CIS Air Box Issues
 

I'm aware of the 911SC air box issue and the aftermarket pop-off valve as a precautionary measure, but did that just start in MY 1978 with the 911SC, or was it also a problem in the previous years CIS-equipped 911's?

It was a problem from the beginning of CIS.
Bruce

And about that pop off aka toilet seat valve, read this

http://forums.pelicanparts.com/porsche-911-technical-forum/976754-pop-off-not-pop-off.html#post9808920

The Airbox was first used on the 1973.5T until 1983SC. If you do not have a pop-off valve on your Airbox I HIGHLY recommend you install one, because sooner or later, your going to hear a loud "explosion" and find your Airbox, torn into pieces or cracked beyond repair and your car stuck on the side of the road!!!!! Cheap to buy and install and one of the best insurance actions you can take. While your at it with your 1978SC change to hydraulic chain tensioners. Peace of mind for the SC's.

Bob
1973.5T