Difference between 77 & SC ventilation?

[frankc]

Some years ago I removed the AC from my 1977 911S. As you probably know, starting in 77 the ventilation system on 911's was changed to include a center dash vent. Cars with AC were plumbed such that the AC air was dedicated to only the center and 2 side/door window dash vents (that is, these vents could not be used for fresh/heated air) rather than using the older under-dash mounted vents.

For cars w/o AC, the center & side dash mounted vents can (obviously) be used for fresh air. In fact, from looking at the parts diagram, it appears that the center vent can only output fresh air from the blower, while the side vents can output heated air if so mixed in via the controls.

Anyway, I would like to change the plumbing on my car to that of a non-AC car so that the center & side vents can once again be functional. Looking at the parts CD, I will need to change the "control boxes" (devices that mix the hot/fresh air) along with some other plumbing. These need to change because the non-AC versions have the output for the side vents attached directly to them, and the AC versions do not. Actually, it's just the plastic piece on top that needs to change, but this is not a separately orderable part. The same non-AC control boxes are used from 74-77.

Given that the control boxes are a bit pricey, I figured I would try to pick-up a used pair from a 74-77 or SC, however I noticed something odd. The part numbers for the non-AC control boxes for a 74-77 and SC are not the same as show below:

74-77(L): 911 571 049 01
74-77(R): 911 571 050 01

78-83(L): 911 571 049 07
78-83(R): 911 571 050 07

In fact, for the SC it looks like Porsche got smart and simply capped off the opening for the side vent from the control box using part 911 573 207 00 for cars w/o AC rather than having two separate control boxes for AC/non-AC.

So, my question is - in order to increase the chances of me finding a used pair of non-AC control boxes, can I use the -07 part number from an SC on my 77? The pictures of the two on the parts diagram look the same (in fact the same diagram is used for both 77[813-05] & SC[813-00]), however I realize that these pictures are not always accurate. The only thing I can think of as to why the two might be different is that somewhere around this time the dash vents were made larger (I know the 84 center vents were), so maybe the plumbing from the control boxes to the side vents was made larger? Or did this coincide with the foot-well fans?

When searching, I found this thread, but it seems to be for later models:

86-89 Carrera w/out AC -- what comes out the dash vents?

I would appreciate any help.

thanks,
Frank

[dm36415]

Frank,

Great question, I'll be watching this thread with great anticipation as I am in the same situation. I bought a 77 about 6 months ago that had the AC removed, and was perplexed as to why I couldn't get any air to come through the dash vents (assumed I'd figure it out before summer comes). Now I know I have (more) work to do.

[frankc]

Just an update...
I called the dealer to order some parts for my brake line replacement project, and I figured I would ask about the price on the -01 control boxes. He came back with some interesting news. It turns out that the -01 parts are NLA and have been superseded with the SC -07 part. So, I guess that makes the SC parts ok to use in the earlier cars, but I have heard of stories where these types of retrofits don't always work out too well.

If I end up getting the -07 parts I'll report back on the outcome.