I own Mazda RX-7s (three of them at this point in time). It was the FSM (Factory Service Manual) which referred to the plastic spacer as the plenum. I only bring it up, because the RX-7, starting in 1989 did indeed have a VDI (Variable Dynamic Intake), or two paths through the intake manifold. A longer path for the lower RPM range, where the inertia of the air mass would help charge the combustion chamber. Actually to the point where it could reach slightly higher than atmospheric pressure. Then at higher RPMs, 5500+, it would switch to the shorter path, because at this point the engine is gulping down so much air the longer path is a restriction.
Mazda did the same for the exhaust ports starting with the GSL-SE (a Special Edition) of the 1985 RX-7. A rotary engine behaves more like a 2-stroke than a 4-stoke. Changing the position of a port changes the engine's timing. At higher RPMs, 3600+, auxiliary exhaust ports open, extending the port timing slightly to help purge the chamber which is only open for a very short period of time. This extended time causes over-lap between the exhaust and intake, while adding power, allows some unburned mixture to exit the tail pipe. The aux ports were not used on the turbo models (no sense in having the pressurized intake blowing straight out the exhaust), nor the RX-8, which had to meet stricter smog specs.
