Today seems to be my day for asking questions. Here's this one:

My 90 S4's A/C was converted to R-134A when I bought it. My mechanic says that the R-134A service fittings were merely added on top of the R-12 fittings. The car also has a receiver dryer bottle that he's never seen before. He recommends replacing the dryer and converting back to R-12. He believes the compressor will be more efficient with the R-12, which will be nice since our temps routinely get above 110 degrees this time of year.

My mechanic and I are playing phone tag. While I'm waiting to talk with him, any thoughts of the pros and cons of this recommended approach?

 

If it wasn't well converted its likely R12 will work better in it, the piggy-back fittings would suggest that shortcuts might have been taken. A new dryer makes sense anyway.

Alan

 

I used the piggyback fittings when I did the conversion on mine 15+ years ago. Short of buying GTS hoses and fitting up a whole new GTS compressor and mountings ($$$), this was the only practical option at that time.

The drier replacement options at that time included the 944 drier, which has identical fittings but is slightly smaller diameter and slightly taller. And a whole lot less $ and more common in the market at that time. Of course, the selection for your car may be/have been different.

The compressor may be "more efficient" with R-12 but I'm not sure how one would determine that. The limiting factor in cooling the car is the condenser capacity, which won't change with R-12 vs R-134a. Greg Brown has fitted a mega-condenser setup to naked cowboy Dan's car in that area, something you might want to look into if you find vent temps aren't low enough. The bigger condenser capacity won't solve heat-soak issues in the cabin, but may buy you some colder vent temps to help with faster initial cabin cool-down. Remeber also that the anti-freeze switch prevents the evaporator from icing, so center vent temps won't drop much below 40ºF with that switch working, regardless of which refrigerant you use or how awesome the condenser is performing.

I have the anti-freeze switch jumpered in my car, since virtually all of my recent driving has been in arid SoCal weather and trips short enough that a little ice formation wasn't critical. With R-134a, center vent temps would be under 20ºF on 95º ambient days at cruising speed, fan set on second speed. I drove up through the central valley of California in 100º+ June temps, and had to cycle the compressor off manually once in a while to allow things to thaw. Black car with no tint, BTW. Freezes the fingers on the wheel if I let the vents blow towards me too long. So before you go through the trouble of un-converting the car back to R-12, consider what your goals are. If it's performance-related, R-12 will provide no noticeable advantage. If it's budget related (yours) there is a financial disincentive. Putting your mechanic's kids through college and want to be stuck buying R-12 for the life of your car again? Why not just set up an endowment fund for them now?

Most folks take their cars to a mechanic for AC stuff that isn't working. The path to leakless R-12 is exactly the same as the path to leakless R-134a. You can buy R-134a almost anywhere for under $10/lb in small cans, under $3 a pound in cylinders. In my opinion, there are enough good reasons to stick with R-134a.

 

Everyday I'm learning as I work on these fine cars and if there is something else out there asides from the piggy back adapters when converting to 134a I'd love to know about it. I use these.



Aside from removing the entire line and installing a custom one, I can't think of a better solution but am open to learn.

As to driers, these are the three I have on hand, would be interested to know what was used on your car.



As to the OP, I would not convert back if the job was done correctly the first time, proper oil, hoses, seals etc. If 134a is not cooling as well as the R-12 it's due to improper conversion and going from -12 to -134a back to -12 is going to cause issues in the long run. Have the mech tear it down and do it right the first time and keep with it. Yes, our cars could use some more condenser but they will still cool well either way. Low vent temps with proper pressures is an indication of moisture in the system.

I stayed at a few Holiday Inn Expresses this last couple of weeks so I am feeling really smart today.

 

There are reasons to doubt that the original conversion was done properly: the hoses were not replaced with barrier hoses (I know some feel making the switch is unnecessary), the drier is one that was not used for any of the 928 modely years (nor is it from a 944), the drier doesn't have a sight glass. Of course, the drier may be a quality aftermarket product.

Here's the current plan: My mechanic will evacuate the system, add some dye to the oil, and refill the system with the existing refrigerant. He'll then top it off (if necessary) with additional R-134a and we'll see if there are any leaks. We'll go from there. I suspect that I'll switch out the drier and hoses and keep it as a R-134a system.

Thanks for everyone's input.

 

Sean, you're the only other person I know who has a granite bench.

 

I had always assumed (not sure why really...) that you'd braze on new R134a fittings for security and one less mechanical failure point. Obviously I'm not an expert at this. Aren't the 928 R134a condensers larger than the R12 ones though?

Alan

 

The condensers may be, but I can't say that I've ever taken the time to compare capacity from one to the other. I'd guess from the ones I've got here that the condensers that I've had my hands on were no different. We've all talked about it as far as cooling capacity but I've got no real world comparisons.

Right now, what is here in the shop, are a 1984, 1988, 1993 (ROW 1992) and a 1994 and each of them are all different. The only real mechanical difference is the drier and the connections to it. Condenser look the same............Dunno. All are here for A/C work and believe me that ordering parts has been an interesting issue.

