[ame="http://en.wikipedia.org/wiki/Velocity_stack"]Velocity stack - Wikipedia, the free encyclopedia[/ame]
Modern fuel injection systems with a plenum and single air inlet typically incorporate some sort of radiused entrance, designed to improve power, based on air flow increases. Power gains are usually at higher rpm.
In amateur and professional racing, aftermarket velocity stacks are often used, as rules allow, and gains in the order of 2% to 4% can be obtained when inlet radii and stack lengths are optimized for that engine.
OEM automotive manufacturers have many regulations to adhere to. One of these is noise.
A properly tuned intake tract produces a rather audible intake noise under high airflow conditions - that is commonly "untuned" to allow the vehicle to pass EPA and DOT noise regulations.
The length of the stack is known to have a direct effect on a particular engine's boosted power range.
"... and single air inlet typically incorporate some sort of radiused entrance, designed to improve power, based on air flow increases. Power gains are usually at higher rpm..."
Ah yes, we do this with Hovercraft, on the lift fan inlet and thrust duct. The rounded entry allows and 'welcomes' the extra air that is persuaded to join the stuff already heading into it in a straight line. (Simple explanation)
Fossicker
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