Overrun control valve

A valve in a crossover pipe between the compressor suction and discharge sides of some turbocharger systems. When the manifold pressure drops, as during deceleration, the overrun control valve opens and allows compressed air to circulate through the compressor again; this maintains turbo speed on the overrun, minimizing turbo lag when the throttle is re-opened; not to be confused with a safety-relief valve in the compressor discharge line or with a wastegate valve