Variable Dynamic Induction System (VDI)

Mazda RX-7 engines not equipped with a turbocharger have a special intake manifold assembly designed to improve engine performance. Mazda calls this system the "VARIABLE DYNAMIC EFFECT INTAKE SYSTEM" (VDI for short). The system is designed to take advantage of oscillations inside the intake manifold, created by the opening and closing of the intake ports, to improve volumetric efficiency especially at high engine speeds.

VDI System Manifold Assembly:

The intake manifold is constructed in three sections. The lower section, simply referred to as the intake manifold, feeds the intake air charge into the rotor housings. The middle section, called the "EXTEN" manifold (probably short for "extension" manifold), is where the dynamic effect takes place. The upper section, referred to as the dynamic chamber or common chamber, is where the intake air enters the manifold assembly from the throttle body before being separated into the individual manifold passages.

Variable Dynamic Effect Intake (VDI) System:

When one rotor has completed its intake phase, air in the intake runners still has velocity and momentum toward the ports, which are now closed by the rotor. The air compresses at the closed ports, and then begins to rebound, creating a pressure "wave" that travels back through the intake runners. The exten manifold contains a valve (VDI valve) between the intake runners of each rotor housing. When the valve is closed (at low engine speeds), the pressure wave from one rotor travels a long path all the way back to the dynamic chamber, and has little or no effect on the air in the intake runners for the other rotor. At high engine speeds (above 3,850 rpm) the VDI valve opens, creating a short path between the intake runners for each rotor housing. With the valve open the pressure waves, created when one rotor closes its intake ports, travel towards the open intake ports of the other rotor housing, compressing the air along the way. This helps "pack" the other rotor housing with a denser air/fuel charge. The "dynamic effect" reciprocates back and forth between intake runners for each intake phase.

VDI Valve:

VDI Valve Actuator:

The VDI valve is operated by a pressure actuated diaphragm mounted on the end plate of the valve. Air pressure is supplied to the diaphragm by the air pump. The air supply to the actuator is regulated by a computer controlled solenoid valve, so that no pressure is applied until the engine speed excedes 3,850 rpm.