|
When you lift off the throttle, the throttle plate shuts but the engine still acts as an air pump creating suction (i.e a vacuum in the space between the back of the throttle plate and manifold). This vacuum opens the DV. Since the turbos are still spinning and creating boost, the open DVs allows the boost to recirculate through the intake system or in your case vent to atmosphere. If the DVs did not open, the boost would slam into the face of the closed throttle plate and cause a pressure wave to bounce back against the turbos' compressor wheels causing them to slow or even stop. This can seriously damage the turbos.
|