A velocity stack is used to smooth the flow of air into the carb's throat and help tune the intake tract length to the desired power band.
Intake runner length can be used to tune an engine based on pressure waves or sound. The shorter the runner length, the higher the engine rpm in which the tuning effect will take place. Runner length is measured from the carb entrance to the valve seat. The length of the intake manifold, carb spacer, carb body and velocity stack or radius ring will vary runner length and change the point of peak horsepower and peak torque. Knowledgeable engine builders use intake tract components as "tuning" tools to shift the engine's power band either lower or higher, help diminish reversion pulses, stabilize the air/fuel mixture, and improve the combustion process.
In this Motor Series installment we covered the induction system for carbureted engines. Next month we will conclude the induction system with a discussion about electronic fuel injected engines.