I was planning on writing this week about installing the evaporator into the ’79 Euro 635CSi—I see you shiver with antici…pation!—but I was temporarily derailed again by the compressor.

Here’s what happened. After I’d finished mounting the condenser and fan in the car’s nose, I reinstalled the radiator and refilled the cooling system so that I could move the car from its hallowed spot on the mid-rise lift in the garage. But then, I thought, there is one more thing I should do before I take the car down off the lift: finish installing the compressor.

If you remember, a few weeks back, I had test-fit the Sanden-clone compressor on the adapted wing-cell bracket and found that I couldn’t slide in the pivot bolt from the rear because the hex head of the bolt hit a mounting boss on the block, and I couldn’t slide it in from the front because the hex head hit the compressor’s pulley. I solved the problem by grinding a side off the hex head sin order to clear the pulley, but I agreed with someone’s comment that an Allen bolt might be a better choice, so I ordered one just in case. I said that with the pivot-bolt problem solved, the final compressor installation was waiting on just one thing: my draining out the R134a-only oil that the compressor came with and re-filling it with the correct oil for my choice of refrigerant.

With new oil in the compressor, I raised it up to the bracket, tried to put the pivot bolt through the front, and had an unpleasant surprise: I could no longer get the bolt through the front because the hex head now hit the newly-reinstalled radiator. Damn.

I tried the Allen-head bolt that I’d ordered, installing it from the back, figuring that the smaller head might clear the boss on the block. It didn’t. Double-damn.

I sat there under the car, thinking that I could drain and remove the radiator; that I could remove the compressor bracket, install the compressor on it, and mount the bracket and compressor as a pair—but I wasn’t sure if all of the mounting bolts had sufficient clearance for me to do that—and that this was waaaaaaay more trouble to go through than it was worth for a freaking pivot bolt. Next time, I’d either use Layne Wylie’s bracket, or pair the wing-cell bracket with an actual wing-cell compressor instead of a Sanden clone.

Then I tried one more thing: tilting the front of the compressor down, putting the bolt through the front, putting the spacer on the bolt, then tilting the whole thing up and sliding the bolt through the bracket. It worked; with the bolt partially slid through, there was just enough clearance, between the bolt head and the radiator. So the compressor is finally mounted.

I did not get the evaporator installed, but having deal with the compressor oil, I’ll talk about that and the choice of refrigerant instead.

There are several factors that affect a/c performance. Among them are condenser size, the amount of air moved by the condenser fan, replacement of the old piston compressor with a modern rotary-style unit, the amount of air moved by the evaporator fan, adjusting the thermostat to reduce compressor cycling, and reducing hot ambient air entering the cabin.

But when dealing with a/c in vintage cars, the choice of refrigerant may dwarf all of these.

Now, I am neither a chemist nor a licensed air-conditioning technician, and the topic of refrigerant can fill far more than this one column, so you should take all of the following with a bright, flashing “The Hack Mechanic’s Opinions On This Matter May Be A Load Of Do-It-Yourself Non-Professional Malarkey” sign attached—but this is a nutshell of the refrigerant issue, or at least my understanding of it.

Nearly all refrigeration and air-conditioning systems function on the principle that a gas cools when it expands (more specifically, when the refrigerant in its fluid state boils and turns into a gas). We’ve all experienced this when we feel the propane bottle attached to a barbecue, or a can of paint being sprayed; this is why it’s cold to the touch. An air-conditioning system uses refrigerant that alternates between liquid and gaseous states; the aptly-named compressor compresses the gas so that it can then expand in the evaporator and cool. A fan blows over the cooled evaporator pipes to send cold air into the cabin; the condenser dumps heat to the outside.

In order for the refrigerant to work, it has to have certain requirements, such as a low boiling point. Some people actually use propane as a “redneck refrigerant,” but it is flammable, so add to the list of requirements that the refrigerant, combined with a leak and a stray spark, shouldn’t kill you.

Starting in the early 1930s, the refrigerant of choice was a chlorofluorocarbon (CFC), specifically dichlorodifluoromethane, known as Freon or R12. Its lack of flammability and toxicity resulted in its being used in many refrigerant and air-conditioning applications—until it was discovered that the widespread use of Freon and other similar chemicals appeared to be degrading the Earth’s ozone layer. By international agreement in 1987, use and production of CFCs such as R12 began to be phased out. I believe that automotive manufacturers were required to stop using R12 by the end of the 1994 model year, although the exact transition point varies not only by manufacturer but by model. Production of R12 ceased in 1996, although it can still be reclaimed, purified, and resold, and small cans of New Old Stock R12 are still widely available on eBay and Craigslist.

