I just took an N/A plenum apart today. Those are the longer runners in mitsu's variable runner length design. It's pretty cool how it works. The physics behind it is even cooler - namely the organ pipe theory. When the intake valve opens, it releases a negative pressure pulse (i.e. sound wave) up the runner. When the wave hits the top of the runner, most of it continues into the plenum but a certain % of it is reflected back down the runner as the inverse - a positive pressure wave. When the engine is at the right RPM, that positive pressure wave hits the intake valve right when it closes, thus forcing a little extra air into the cylinder and improving volumetric efficiency (POWER).
The reason for the dual lengths is that at lower RPMs, there's more of a lapse between intake valve openings, therefore the longer runners allow the stronger first or second pulses to synch up ... whereas at higher RPMs, the valves within the N/A plenum open, cutting down the runner length considerably, and working with the shorter lapses between intake valve openings.
Pretty cool huh??
