Velocity stack

A velocity stack, or air horn, is a trumpet-shaped device of differing lengths which is fitted to the air entry of an engine's intake system, carburetor or fuel injection. It is designed to:

Modern engines universally have tuned intake tract volumes and associated resonance frequencies, designed to provide higher than atmospheric intake air pressure while the intake valves are open - increasing the density of the trapped air in the combustion chamber (higher compression). Modified engines often have the original air box and associated ducting removed and velocity stacks are installed as accessories.