'74 911 a/c upgrade

vgarza · 03-04-2002, 01:36 PM

Hello,
I'm trying to determine if I need to upgrade my
existing a/c in my '74 911 Targa. I've heard
that I should replace the existing receiver/dryer
and perhaps the condenser to get decent performance here in Houston (can get to 100 deg).

Any thoughts ? Any help would be appreciated !

Hladun · 03-04-2002, 05:08 PM

If your system dates from 1974 it will have (probably "will have had" because its gone) R12 refrigerant. You'll need to convert to 134a which I believe would need a new dryer. Your compressor may be gone as well. You’ll need to use an a/c shop.

I've studied the system because I'm going to get the a/c working in my 78SC this summer. I think the big (but totally unnoticed) problem with 911 a/c systems is the lack of air flow through the condenser; particularly at idle. We all look at that engine fan and think it pushes a lot of air but it doesn't. At 6000 rpm the fan pumps about 3000 cfm and the engine draws about 300 cfm. At idle (1000 rpm) that's 550 cfm. That's an air flow of 4 mph through the trunk lid opening at idle. Not enough!

Get a 9" pusher electric engine cooling fan (J.C. Whitney) and mount it on the left side of the condenser. Check very carefully so you have enough room. This fan blows at 660 cfm and since what goes out has to come in, at idle you'll get 2X660 plus 550 = 1,870 cfm. That's about 13 mph air speed through the condenser. That should make a big difference in your a/c performance.

vgarza · 03-05-2002, 09:17 AM

Where can I find more information about this
electric fan and how to install it ? Where is
the switch ? ....etc...

Thanks for your help !

Hladun · 03-05-2002, 06:43 PM

Here is the fan I was talking about;

http://www.jcwhitney.com/item.jhtml?ITEMID=2127&BQ=jcw2

Mount it under the trunk lid condenser on the left side. It mounts directly to the condenser with the nylon ties. Just make sure you have enough room under the condenser so when you close the trunk lid so you don't crush the fan/condenser.

Since it only draws 5.5 amps, I'd wire it directly to the a/c compressor clutch. You can use an interposing relay if you're electrically adept. However, if you don't just make sure the clutch is engaging (compressor turns) and it’s not blowing the fuse. The fan should run only when the compressor clutch is engaged.

jclc · 03-05-2002, 10:51 PM

"At 6000 rpm the fan pumps about 3000 cfm and the engine draws about 300 cfm. At idle (1000 rpm) that's 550 cfm. That's an air flow of 4 mph through the trunk lid opening at idle. "

Hladun, I was surprised by this result and ran the numbers. Turns out your right, maybe even a little optimistic. I get 3 1/2 mph IF the car is standing still AND if the entire airflow goes through the grille, neither of which is likely.

Its a wonder it works at all... No wonder they added the extra condenser under the trunk.

Jeff Craddock
Orange, CA
74 911 coupe

Steve W · Steve W's Garage

Since you have a R12 system, redoing your system with R134a will reduce the efficiency of your system, unless you upgrade or add to the condenser size. An alternative you might want to consider is to use propane instead of freon12. It is actually a chemically perfect substitute and its properties in compression and expansion are virtually identical. Actually the ideal combination is a 80:20 mix of propane and isobutane. I use the propane out of a propane torch bottle, and the isobutane out of a 8 oz MSR camping stove fuel can. Your system will actually run more efficient than R12 and be colder. Its what I'm currently using in the ac system of my Toyota truck, and it works super cold.

vgarza · 03-06-2002, 06:00 AM

When you say TRUNK I take it that you mean the
front part of the car where the 2nd condenser
is usually installed ? My '74 doesn't have
this secondary condenser - just the one in the
back above the motor. Are you saying I could
also mount it there ? Seems like it would be
a really tight fit. Do you have any pictures
you could email me on how this would fit ?

Thanks again...

vgarza · 03-06-2002, 06:37 AM

Something else that I've found is to replace the dryer.... go to www.procooler.com and look under the a/c section for 911s......

