The turbo-based intake boost system needs some means of dissipating unnecessary boot pressure (such as when the throttle body is closed, like during shifts or deceleration); the boost needs an escape route. If the boost cannot escape during these times, it can back up and cause air-pressure shocks to the turbo.
The difference between the OEM diverter-valve (DV) and a blow-off valve (BOV) is that a Blow-Off Valve (BOV) performs the same task as a DV, but instead of returning the boosted air back into the intake, it vents it out of the intake system to the atmosphere.
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