Dump Valves are the ones which are fitted to the engines which are been turbo charged cars which is of the usual order and they allow to sit between the turbo outlet and the throttle body. The turbo while transferring from a stage of boosted state to a closed throttle state which would be in between the shifts due to the inertia the turbo would continues to pressurize the air but the closed throttle prevents the air which is been compressed from entering the engine. In this case the pressure which is been created exceeds the preset spring pressure in the dump valve and the excess pressure is been bled off to the atmosphere. The compressed air which is been dumped in the valve at times acts as a brake on the turbo which slows down the process and because of the pressure on the back side of the turbo which is been set at a high pressure in the dump valve and the excess pressure which is been created is been bled off to the atmosphere.
The pressure is been created on the back side of the turbo at a real high pressure rather than on a higher side on the front side, and the air which is been actually would flow through the backwards of the turbo.
A blow off valve is a solution which is more elegant one to this problem by allowing the turbo to the free wheel when the throttle of the engine is closed which equalizes the pressure on both sides of the turbo. It is unlike a dump valve a blow off valve can be used at a multiple boost the settings without any reconfiguration. The blow off valves are even sometimes in correctly which is called as dump valves because they serve a equivalent similar function but they are clearly and very different solutions to the same and exact problem.
Most of the turbo charged engines are fitted with a dump valve. This dump valve is been fixed to vent away the excessive turbo charger which will be a boost pressure when the throttle is completely closed. Some turbo chargers have their own integral unit also. The dump valve is the black plastic sauce pan which is a shaped object which will see you connected between the inner cooler and the air inlet hose to the turbo charger. There is a small hose leading from the top of the valve to the inlet plenim of the engine.