Let me know what you think...

bonedaddy · 03-06-2002, 06:51 AM

I don't know if your system is factory or an aftermarket version. I do know that the major weakness of 911 a/c is lack of condenser capacity, which is more acute with 134A due to a lower latent heat of vaporization.

I recommend you check out Performance Air in California and give them a call. They are very knowledgeable about all the permutations of both factory and aftermarket A/C and can provide you with information on obtaining and mounting the static condenser that attaches underneath the car. They have done extensive testing with various combinations of compressors and refrigerants, also.

Hladun · 03-06-2002, 03:04 PM

The early 911's weren't made for the front condenser. The later cars had a fan inside the trunk and the bodywork accommodates the condenser. It's such a vulnerable part to stick out under the car.

If you need a second condenser this is much better;

http://www.griffiths.com/porsche/ac/porsac911.htm#Kuehlcondenser

But bring lots of money!

Thanks to Jeff for verifying my airflow calculations. Bonedaddy reinforces the poor condenser problem. Don't overlook puting in the fan I suggested as a very cheap first step. It may be all you need.

Hladun · 06:46 AM

Quote:

Originally posted by vgarza
Something else that I've found is to replace the dryer.... go to www.procooler.com and look under the a/c section for 911s......

Let me know what you think...

My first comment is, if this worked everyone would do it. You are taking heat from the HP side and putting it in the LP side, so I can't see how the low side pressure goes down. To make a system cool better physics says you have to reject heat (the condenser again). This looks a bit like a thermal perpetual motion machine.

March 10/02

I've given this some thought and there may be something here. When the refrigerant comes out of the evaporator it's cold. On the return run to the compressor it will pick up heat from the surrounding air. On a Porsche this is a long run and the heat transfer may be significant. (Incidentally, don't put any a/c hoses next to the 200F oil lines.) With this you will also get a higher compressor inlet temperature and therefore a higher outlet temperature, but this should help heat rejection in the condenser. Maybe these guys are on to something. Anyone haveone of these?

scottb · 08-09-2002, 10:18 PM

If the JC Whitney fan is installed, will the rear deck lid still stay up, or do you have to add a second strut like when adding a "whale tail?"

Thanks.

Hladun · 08-10-2002, 06:57 AM

My strut is quite new and it holds the rear deck lid up with no problem. I have the regular deck lid, not a whaletail.

Some added information, I installed this fan when I rebuilt my a/c this summer. I have a 78SC that had a non-working aftermarket system. I have the regular deck lid condenser, no front condenser, a $20 used Sanden compressor from a Saab and run R134. With 90F and low humidity outside temperatures, I get 47 to 50F outlet temperatures. Since I live in Calgary this is adequate for me. I just can't overemphasize the importance of good condenser air flow.

scottb · 07:20 AM

I'm intrigued by your concept. I agree, one of the problems with the 911 a/c is lack of airflow over the rear condenser (I have an 84 with the dual condenser system).

Do you know if the cooling fan will fit the engine compartment of an 84? The catalog says it is 10.25" in diameter. How deep is it? Also, does the fan at all interfere with air flow and the cooling of the engine when the fan is not in use? (i.e., when the a/c is off?). Have you noticed any appreciable increase in engine temp?

Thanks much for your response.

Hladun · 08:57 PM

The fan is about 2 inches deep at its center. I mounted it over on the left side of the condenser on my 78SC. I had to move the coil and cut off the inlet cone on the air cleaner housing to make it fit. It takes a little playing around to get it in (and don’t slam the lid to find out), but it works. You can even run the a/c with the deck lid up.

The condenser itself would probably cause much more air resistance than the fan when its not running. When its running I don't think that it starves the engine, but if it bothers you get a puller fan and have it blow in... electric supercharging!

My engine runs hotter than it did, but the compressor load is probably more the cause. The trombone cooler was a silly idea. Like the a/c you need airflow to cool and the Porsche designs without an electric fan just don't work. I am upgrading my oil cooling with a $130 JC Whitney oil cooler/fan. I'll post the results when I install it.

P.S. The hotter running problem was that I got a smaller diameter crankshaft pulley from Parts Heaven and that slowed my fan speed. To their credit, they got me the right pulley, no questions asked.