What Is The Secret Importance Of An Intake Airbox?

Описание: Think of a motorcycle airbox as a glorified beer bottle. We’ve all blown across the mouth of a bottle and heard the “whoooo” of bottle resonance. If the volume of the airbox and the mass of the air in the intakes are correctly chosen, the box will hum like a beer bottle—and boost engine torque.

If you have ever had the gas tank off your late-model sportbike, you will notice that the front of the fuel tank doesn’t hold fuel; it holds an airbox. In the old days, when you bought a new bike, it had an air-filter case attached to feed the carburetors or the fuel-injection intakes. All the sharp, young guys would immediately rip off the filter case and replace it with four sock filters. Reduced airflow resistance. Much better performance.

One day in the late 1980s, they began to rip off the airboxes of their new bikes and their engines fell on their faces. They lost a bunch of performance. “This can’t be happening! Putting on sock filters always worked before.” But it turns out the industry found a way to boost performance by making what is known as a resonant airbox.

We have all in an idle moment blown across the mouth of a beer bottle and heard the "whoooo" of the bottle resonance. As air goes across the mouth of the bottle, it creates a low pressure, which causes air to flow up. That deflects the air away from the mouth of the bottle. Then the air goes back in, the airflow from your mouth goes back across, and the cycle repeats, rapidly fluttering and producing that deep tone. The compressible air in the bottle is acting as a spring, and the slug of air in the neck of the bottle is the mass that vibrates against that spring.

This intake airbox from a Honda CBR600RR is just a glorified beer bottle. Instead of the engine blowing across the mouth of it, its four throttle bodies are sucking from the box, pulling its pressure down. Air rushes in through the ducts in the fairing to fill up that low pressure. The next cylinder sucks the bottle pressure down and more air rushes in and restores the pressure. If the volume of the box and the mass of the air in the intake pipes are correctly chosen, the box will hum like the beer bottle.

The trick is to get your engine to draw air from the box when the pressure is up and then the box refills when the pressure is down. And that is why ripping the airboxes off and putting on old-time sock filters resulted in a reduction in performance. In a specific zone of rpm, a resonant airbox can boost your engine’s torque by 10 percent. That’s worth having!

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