Hi guys, fred from Evolution racewerks was v kind to let me understand how the bov works and explained the observation as discussed earlier..
"The job of the BOV is to release boost pressure from the turbocharger. *When the throttle plate in the throttle body closes (when you lift the gas pedal), the turbocharger is still spinning and pushing pressure. *This pressure hits the closed throttle plate and the pressure has to go somewhere (because the turbocharger is still spinning). *Thus, that's why a diverter valve or BOV is required on a turbocharged car. *It's job is to release pressure when the throttle plate closes. *When you shift at WOT and change gears, you're usually shifting very quickly. *Thus, you either don't lift the gas pedal or you lift very quickly. *Because of this, the throttle plate either doesn't close during shifts or it closes and then opens so quickly that the BOV does not need to open. *That is why you do not hear the valve when you're drag racing."