A dump valve, also known as a blowoff valve or bypass valve, is a modification system that releases pressure build-up in turbo charged engines, such as Ford Fiesta Mk3 The main purpose of the dump valve is to prevent compressor surge and protect the turbocharger and engine from excess wear.
As the throttle plate closes the pressure builds up between the engine and turbo, and because the turbo is still spinning it is continuously pumping air into the pipes, even though it’s not needed. This creates a vacuum on the other side of the throttle plate. The dump valve is connected between the engine and turbo, which opens and closes allowing air to be vented from the pipe.
The top part of the dump valve is connected to the engine intake manifold via a silicone hose. The differential pressure on either side of the dump valve means it will open and close when needed. Without this venting the trapped air will force its way back past the turbo causing it to stall. Over time, this stalling will wear the bearings and seals and cause a juddering movement in the main turbo shaft. With dump valves by brands like Forge from LK Performance you can protect your engine and turbo from premature wear and tear.