Abstract: An apparatus for improving engine performance at wide-open throttle or full load by supercharging the induction system. An expanded/enlarged volume plenum is divided into two chambers. each containing a group of three openings for flowing air past inlet valves to three cylinders of a six cylinder engine, for example. A pair of adjustable length intake runners are connected to the upstream end of the plenum and are adjusted with changes in engine speed to resonantly tune the induction system for the speed changes to effect greater input of air to the cylinders. A valve can be opened to communicate the chambers with each other to further resonantly tune the system at intermediate speeds. At the higher engine speeds, resonant tuning in conjunction with the ram induction of air into the cylinders further increases the mass flow of air.