the intake duct with the flow stream adhering to the pipe walls.
- Modify the dynamic tuning range of the intake tract slightly by functioning as a small reverse megaphone which can extend the duration of pulses within the tract.
- Alter the dynamic tuning speed if it includes extra length thereby changing the tuned length to a lower rpm.[1]
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 is 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 in some cases.
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.[2] The flow coefficient of a perfect entry would be 1.0 while the coefficient for a sharp edged entry would be 0.6 and a re-entrant plain pipe 0.5.[3] In practice these latter types of entry are never used for engine intakes. There is always some attempt to provide some radius at the entry. This means that total engine airflow would not increase by the amount suggested by these figures, which apply only to the entry alone, as the inlet end is never the smallest or most restrictive part of the system. Because the greatest losses to flow occur near the valve seat, actual overall gain from any improvement of the entry flow would be much less.
References
- ^ Design and Simulation of Two-Stroke Engines II -Dr. Gordon Blair
- ^ DESIGN OF AN INTAKE BELLMOUTH -Dr. G Blair http://www.profblairandassociates.com/pdfs/Bellmouth.zip
- ^ Internal Flow Systems By Donald S. Miller