according to wiki :
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.
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.