I can say that I'd never think about brazing an R-134a connection to an r-12 for a retrofit. Once you go down that road, if the modification fails you are pulling some serious time to get the lines out of the car. I just spend 20 hours pulling each and every line and connection out of a '94 to clear a failed drier out. I'd not want to do that again and I'm sure the customer would not want to either.

 

Alan - My understanding is that Porsche made most of the A/C components larger when they switched to R-134a in 1993. (That's what Dan told me today.) R-12 is more efficient at carrying away heat, so R-134a needs a larger area in which to do its work. I had an engineer who designed these systems say that the difference between freon and its substitutes is similar to the difference between copper and aluminum wiring. Both will carry electricity, but you can get away with smaller wires when using copper. (That's right, copper is more efficient than aluminum isn't it?)

He said that it would be possible to use a freon substitute in a system designed for freon, but it wouldn't work as well. That's fine if the system was over engineered such that there was excess capacity. In such a case, using a less efficient refrigerant would just mean that the system would need to run a little longer or harder than before. The problem would be for a system that was barely large enough to do its job with freon. Then, converting to a less efficient refrigerant would mean the system would be unable to do its job. (By the way, this discussion was concerning the A/C unit for my house in Texas. One of my firm's clients was a major manufacturer of A/C equipment and I spoke to one of their engineers about whether it made sense to convert my old system, upgrade to a new heat pump, or to keep using freon. I assume that the same principles apply to cars.)

My current quandary is whether the pre-1993 928's A/C system has the excess capacity necessary to use R-134a. I suspect the answer to that may depend on where and how the car is used. I also suspect it might NOT have much excess capacity for my use. If I park my car outside in the sun, the inside of the car heats up to the point that my iPhone will turn off if I let it rest on the center console next to the gear shift. Did you know iPhones turn off when they get too hot? I didn't before June.

 

I converted my car to 134 years ago (an '82 euro) with a properly evacuated system, properly installed connectors, new o-rings at all junctions, a new drier and the proper oil charge it has worked well in our Texas heat... It worked well until the heater valve failed and then once replaced worked well again!
There is no reason why a properly converted system should ever be switched back to R12.

The real key is being sure that the charge level is correct, there is a change in coolant capacity with the conversion... if I remember correctly you will use slightly less 134 than R12.

 

Drove my GTS in the Texas heat with a properly converted R134a system, no problems (tambien en España ahora, no problemas ).
If the system was not converted properly it can´t hurt to do it again. New receiver drier with new o-rings and new R134a service port adapter fittings. Important: empty the system and make sure that the old Schrader valves are REMOVED before the new R134a adapters are used!

Evacuate the system and refill it to about 80% of the original R12 capacity, that´s it.

BTW the condenser is the same as of MY 87. What changed is the compressor and the service port location for cars with factory equipped R134a AC.

 

My 81 US which I converted to R134 with a griffiths kit (all new lines, compressor, expantion valve etc.) cools okay but it would cool much better if I cleaned out the condenser as it is filthy. I would recommend anyone to clean their condenser before assuming something has failed.

Also, I would like to know if the later S4+ condensers are more efficient than the earlier ones? As the 78-79 and 87-95 have a round pipe design that is comprised of multiple layers of these pipes and the 80-86 has a flat pipe design that serpentines up to the top from the bottom and is only one layer.

Sorry to add another question to the thread, just curious.

 

Converted from R-12 to R-134a about ten years ago using one of Griffith's R-134a conversion systems.

http://www.griffiths.com/porsche/ac/928ac/index.html

It still works great today.

Great support from Griffith's during the conversion.

 

"Piggy back"... hmmm. That's a new one, lol. To convert from R12 to an EPA approved refrigerant, such as R134a, there are particular charge port adapters used for each type of approved refrigerant. There are 2 types of charge port adapters. Those that do not have their own schrader valve core, rather they have a pin that pushes down on the existing R12 valve. These are not that reliable. Then there are the preferred type that do have their schrader valve (refer to Sean's steel with yellow chromate examples) , you remove the existing schrader valve from the original R12 service port, screw on the R134a charge port adapter and you are done! Depending upon the location of the original R12 service port you would use either a straight adapter or 90 degree adapter for accessibility. This is how its been done in the industry, whether the procedure is done at the dealership, at a repair shop, or by the DIY, since the introduction of alternative refrigerants many years ago.

I'm getting the feeling that maybe your mechanic has not seen that many R134a conversions on sharks.
The drier used on the 1990 is the same style used on 1987-1992. Shape is a beer can with the connection ports on the top opposite each other, alike 944/968 drier used up through 1992. The OEM drier was silver and typically an aftermarket drier is black. You can use any same fit/form aftermarket drier so long as it has XH-7 or XH-9 desiccant, the later is preferred.

