[zuffenhausen]

The large casting above the throttle body is actually the pop-off valve. It bleeds excess charge pressure to the intake side of the turbo. I'm don't remember the plug you're referring to when I did the top-end rebuild on a '78 930, but I don't doubt that it's there. My question is: Why do you want to tap into that for your boost gauge when the factory already provides a boost gauge and a tap for it? Also, there are several hoses that I recall going to this casting and you could easily Y into one of those.

-zuff