The general idea is that because a turbo is positioned before the throttle body, when you lift off to change gear/decellerate the throttle obviously closes, but the turbo is still spinning.
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The effect of a closed throttle will effectively stall the turbo, so a
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'dump' or
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'blow off valve'
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(same thing) releases any excess pressure created by a spinning turbo and a closed throttle, to enable the turbo to still spin or really to stop it having to cope with the stresses of stopping suddenly.
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Theory is that it helps spool up times on the turbo and reduces lag.
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There are two types, one is the vent to atmosphere whereby the BOV vents straight to atmoshere with the accompanying PTTTTTSSSSSSSSSCCHHH sound or the more common with car manufacturers recirculating BOV which allows the air to recirculate through the intake and thus relieves the pressure in a more civilised way
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(ie its quieter).
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So yes they do have a job to perform.
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How much of an actual effect they have is enough for quite a long debate.
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HTH