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 preset 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).
A blowoff 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.
Blow off valves are sometimes incorrectly called dump
valves.
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