In the automotive world, the replacement for R12 was R134a.

You can spend long evenings reading about the differences between the two refrigerants. As I understand it, the molecular size of R134a is smaller than R12, so there is some concern that it can leak out through a car’s original non-barrier hoses and through the flare fittings often employed on old R12 systems. R134a systems run at higher discharge-side pressures. The two refrigerants use different oils (more on that below). Somewhat counterintuitively, you use less R134a than you would R12.

Most important, bottom line, R134a does not cool as well as R12.

As R12 was being phased out in the early 1990s, there was a lot of conflicting information regarding whether you needed to convert an R12 system to R134a. Initially, the cost of R12 spiked to $100 a pound and higher; since R134a was around two bucks a can, conversion seemed to be the thing to do, but there was no agreement on the steps necessary for successful conversation. At one end of the spectrum was the intimidating and expensive recipe that said that the compressor and the expansion valve had to be replaced with R134a-specific versions; the hoses all had to be changed to barrier hoses; and any flare fittings or black O-rings had to be replaced with R134a-compliant green O-rings, or they’d leak; and the evaporator and condenser had to be flushed to remove any traces of the old oil prior to adding the correct R134a-specific oil. Of course, the receiver/drier had to be replaced, as it does any time you open the a/c system up.

At the other extreme were the inexpensive kits sold at auto-parts stores, containing just R134a charging fittings and a bottle of oil you were supposed to dump in without any flushing. These were derisively called “death kits” by refrigeration professionals, as they had a reputation for causing the compressor to seize within a few months.

Over time, a credible middle ground emerged, in which as long as the system had been operating and wasn’t leaking refrigerant, you could drain the compressor of its old oil, flush the system in place (meaning the evaporator wasn’t removed), refill the system with new oil, and change only the receiver-drier. Of course, the magic words are “as long as the system had been operating and wasn’t leaking refrigerant.” But if that were the case, who in their right mind would go to the time and expense to convert a tight, functioning R12 system to R134a?

Thus we see the problem: R134a conversion became something that was often done to a car whose a/c was working marginally, if at all.

Really, the issues of conversion and performance need to be separated. If an a/c system isn’t working, it has to be diagnosed. The overwhelming majority of the time, the cause is that most or all of the refrigerant has leaked out, in which case the source of the leak must be located and fixed. If you’re lucky, it’s a leaking connection; you tighten it or replace one bad O-ring, and bam, it’s repaired, and it can be recharged with whatever refrigerant it had in it.

But you’re rarely that lucky. Most often, there’s a major failure in some component.

Now, in truth, with the changeover to R134a 23 years in the rearview mirror, there aren’t a lot of cars still running around with R12 in them, so demand is much lower. Today, new-old-stock cans of R12 sell on eBay in the $25 to $30 range (and nothing verifies “new old stock” like an original-price sticker from Checker Auto Parts for $1.69). You technically need an EPA 609 certification to buy R12, even from a private party, but really, no one cares; and besides, certification is easily available by taking an online exam.

More to the point, have you bought a can of R134a recently at Autozone? It’s about fifteen bucks. So if you do your own air-conditioning work, the price difference between R134a and R12 is only a minor consideration.

However, in terms of environmental hazard, R12 is an ozone-depleting agent. R134a is not, but it is a greenhouse gas. In either case, whether you’re a professional or a do-it-yourselfer, you’re not supposed to discharge either R12 or R134a into the atmosphere; you’re supposed to use recovery equipment to capture it; there are, I believe, potential five-figure penalties if you are caught venting refrigerant. If a system is leaking, you’re not supposed to simply “top it off”; you’re supposed to recover the refrigerant, find the leak, and then recharge it.

Because separate recovery equipment is necessary for R12 and R134a to prevent cross-contamination, and because it’s now been 23 years since a new car had R12, fewer and fewer service stations maintain R12 recovery equipment, so it’s harder to find places that will work on R12 systems, even if all you want them to do is capture yours so that you can work on the car’s a/c with a clear conscience.

With that background, in my opinion, when choosing which refrigerant to use for an a/c retrofit or rejuvenation, the primary factor should be the desired performance in the environment you plan to drive.

While new a/c systems designed for use with R134a perform very well (my E39’s a/c is great), on cars like 1970s-era BMWs, whose a/c was marginal at best even when they were new—as Mike Miller once said, a 2002’s air-conditioning was like “a hamster blowing on a snow cone”—there is the real possibility that you can go to the time and expense of R134a conversion, and even with a rotary-style compressor, a parallel-flow condenser, and a big cooling fan, not be happy with the results. If you are considering an R134a conversion, I strongly recommend posting the question to web forums specific to your car. (“Hey, anyone in Phoenix convert an E30 to R134a? Are you happy with it?”)

