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A velocity stack, trumpet, or air horn[1] is a typically flared, parallel-sided tubular device fitted individually or in groupings to the entry of an engine's air intake system to smooth high speed airflow, and allow engine intake track tuning to incorporate pressure pulses created by its internal components.
Velocity stacks can be of differing lengths and fitted to both carbureted and fuel injected engines. They are not to be confused with the similarly shaped sound-producing pneumatic air horn.