VICS
The FE-DOHC featured Mazda's VICS system, short for Variable Inertia Control System, a variable intake setup to optimize runner length and resonance at different engine speeds. Much like Toyota's Acoustic Control Induction System, it had two sets of intake runners, a long set for low-medium RPMs, and a short set for high RPMs. It was operated by a vacuum solenoid based on the engine's current speed, actuating a pair of butterflies inside the manifold to open or close the short runners past 5 400 RPM. This system has been used on many Mazda engines since including the BP. The K-series V6 engines used a different principle to the same effect dubbed VRIS.
Read more about this topic: Mazda FE-DOHC Engine