A dump valve is found on high-performance turbo charged engines to reduce lag during gear changing and other times when the engine revs may drop briefly, and then be required to pick up again quickly.
When driving a turbo charged car, there is often a lag between pressing the gas pedal, and the turbo kicking in for full boost - this is due to the fact that the turbo charger is usually exhaust-driven, and therefore doesn't start spinning at full speed until the engine has already begun to rev higher. Furthermore, the turbine has inertia which causes it to pick up speed relatively slowly.
On a constant run, the turbo spins at a pretty constant rate, and is effective. When the throttle is closed, a problem occurs. The spinning turbo pushes air into the carburetor's air inlet, but the airway is blocked by the throttle butterfly. This causes a build-up of pressure between the turbo and the carb, which will stall the turbo - losing all of its inertia.
When the throttle is opened up again, the lag is apparant, as the turbo has to spin up to speed again. This will happen every time the gears are changed, every time the throttle is released then pressed again.
The dump valve exists to vent pressure from the pipe between the turbo and the carb. It remains closed during normal driving, but when the throttle is suddenly closed, the valve is opened by the vacuum between the carb and the inlet manifold. The pressurised air from the turbo is dumped to the atmosphere - with a very characteristic noise.
If you don't know the noise, imagine all the air from a set of air brakes on a truck being released in a third of the time... that gets close. It's a sharp burst of air, slower than an explosion, but not by much. The easiest way to experience it, is to get down to your local cruise, where people who have dump valves will be driving in an atrocious stop-start manner to make them go off!
The advantage of this setup is that the turbo will no longer experience back-pressure, and its inertia will keep it spinning. When the throttle is reopened, the turbo will already be spinning, and little or no lag will be apparant.
Installation and Maintenance
The dump valve is a simple item to source and fit to a vehicle, although the type of valve should be matched carefully to the boost characteristics of the turbocharger. Apart from occasional dismantling and cleaning, very little maintenance is required.