Stock boost controler with two pressure sources

[Deathmug]

My stock boost controller setup has a pressure source from the turbo and also from the intake.
Can anyone tell me why it's setup this way?

 

[zippyshoe]

The stock boost controller (aka boost control solenoid) has two ports on it - the lower port connects to a tee. One leg of the tee then runs to the turbo compressor housing which is the pressure source, while the other leg of the tee runs to the wastegate actuator. The stock boost controller is basically a bleeder-valve that allows a small portion of air from the pressure source to "bleed-off". This bled air is then routed out the upper port of the stock boost controller and fed back into your intake. Hence the upper port is not being used as a pressure source.

 

[Quasimondo]

That's the Boost Control Solenoid. Like previously said, the boost is bled off by that solenoid. By doing so, it delays the opening of the wastegate and allows the turbo to push more boost. The solenoid closes up and lowers boost as a way to control knock along with cutting back the ignition advance.

 

[wret]

Yeah, the designers were pretty anal about not letting any "counted" air escape. Even the small amount released by the bcs is fed back into the system after the MAS.