What is the valve on the inlet to the intake manifold on a 1981 Mazda RX7 It is operated by a vacumm module and is on the rear primary?
not enirely sure this is what you mean but i think you meen the 5th and 6th intake ports.
only NA Rx7's have them, all turbo version have 4 intake ports.
NA 13b engines have these ports so that the engine can take advantage of a smaller intake tract at lower rpm creating torque however later on the 5th and 6th ports are opened by actuators to release high rpm horse power. Without the 5th and 6th ports the engine would be sluggish when rpm is fairly low.
These 5th and 6th ports operate in a similar functionality to the V-tec engine of Honda civics, the 13b engine like the V-tec can change the internal dimension of the intake tract for its own benefit to get the best of both worlds, only v-tec engines do this with exhaust ports as well but that's beyond the scope of this reply.
Turbo 13b engines have no need for 6 ports, due to air being compressed 4 ports is quite sufficient to get the air into the housings.