Hello all. Sorry for the length of this post, but I have some questions for the community about my ‘74 914 2.0L’s heating system, which was removed in its entirety by my PO (he wanted to reduce weight to improve racing the car).

I am now inclined to build it all back in. I believe I have all of the major components, except heat exchangers (I am in the market for a pair still in very good shape or restored; too damn expensive new) and a bunch of the flexible pipes (which probably need to be replaced with new and better tubing anyway). The main Behr intake box under the cowl; the fan and motor beneath that; the 2 black plastic control boxes; the 2 black plastic dashboard defrost funnel vents - - I have all of these and they appear to be in good condition and generally just need a cleaning (the round rubber / foam seals on the valves are a little fried but I think I saw replacements for those being sold somewhere). The metal control boxes (with the “mushroom”-shaped heads) that connect to the heat exchangers in the back seem pretty well cooked and beat up, too, so those maybe need to be replaced as well.

Regardless, my questions are as follows:

1) I have located 2 exploded diagrams for the system looking through books and on the web. One diagram shows the heat exchangers, metal control boxes and other related equipment in the lower back of the car. The other diagram shows the main intake box and main blower motor under the cowl, as well as their related hoses and equipment. What I cannot find is a diagram detailing the related equipment that belongs in the engine compartment, nor a diagram that shows how these 3 sections all connect up with one another.

Is it fair to say that in the engine compartment, the only heater equipment is the “additional blower/motor” that attaches on the left side, behind the driver, next to the relay box, along with one (or two, if modified) hose(s) to holes at the bottom of the engine compartment? (I have seen recommendations for this mod/ update, i.e., from one hose to two hoses, using a splitter.). Thoughts on this?

If I am understanding this correctly, this motor in the engine compartment would seem to be sucking air from the engine compartment and then pushing it through those pipes towards the heat exchangers? Right? Well, if so, then that heated air is then rerouted and somehow makes its way through the system to the passenger cabin, no? Is that safe (is that air clean enough and safely breathable)? Or do I have this backwards somehow?

2). Moreover, in studying the diagrams, I am having trouble understanding how that rear portion of the heating system air flow connects to the front portion of the heating system air flow. Are there separate hoses that are supposed to connect from each side corner on the rear firewall (see pictures below) up to the pipes behind the speakers (see picture below) and connecting up to the portion of the ventilation system in the front compartment)? Or, on both sides of the car, are there essentially metal pipes /passageways that are built into the frame / chassis / body of the car that makes this connection?

If I am understanding this connection correctly, the main blower box under the cowl would seem to me to pulling fresh air from above, outside by the windshield, down these pipes towards the back of car, no? But, isn’t that backwards? Doesn’t it actually need to suck hot air from the back of the car up to the equipment in the front under the cowl and then somehow simultaneously push that hot air through the defroster and side vents in the dashboard?

Here are some pics:

In two of them you can see round metal openings in the top left and top right of the pictures that I refer to above, which seem to lead into the frame, and seem to lead to the pipes that come up behind the speakers? Is that correct? Are there supposed to flexible pipes in there somewhere that run down each side of the car - -or are the “pipes” / passageways built into the frame connecting these spots?

Sorry again for the long post. Really appreciate any insights y’all can provide add here.

Thanks,
Steven

Those are your heater tube connectors. The tubes themselves are flexible but can't be removed without cutting into the longs.

Dy you have the PET ? That's a complete breakdown of ALL the parts that make up a 914. It's too large for me to add to this note but it shows in diagrams everything you need to know.

Barefoot

I didn’t but found it on the Porsche website just now. Thanks for the tip!

Here's some pics I took that I posted on Pelican a while back that will hopefully answer your questions about how the parts under the engine go together:

http://forums.pelicanparts.com/porsche-914...html#post546504

The heater system uses air from the main engine cooling fan. That gets pushed into the heat exchangers, and either dumped out under the car (by the "flapper valves") or routed into the pipes that go through the longitudinals and then up into the dash. The one slider up on the dash moves flaps inside the air distribution stuff to send the air to the windscreen or to your feet.

The "fresh air" system takes air in through the vent in the cowl, and directs it up to the windscreen or down to your feet. Air does not go from the front to the rear inside the system at all. I don't believe that warm and cool air even gets mixed inside the dash, though I could be wrong on that. But two separate levers directing warm and cool air says to me two different air-handling mechanisms.

--DD

The engine bay fan is the only one pushing warm air, thru the heat exchangers and then forward inside both sills, up behind the speaker grilles to mixing box/ the dash vents.

The front fan under the cowling only moves fresh (cold air) into the cabin.

As for the safety aspect of the engine bay air. It was probably deemed ok in the 70's when all seals were new etc. Today its probably more of an owner judgement call.
The main engine fan moves alot of air while driving (say 3k rpm) and the engine bay air is swapped out many times each minute. So if your seals between body and engine tins are good then its not much of an issue.

If the switch is set up correctly, you can turn the heat on without kicking the fan in the back on as well. Just don't pull it all the way back. Won't be max heat of course.

Many thanks to all for your responses.

In the meantime, I came upon an older post on 914 World regarding heater system mods that suggested that removal of heating system and blocking off the hoses might result in some gained HP. Not sure I understand why that might be, or maybe I am misunderstanding the thread.

The engine im my car is running very well, which I attributed to the recent rebuild. Seeing that thread got me thinking that putting the exchangers, control boxes, pipes, blower, etc. all back into the car back in might actually reduce the car’s HP and therefore not besuch a good idea after all. Thoughts on this?

Maybe adding back the fresh air system in front all, without the heating equipment in the back, would be sufficient for improved cabin comfort and defrosting.

Here is a picture of the basic looking exhaust system on the car now, in case it helps

Thanks,
Steven

Not really sure that (unless you are running a tuned exhaust and more than stock HP) that the removal of the heat exchangers would result in any noticeable HP.

I would say that if you intend to drive it on a cooler day or in the rain, you should have heat for defrost.

If heat isn't going to be an issue leave as is.