RX-8 Variable Dynamic Intake Geometry System

Like most modern engines, the RX-8 has valves inside of the intake manifold to vary geometry to maximize torque and  power throughout the rev range. In other engines these might be referred to as intake manifold runner controls (IMRC) like the system Ford uses. 

In the RX-8's case, it has 3 valve sets. VDI, SSV, and APV. 

Towards the top of the engine there's the VDI valve (Variable Dynamic Intake). This is the last of the valves to open, on the 6 ports 7250, and 4 ports 5750. This valve is vacuum operated with a solenoid behind the intake manifold which gets a signal from the ECU. This one is the most crucial for maximizing your torque when you change your intake for an aftermarket one, or port your motor. Without tuning it you may feel a kick of power or a jolt at the opening rpm I call Rotary-VTEC. this jolt means that you can open that valve sooner to gain the power. or vice versa, with some setups you can hold it closed longer to keep the torque higher. This is best done on the dyno where you can see spikes, but can still be done with e-tuning. 

The SSV valve (secondary shutter valve) is very similar to the VDI valve, but is the first to open. This one opens at 3250rpm. These tend to get stuck with carbon being down so far in the intake manifold (located behind the thermostat housing) and can get stuck open or closed causing an unstable idle, lack of power, and an engine code. To control this valve its a bit different, instead of one set number, its a table that modifies its opening rpm from 3250 per engine coolant temperature increments. Not much work is usually needed but can be done depending on certain modifications. This valve operates identically with vacuum to the VDI valve.

Finally, there's the APV valves (6 port ONLY). These operate much different to the aforementioned ones. Two electronic motors move their own respective valves to the auxiliary ports (outside 2 of the 6 intake ports). Under 6100rpm the motor functions like a 4 port (only with some more intake turbulence which is why you see some people prefer 4 ports for turbocharging). At that RPM one electric motor opens the valve through a gear until 6250rpm when the other motor opens the last channel to the auxiliary port.  This valve can have the same effects as the VDI valve when intake modifications change the airflow which requires tuning. As mentioned these are fully electronically controlled so no vacuum solenoids to worry about, and are fully tuneable.

Also, there's the VFAD (variable fresh air duct) system which is the valves and vacuum lines you see running to your airbox and in front of it. This is for noise abatement, like the intake resonator (large piece of plastic on the rubber intake tube) and I usually don't bother touching it unless the user removes the system for simplicity, which I set the RPM to 10,000rpm to avoid engine codes. Alternatively, you can keep parts of the system to make other valves/devices open at a certain rpm for custom purposes whatever they may be.