Originally posted by Diablo
I reckon that it might be worth a couple of hundred rpm if adjusted for proper tuned length. A velocity stack smooths the flow of air entering the carb and can also provide a stronger "sonic" ram effect. Not practical on engines with the carb sticking out the side of the cylinder, but fine for the rear carb motors. I calculate a 7 inch length intake track should tune it for a boost at 6500 rpm.
Some of the weedie conversion people are putting velocity stacks on the side-carbed engines (e.g. Homelite), supposedly to minimized the pressure reduction caused by the venturi effect of the incoming airstream.
http://www.carrprecision.com/Pages/prod02.htm
I don't think these should be called "velocity stacks" since they don't function per the namesake. Intake tuning is akin to exhaust tuning. The difference is that whereas exhaust is tuning for negative pressure to optimize scavenging, intake is tuning for positive pressure to maximize intake charge filling. These days, most auto engines are using a resonant intake chamber to achieve the same goal but with a wider powerband. This is why those who replace the OEM airbox in their cars with a supposedly less restrictive cone filter often end up with a net power loss.