A waste-gate also called a waste-gate is a pressure release system that is present on most turbocharged engines. Its purpose is to release excess pressure from the turbo when the throttle is closed quickly in boost conditions, extending its useful life and, in extreme cases, preventing damage to the compressor.[1]​.

According to its design, we can differentiate 2 types of discharge valve, recirculation valves and atmospheric discharge valves:

The wastegates were designed to prevent unwanted overpressures in the compressor, a phenomenon that occurred when releasing the throttle on a turbocharged engine that had no way to expel the accumulated excess air compression. When the throttle valve is closed on a turbocharged engine, with the turbine spinning at high speed, flow is reduced beyond the surge line of the compressor. At this point, the pressure differential (delta-P) across the compressor is reduced and leads to a collapse in flow and possibly even a reversal of flow and a collapse in the pressure chamber.[2] As the compressor continues to rotate at high speed, once the flow has reduced enough, the delta-P across the compressor begins to rise and flow is restored in the chamber. This continues until the chamber is once again pressurized and the flow begins to drop until the overpressure limit is again exceeded and the cycle repeats.[1]​[3]​ This unstable flow leads to the cyclic noise sometimes heard in high boost engines without a bypass valve. With a valve installed, flow is maintained by preventing the compressor from entering the overpressure cycle.

Repeated high speed cycling will cause cyclic torque on the compressor and can lead to increased stresses on the compressor bearings and impeller.[2]​.

A wastegate is connected to a vacuum hose that goes to the intake manifold, after the throttle plate. When the throttle is closed, the relative pressure in the manifold drops below atmospheric pressure and the pressure difference moves the wastegate piston. Excess pressure from the turbocharger is expelled to the atmosphere or recirculated in the compressor circuit itself, depending on the type of valve.