Blow off valves/Diverter Valves

Dump valve

Dump valves are fitted to the engines of (usually older) turbo charged cars and sit between the turbo outlet and the throttle body. When transitioning from a boosted state to a closed throttle state (as in between shifts), due to inertia, the turbo continues to pressurize air, but the closed throttle prevents the compressed air from entering the engine. In this case the pressure exceeds the pre-set spring pressure in the dump valve and the excess pressure is bled off to atmosphere. Even with a dump valve the compressed air acts as a brake on the turbo (slowing it down), because the pressure on the backside of the turbo is at a higher pressure than on the front side (and the air actually wants to flow through the turbo backwards).

Blow off and Diverter valves

A blow off valve is a more elegant solution to this problem by allowing the turbo to "freewheel" when the throttle is closed (equalizing the pressure on both sides of the turbo). Unlike a dump valve a blow off valve can be used at multiple boost settings without reconfiguration. They work as follows; when the throttle closes it creates a vacuum in the inlet manifold, this sucks the diverter / blow off valve open and the excess pressure is released out of the system (and allowing the turbo to spin without load). When the throttle is opened again, the vacuum disappears from the inlet manifold, the diverter / blow off valve closes as it is sprung loaded, and the system pressurises again. The only difference between the two is blow off valves release the air out completely ('pssshht' noise) and diverter valves divert the pressurised air back into the system near the air filter somewhere (quieter 'psst' noise).

Blow off valves are sometimes incorrectly called dump valves because they serve a similar function, however they are very different solutions to the same problem.