Results from my A/C project
 

What do you think?

Now that it's the dog-days of summer, I thought y'all might like to know how the 100% stock, R134a filled, A/C in my '88 is performing. (Some may recall that back in the spring, I decided to revive the A/C - Thanks Jim!)

The temp today was 97 degrees, with a soft breeze. Here is a pick of the t-stat at 11:30am today. (Windows down, sunshield in the windscreen.) It's reading between 115 and 120 degrees!
http://forums.pelicanparts.com/uploa...1151958787.jpg

Here is the vent-temp after driving for 5 minutes. Okay, maybe 70 degrees doesn't sound too great, but I'll take 70-something cabin temps any day. It's a far sight better than windows-down in this heat and humidity.
http://forums.pelicanparts.com/uploa...1151959343.jpg

And here's the temp at the condenser. A bit over 100 degrees. (The angle skews the reading.)
http://forums.pelicanparts.com/uploa...1151959392.jpg

So now I'm wondering what would happen if I put a bit more R134 in? I'm also thinking about upgrading to a pro-cooler (refrigerated dryer) next year.

Thanks for posting the data point.

The Rennaire Procooler is a very easy modification/installation and the Griffith's fender mounted condensers are also relatively easy to install (the rearward unit is especially easy to install). If I elect to add more condensing to my '76, I'll probably install a Griffith's Keuhl rearward condenser/fan (piccolo type) unit.

Jim, What is a piccolo unit?

No, no, Jim. Thank you for your guidance. Your help, my patience and some effort, along with a small touch of money got my A/C working again.

A Piccolo condenser is a multiple pass unit that gets it's name from the form or shape of the manifold; it isn't quite parallel flow but it has it's virtues as noted below.

TUBE and FIN - Oldest industry standard, 3/8-inch diameter round tube condenser. Ok, for R12 but not recommended for R134a. This type of condenser is flushable.

6 mm PICCOLO multi-flow: smaller, lightweight and more efficient than 3/8-inch tube and fin and serpentine. Used in many domestic OE applications. This type of condenser is flushable.

SERPENTINE: All aluminum, more efficient than tube and fin, used on smaller imports where space is limited. This type of condenser is not easily flushed, replacement is recommended.

PARALLEL FLOW: all aluminum, this efficient design breaks up flow into tiny streams that give up heat more rapidly. This type of condenser is not flushable, replacement only!

PARALLEL FLOW with SUB-COOLER. Oval tube multi-flow and sub-condenser(sub-cooler) stacked up. Refrigerant flows through the multi-flow into the drier (called a modulator) then continues as a liquid into the sub-cooler. This type of condenser is not flushable, replacement only!

Dear Captain,

At what section are you attempting measure the deck lid condenser temperature perse, upper RH corner, top row,
bottom row, left and corner?

Jim - thanks for the education!

"Piccolo" condenser manifold:

http://forums.pelicanparts.com/uploa...1151988086.jpg

Non-manifold side of "Piccolo" condenser showing return bends:

http://forums.pelicanparts.com/uploa...1151988254.jpg

A couple of weeks ago in LA. about 95 degrees, and slightly humid. I was cruising, not stuck in traffic. In traffic, it goes up closer to 45. Gotta work on that.
http://forums.pelicanparts.com/uploa...1149043834.jpg

I'm measuring bottom center. The gauge is about 3/4 to 1 inch above the condensor. Lid is down.

So, Does anyone think a bit more R134a would help the performance? I chickened out adding more with the high pressure gauge reading 315 to 320 at 2,000 rpm.

Captain,

1) Normally you clamp a thermocouple onto the inlet pipe of the condenser, take a reading, and then clamp it on the outlet pipe and take your reading, and then you can see the delta... for what it is worth.

2) Before you go totally P&T only on your refrigerant charge:
A) How many ounces (or grams) of R134a do you think had in the system when you observed the 315-320 high side.
B) Was the ambient (outside temperature) still 97F?
C) What is the high and low side and ambient temperature (outside) at idle with the current charge?

3) No comment on the 27 F delta (ambient vs. cockpit vent) yet.

[ That's what I love about the 'Net. I get to learn stuff every day. I had no idea that was the procedure.
Any specific thermocouple?
Any 'typical' technique I can reference?
I presume I measure off the main deck lid condenser?
My highly inaccurate shade-tree measuring indicates I have about 34 oz of R134a in the system. (The R12 charge on an '88 is 47oz pf R12.)
Yes, to the best of my knowledge.
The synopsis question. As outlined above: High and low? I do not know. Ambient, 97 degrees to the best of my knowledge.
Come on… The suspense is killing me… I think it needs more R134a. But the high-pressure reading scares me. (If I saw those pressures on my other cars that are designed to run on R134, I'd panic!) Do I dare add more and blow the lines off the car? Am I being needlessly cautious?


What I can tell you is I'm learning a lot, and having a fun time doing it.

For Kuehl's question C, "high and low side" is the high side pressure and low side pressure. You have already stated 315+ psi on the high side -- what did the other gauge say?

