I was really currious about what psi mine were opening at, so i took some videos. I figured they may help people, and also help explain how they work. I have a 90 S5 Vert BTW. Any ways, here you go. The first is the VDI, and the arm is located in the middle of the screen. the second is the aux port in the rear of the engine.

 

Oh I forgot to say dont pay attention to where the gauge starts at. It was dropped and thats why it reads like 5psi of vacuum when its not even hooked up.

 

Damn not many looks, Oh well i thought it was interesting, anyways, its here for people if they're curious...

 

hey about vdi and aux ports, these ports are operated by a solenoid. the solenoid is controlled by the ecu. if your woundering when they open then you might check when the solenoids open.