Having done six retrofits and rejuvenations on 1970s BMWs, half of them using R12 and half using R134a, I can say this: It isn’t even close. You turn on an R12 system—I’m talking about one that I’ve updated with a rotary compressor, a parallel-flow condenser, and a big fan—and it blows cold almost instantly, and gets so cold that the biggest problem is that your right knee is in danger of frostbite. Seriously.

In contrast, on the R134a systems I’ve done, even with the updates, once they’ve run for a while, they get adequately cold, but not freezing—as long as the temperature and humidity don’t hit the upper 90s. If you’re going to be daily-driving a car in a wicked-hot (or worse, wicked-hot and disgustingly humid) climate, and if you can either deal with the a/c service yourself or find one of the shrinking number of places that service R12, go with R12.

Note that because R134a is a greenhouse gas, there is talk of phasing that out in vehicle a/c applications, too, possibly in the early 2020s. One frequently-mentioned potential replacement for R134a is R152a; this is what comes in canned compressed air used to blow out computer keyboards and other electronics, sold commercially with names like Duster and Dust-Off (ironically, R134a used to be sold the same way). You can find many intriguing links on R152a, including how it is a “drop-in replacement” for R134a—that is, that it works with the same oil—as well as scientific papers claiming that it cools better than R134a, but not as well as R12.

It is, however, flammable.

You’d think that would rule it out. But then again, you can find articles about how, although R12 itself is not flammable, R12 with mineral oil dissolved in it is—and how, when it burns, it produces deadly phosgene gas. Remember World War I? Not good.

Let’s talk specifically about refrigerant oil. Any a/c system needs oil to lubricate the compressor’s moving parts. Unfortunately, the oil won’t simply stay in the compressor; it winds up circulating around the system along with the refrigerant. Thus the oil has to be compatible with the refrigerant, dissolving in the liquid refrigerant and being carried as a fine mist in the gaseous refrigerant. R12 systems use mineral oil because R12 is a CFC, and mineral oil binds with the chlorine atoms. When the new refrigerant, R134a, was introduced, mineral oil wasn’t compatible with it—it wouldn’t dissolve in it and be carried through the system. Thus, along with R134a came a new oil: polyalkylene glycol, or PAG oil. This is the type of oil found in the a/c system in most new cars, and which is shipped inside most replacement compressors.

The problem is that PAG oil is hygroscopic—it absorbs water—and R134a reacts with water to form hydrochloric acid. This, needless to say, has a damaging effect on the compressor and other components. Any contamination of the system, either through use of an open container of hygroscopic oil or a bad seal on an a/c component, can cause the system to self-destruct. (Jumping ahead to a later installment, this is also one of the reasons why it is imperative that an a/c system be evacuated—pumped down—so that any moisture in the system is boiled off.)

There is another commonly-used type of a/c oil: polyol ester oil, or POE. Ester oil has the advantage that it binds with both R134a and R12, so its use was advised in retrofit applications where there may be some trace amounts of R12 left in the system. However, like PAG oil, ester oil is hygroscopic, so when it’s used in an R134a system, you have the same problem with acids forming if the oil has been left open to the air.

So, to come full-circle to the beginning of this piece: The only thing preventing me from permanently mounting the compressor on the bracket was filling the compressor with the correct oil.

The compressor came new filled with PAG oil, making it suitable for use only with R134a. On my tii and my Bavaria, both of which I knew I’d be charging with R12, I filled the compressors with good old-fashioned mineral oil. On the Shark, I’m still hedging my bets.

My oil of choice was ester oil. I drained the PAG oil out of the compressor, poured in a few ounces of ester oil, rotated the compressor to distribute it inside, drained it again, then filled it with about six ounces of ester oil, and mounted it permanently on the engine.

So, yeah—this is a bit of a shaggy dog story. I titled it “The Choice Of Refrigerant And Oil.” I chose the oil, but I haven’t actually chosen the refrigerant. With ester oil in the compressor, I can use R12, R134a, or even the dark horse in the race, R152a. I’ll make my final decision in the coming weeks.

Hey, it occurs to me that there is an upside to the potential flammability of R152a: The system could function as an a/c and as a heater!

(Next week: The evaporator. For sure. I think.)—Rob Siegel

Rob’s new book, Ran When Parked: How I Resurrected a Decade-Dead 1972 BMW 2002tii and Road-Tripped it a Thousand Miles Back Home, and How You Can, Too, is now available on Amazon. Or you can order personally inscribed copies through Rob’s website: www.robsiegel.com.