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motocrossactionmag.com

ASK THE MXPERTS: WHAT IS A VELOCITY STACK?

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MXPERTSvelocitystack2These are classic velocity stacks, often called flutes or trumpets.

Dear MXA,
I am always reading in MXA about how this bike has a velocity stack and this one doesn’t, and I don’t have a clue what it means. Can you explain what a velocity stack is and how it works?

MXPERTSvelocitystack2010YZ250FIn motocross, the velocity stack has to be build out of rubber between the carb/throttle body and airbox. This is a good example of the proper shape from a 2010 YZ250F.

Velocity stacks have been around for decades, although they reached their zenith during the 1960s in Formula 1. Think of a velocity stack as the flute of a trumpet. It attaches to the intake side of the carb or throttle body and maximizes Bernoulli’s principle to increase air speed (velocity) through the venturi. Originally derived to explain how fluid moves, Bernoulli’s principle says that as a tube narrows, the air (or water) in it must speed up to keep the same amount of fluid traveling through a smaller space. Think about what happens when you put your thumb over the end of a water hose. The water sprays with more force. The same concept can be used to force-feed air into a motorcycle engine. Most motocross bikes, although not all, have incorporated a velocity stack into their air-boot design. It has a larger diameter where it connects to the airbox and gets smaller as it approaches the throttle body.

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