Acoustic Control Induction System (ACIS): 4Runner
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The Acoustic Control Induction System (ACIS) is realized by using a bulkhead to divide the intake manifold into 2 stages, with an intake air control valve in the bulkhead being opened and closed to vary the effective length of the intake manifold in accordance with the engine speed and throttle valve opening angle. See Fig 1. This increases the power output in all ranges from low to high speed.
For components and construction of ACIS, see Fig 2.
For description of operation, see Fig 3.