BINGO exactly, there are dual stage intake plenums in the H23 at the IM (intake manifold) and also at the resonator box, once the engine gets up to around 4500 RPM, they are opened by a vaccuum line.

Another good use for the H23, is autox: the torque lower in the rev band allows the car to pull out of tight corners. On tighter courses, it is an advantage over higher revving engines.