Variable Inertia Charging System (Protege & Protege5 2.0L)
WARNING: This page does not describe the selected car, but rather 10 other vehicles, including the 2002 Mazda Tribute, 2002 Mazda Protege5, 2002 Mazda Protege, 2002 Mazda Millenia, and 2002 Mazda MX-5 Miata. However, it is still accessible from the selected car via links, so may be relevant.
Shutter valves in the intake manifold control length of intake air path, depending upon engine RPM. With engine speed below 4750 RPM, the shutter valves close, improving torque output. At more than 4750 RPM, the shutter valves open, increasing airflow and improving high RPM performance.