You would change the drier if you are changing the refrigerant because residual R134a gas would be trapped in the oil in the drier (you don't want to mix refrigerants).
I've owned 3 sharks in the past: 78,84, 89, all of which I converted to R134a and probably drove all of them in near 100f temps and they all blew cold, quite comfortable and reliable too. The problem with the shark is when you let it sit in the sun for a few hours the interior absorbs a heck of lot of heat (you got steel, plastic, insulation, seats, etc... a lot of mass). Get in a shark after its sat in the 90F sun for 3 hours and its well over 130 inside. If you expect it, or any other vehicle with mass, to instantly cool down when you turn on the AC you are kidding yourself. That evaporator core and condenser are going to be running overtime to pull out the btu's. Best procedure is to roll down the windows, pop the sun roof and let mother nature suck the heat out; if its 100F outside and 130+F inside.... the outside air will pull the heat out quicker than the AC. After cockpit temp equals the outside ambient temp you then turn on the AC and slowly close up the windows when you can feel cool enough air coming out of the vents.

In terms of your compressor being 'efficient'? Your compressor is "compatible" with R134a if it has the correct seal materials inside. OEM was buna or nitrile. R134a compatible seals are HNBR or Hydrogenated Nitrile Butadiene Rubber. Although most manufactures of seals and o-rings try to color code them, example "usually" HNBR is green, you can find blue and black ones as well. When we talk about "efficiency" that noun you could relate to mean "with minimal time or effort". A compressor is a pump. There are certain designs of compressor pumps that require less energy or less effort.
If we look at the various types used in range of Porsche models, the York crank piston (in the early 911) was the least efficient. Next comes the swash plate, which requires less energy (a cylinders or two side pistons moved back and forth by a wobble plate on a shaft) to suck and compress the gas. Your vehicle uses a swash plate type compressor. More efficient is a variable displacement which changes the length of the piston stroke based on upon operating pressures, these have advantages at low speed however they are more complicated and have their own 'issues'. Most efficient at the moment is a scroll type. Would you notice a difference between the swash plate and the later types? No. You'd need a dyno to see the difference or very long term mpg testing. Could you adapt the later types, yes... but very very expensive hence the cost benefit is not there.

Where could you see better 'efficiencies' overall in your system? The first place to look is the condenser. Having a condenser with debris covering the cooling fins or many bent cooling fins will reduce the 'heat exchange'. Some earlier sharks did have 'serpentine' coils as opposed to later models having tube and fin. Serpentine coils tend to be more efficient than tube and fin but it all depends upon their design (number of channels, width of the coil, number of fins per linear inch); more is not necessarily better than less, its the overall design that counts. In terms of condensers further, Parallel Flow or PF's can be more efficient than serpentine, however its based upon overall design. PF's require vertical header tubes on the sides hence they lose some real estate value in overall square area; and alike serpentine there are various widths, channels, fin designs. Griffiths offer's PF designs and if you need to replace a condenser you might want to consider the cost benefit (including your driving climate); historically we have converted 100's of sharks to R134a with stock OEM condensers without any 'efficiency' issues, we only suggest it when its needed.

Having a clean evaporator is just as important as a clean condenser, although a pita to pull and clean, if you got the time then go for it.

Now that you have some of the ac basics down, gather your thoughts, prepare your questions wisely, and ask your mechanic the facts.

Stay Kuehl!

 

Conversions get a bad rep because most are done half-fast, using an off-the-shelf "conversion kit" and no real effort to take care of all the issues involved. Start off with the idea that most "conversions" are done this way because the car doesn't cool anymore, and there isn't a ready supply of R12 in consumer-size cans anymore. All the issues contributing to the "car doesn't cool anymore" still need to be addressed as part of any conversion process. Conversion involves a new drier with a dessicant compatible with the new refrigeration oil. It includes new o-rings that are compatible with the new refrigeration oil. It includes completely removing the old refrigeration oil which isn't compatible with the new refrigeration oil. From this pont, it's OK to use a "conversion kit", which typically consists of the port adapters and a bottle of PAG refrigeration oil. You can't leave out the first part of the project, even though the big print on the conversion kit might suggest that you can; the small print on the back says you can't.

I remember reading here from folks who decided they didn't need to use a good vacuum pump to get all the air and moisture out of the system before charging. Without considering the loss of system capacity, they also found out that they couldn't get vent temps any lower that 55º. Use a cheap pump that doesn't pull enough air out, and that number might drop to 50º. Live in a hot climate (over 75ºF in the heat of the day)? 50º vent temps and a system with capacity impaired by air in the system won't cut it. It won't even work well in the winter.

The almost-myth of R-12 "cooling better" that R-134a-- It does in theory carry a little more heat, but not for the reasons you might suspect. The compressor is a pump that can maintain a specific volumetric flow at a given speed. R-134a has a slightly higher pressure at 100ºF than R-12, so the system ends up with a slightly higher suction pressure with the constant pump capacity. With that higher suction pressure comes more mass per volume pumped, almost but not quite enough to make up for the pressure difference at that same 100ºF ambient condition. The most apparent difference with the R-134a is that the higher suction pressure causes the evaporator temperature to increase by a few degrees, but that pumping capacity increase almost makes up for that in the total amount of heat that can be moved. In a properly-functioning system, the only difference is a slight increase in the compressor duty cycle, managed by the freeze switch. In real life, the cool-down time for a heat-soaked car might be slightly (minutes) longer, but the difference is almost impossible to predict because of other factors.

Lots to think about.