Abstract

A variable cam timing phaser for adjusting phase between a first shaft and a second shaft using oil pressure from an oil pressure source including a housing assembly and a rotor assembly together defining a plurality of segments. The segments include at least one operating segment including an advance chamber and a retard chamber, the advance chamber and the retard chamber being oppositely switchable between at least a source of oil pressure and a drain, the vane being movable by oil pressure from the oil source applied to either the advance chamber or the retard chamber with the other of the advance chamber and the retard chamber being coupled to the drain and at least one assist segment including an assist chamber and a vent chamber, the vent chamber being vented to atmosphere; such that oil supplied to the assist chamber assists the motion of the vane in a direction.