Brian

Captain Carrera,

"For what it is worth" means simply if you want to know how many degrees (roughly) the rear coil is dumping
you need the in and out temps, however "for what it is worth" further implies a question for you, what are you attempting to do taking coil temps?
Are you trying to determine the refrigerant charge using a quasi Chrysler Delta method or Subcooling method, or home a/c method?
What specific thermocouple? Well what is your favorite brand and do you want something that is NIST traceable?

Obviously your high side pressure is too high given the amount of refrigerant you indicated. How high should it be? Look it up on a P&T chart.

The "synopsis" question we will assume is <i>taking a
summary given in brief terms that covers the major points of a subject matter</i>.

How about a simple answer: NO. Don't put in more refrigerant until you figure out why your high side pressure is so high!

Jim, maybe you have time for the joke.

Getting outlet temps of 38 degrees using ES-12, the freon/gas mix. Not planning on switching over to R-134 until forced to but lots of good info above.

Thanks for validating that I should be trepidatious at this point. I truly appreciate it.

Well, I'll admit I'm not clear on what the joke is.... But I’m sharp enough to realize it’s ultimately aimed at me. "Fank You" for the attitude. :rolleyes: Hopefully others will see your response before ordering from you.

Rob: 1) Your high side pressure is too high 2) the Pro-cooler is fantastic (unpaid happy customer) 3) the joke is the picture of 28 degree vent temp at 95 ambient air temp outside. That is virtually impossible for the A/C system in our cars.

He was teasing a bit because...while the 27 degree difference is an improvement, it is still not very good for a fully operational system. Under the same 95 degree ambient, my vent temps would drop to 40 within a minute and to 32 soon thereafter. On the other hand, I recently spent a lot of money to get those temps. I have barrier hoses, the dual Kuehl condensers with an upgraded lid condenser, rennaire evaporator and procooler...all installed by the best Porsche A/C man in the southwest.

Keep asking questions. Don't let a little teasing create any heartburn. These guys do this for a living and are donating their valuable time here. They don't mean any harm...they just work around the stuff all the time and forget how little most of us know about A/C.

nicely put flintstone

I wouldn't like to see more than 270 psi on the high side with a 100 degrees F ambient temperature. The 315 psi pressure can indicate several problems: insufficient condenser cooling, refrigerant overcharge, air in the system, expansion valve issues, etc. Do you have a low side pressure under the same conditions? Are the low side fitting/hose just after the evaporator covered with frost or moisture or are they hot to the touch?

Sorry Captain/Sir.
I needed to cool my jets, 16 hour days are now boring.
I keep thinking it is my turn to volley but no one wants to play. But we will share our toys.

Becks in hand:
So you are saavy enough to be cautious and you want to know why your head pressures are above normal for R134a on a 97 F day!

THE FINAL ANSWER IS AIR!
a) Air in the system (inadequate evacuation, not enough hours on the pump or the system iced up during evacuation and there is residual moisture), or
b) Not enough air across the condensers (ie. deck lid down, front condenser motor works), or
c) Fresh air ventilation system is open (all slide knobs s/b left)
d) Heater box(es) are open (heat coming in the cockpit, cables dysfunctional).
e) Left thermostat knob not fully clockwise.

Or. ASSUMPTIONS:
f) Verified equipment: thermometer is accurate within a given range of xx-x F of +/- x F.
g) Vacuum pump pulls xxx microns.
h) Service gauges are accurate (needles are true).
i) Charge hose set purged.
j) That your refrigerant scale is working, or your metering device is true, or you know you used 12 ounce cans and not 16 ounce cans.
k) Some years have "automatic temperature sensors", either located in the dash or in the headliner between the visors, if your vehicle has this feature you have tested them.
l) You noted in the first post that the "Windows down". Do you mean up ? Or are you attempting to chill out the neighborhood. ?
<i> and you have checked all these above items</i>


On the "average" the replacement charge of R134a refrigerant ranges from 70-90% in weight of the R12 OEM amount (according to typical automotive retrofit guides; in like systems: reference MAC's guide publication #44201, or "the good ole" Snap-On Technical Training Systems manual ACT 2798).

Assuming an OEM amount of 47 oz. that would give you a range of 33 to 42 ounces. Why the vast range stated by the guides? All depends upon what heat loads they used the day they tested their charges. Did one fella determine the perfect charge on an 80F ambient day and another on a 100F ambient day... ?

Porsche's shop manual suggests, with R12, @77f, deck lid down, 2k rpms, you could have a high side range of 115-205 psi. (double or nothing!). Compare that to a more finite or average industry guideline for R12 of 184 psi @77 F, or extend the temperature out to 97F and you might have 232 psi with R12.

Jim has pointed out that a high side of 270psi @100F would be near the end. I agree. Fudge in the R134a curve and 270 psi @97F does not sound too far off for a stock 911.

Now if you wish you could drive the head pressures right up there to 375psi, blow compressor nose seal or fry some ribs, but that gets to be expensive.... so keep the head pressures down and extend the life of the compressor.

If you have reviewed the AIR and ASSUMPTIONS, have you considered exploring what your evaporator core temperature is under your testing conditions; replacing the thermometer in place of the thermostat's thermal expansion (aluminum tube) tube (in your evaporator); or simply, did you have frost or sweating on the evaporator outlet pipe?