What is a Plenum Chamber?
Turbochargers deliver air at a relatively constant rate, while cylinders demand it in a varying manner, as the valves open and as piston speed varies through the stroke. Simple direct ducting would give problems where the nearest cylinders received more airflow. The pulsating demand from the cylinders would also show problems of either pressure waves in the duct, or a shortage of inlet air towards the end of the inlet phase.
The solution is to provide a large-volume plenum chamber between the inlet and the cylinders. This has two benefits: it evens out the difference in path restriction between cylinders (distribution across space), secondly it provides a large-volume buffer against pressure changes (distribution over time).
Custom Billet Plenums
Extreme Creations have made bike specific plenum chambers to fit seamlessly into the host motorcycle. This model is for the Honda CB1000R, it also houses a set of secondary injectors to allow additional fuel to be added as required for more power.