B18C1 (GS-R) motor has a dual stage manifold, basically two paths for air to go through. By changing over paths at around 5700 rpm, you keep the right intake manifold for low rpm operation and open it up for high rpm operation. It's done by a vacuum operated diaphram controlled by the ECU. If your installing a B16A, you don't need to concern yourself with it, they use a different single path manifold.