How does this ventilation setting work?

[frankc]

There are two types of 911 ventilation system controls I am familiar with, with the only difference being one or two levers at the top. For 77 (and many other years), cars w/o air conditioning had two independent levers for fresh air and the fresh air fan. Cars with A/C had a single lever which when slid to the right first opened the fresh air ducts from the fan box, and then when moved further to the right engaged the fan speed settings.

This latter mechanism always made more sense to me since why would there be a need to have the fan running if the valves leading out of the fresh air fan box where shut?

Since I am retrofitting a non-A/C ventilation system in my car (Retrofitting non-A/C ventilation ducts) , I started looking into this a bit deeper when deciding whether to replace the dash controls. I ran into this interesting piece of information from the 77 owner's manual (picture in red box):



So, can someone please explain how having the fresh air ducts closed (top left lever to left - "no fresh air supply from the outside"), and the fan at high speed (top right lever to the right) can provide "maximum ventilation of car interior, open center vents"? How is the air getting out of the fan box with the fresh air ducts leading out of the left and right side closed?

I thought I knew every last detail of this system, but there is something different about a non-A/C system that I am unaware of. I believe javadog's post in my thread above which shows a picture of the non-A/C system is providing some clues with the linkage on the splitter duct. I suppose once I get the splitter duct I ordered (2-3 weeks), this may be clearer.

btw, I know the ventilation system has been discussed here ad nauseam, but I did a search and couldn't find anything on this particular subject.

thanks for any insight,
Frank

[DRACO A5OG]

It must be in translation but the picture high lighted in red should not have the top two knobs positioned that way, it is confusing.

Top knob is for Off (left) Full Fan (Right). my 85 has the additional opening of the inlet without the fan until the middle then the fan engages beyond the center of the track.

Middle dictates Fresh Air flow to the Floor (left) Windshield (right) both Floor and Windshield (center)

Bottom is for heat distribution to the Floor (left) Windshield Defrost (right) center for both.

My personal settings are:

Top Center (it increases naturally as I increase speed on the road)
Middle Right
Bottom Left

For Defrost:

Center Heat control to Maximum
Top Right
Middle Right
Bottom Right

For Maximum Fresh Air:

Top Right
Middle Right
Bottom Left

Just my observations.

[javadog]

Well, the only way this would work is if you add the part you have on order with the additional butterfly valve (the one that goes in place of the 'T" shown as number 20 in your original post and that has the additional valve as shown in my picture of the white ROW car) and remove the butterfly valve from the right hand side of the blower assembly, if one is present there.

If you have two butterfly valves in the blower assembly, and one of the simple "T" fittings (part number 20 in your original post) in place of that additional valve assembly, there is no way you can use the control settings as they show them.

I think you understand the flow of the system; who knows what Porsche was thinking?

JR

[frankc]

Javadog,

I started this thread back when I was still trying to figure this out, but your postings on my conversion thread (link at top) cleared things up for me by indicating the deleted right valve in the air-box for non-A/C cars. btw, there is simply a bolt clamping the butterfly valve to the linkage so it can be easily (re)moved. I'll post a follow-up on the conversion thread once the parts get here in another week or so.

Also, regarding the scanned image from the owner's manual - IMHO, I believe the text in the yellow box was simply a carry-over from the 76 manual that they forgot to update for 77. I'm curios what an SC manual might say about that upper left lever.

Frank

[javadog]

I don't know about an SC but I looked at my '80 turbo and it's both in the owner's manual and on a sticker glued the inside of the glovebox lid.

JR