About three years ago, I was putting the finishing touches on my 1968 corvette restoration project. The project had started with a pile of boxed up parts, a bent frame and a long list of missing or useless parts and ended up with a largely accurate, original car based on the tank sheet from the factory. The positive piece of the puzzle was the fact that the major parts of the car were "numbers matching" including the engine, transmission, rear end and the interior as was the body from the windshield back including the original spare!
Back in the mid sixties, air conditioning wasn't standard equipment because it was so expensive. On this particular corvette, the base price was $4663 and air conditioning added an additioal 10% - $412 more! The only options that were more expensive were leather seats and a super-rare engine option (the fabled ZL-1). A little over half of '68 vettes had factory air. Mine is one of them.
As is well documented, the C-3 corvette (1968 being the first year of the third generation) experienced air flow issues through the engine compartment. The factory recalls included literally cutting holes in the nose of the car to create a bigger opening in an effort to get the heat moving through and out of the engine compartment. The idea of proper air management had fallen second to the exterior styling. In '69 they went to a larger radiator when they realized that cutting holes wasn't enough!
As if all of those issues weren't enough, the air conditioner condenser (the "radiator" in front of the radiator) generates considerable heat when in use, effectively pre-heating what little air was moving through. Once it became obvious that cooling was an issue, it wasn't especially uncommon for the a/c system to be removed altogether. Come on, GM.
Being the first year of the C-3, there were more than a few parts that are unique to the car, with a bunch belonging to the air conditioning system. These parts include the muffler for the compressor as well as a crazy system of hoses clamped to fittings with O rings going into and out of the condenser, dryer, compressor, TXV, evaporator and POV valve. Tracking down these parts proved to be a challenge and assembling these without any leaks was even more so.
Finding the last leak proved challenging. I would vacuum the system and see that it would hold indefinitely. I would then charge the system and voila, two weeks later, the R-134 charge would be low again. This proved to be a schrader valve in the charging port (which was sealed up when the charging hoses were hooked up)! Once the system was assembled and leak free, getting cold air from the vents was expected to be fairly straight forward. Oh contraire, mon frere. The first time the system was charged, the vent temps were around 50 degrees at speed and around 60 degrees sitting still. Not very good. A quick call to the Classic Air hot line suggested that 50 degree air at speed was as good as you could expect from this car. Bah. Not good enough for me. Thus the saga begins.
The first thing that I found to be unacceptable was the A-6 compressor. Mine was a remanufactured unit from a reputable corvette restoration parts company. The compressor was leaking oil out the front seal and slinging it around the engine compartment. As it turns out, this is by design! The original seal is ceramic and requires lubrication in the form of the a/c lubricant (PAG oil for R-134). The seal can be replaced with a modern "lip" seal to resolve this so I opened up the compressor and replaced all of the seals, some of which were so old that they were hard and cracked - lesson learned - re-manufactured doesn't mean much.
With a freshly sealed compressor I turned my attention to the excessively high pressure at the compressor output. This was eventually determined to be due to a lack of air flow through the condenser. If I put a running garden hose on the condenser, the high side pressure would drop precipitously and the vent temps would cool down along with it. Back to a focus on air flow. I added an electric fan that cycles on with the compressor which gave me some air movement while stopped but the vent temps were not cold by any stretch.. I also tracked down and sealed off every opening between the body and the radiator and the condenser. Now, no air can do an end run on the radiator and most of it passes through the condenser as well. The "experts" also suggested that I replace the condenser with a newer technology, double pass, high efficiency, new millennium xr 10,000 type but there is none available as a direct replacement for a '68 vette. Ugh.
The mid sixties general motors air conditioning systems used a continuously running compressor approach with two valves to control the flow of the refrigerant through the system. So here's how it works - the compressor pumps up the system which causes the R-134 to get really hot. The hot gas then flows through the condenser which cools the hot gas to the point of condensing into a liquid. The liquid is then pushed along to the TXV or Thermal Expansion Valve which either diverts liquid refrigerant into or around the evaporator (the thing that is supposed to get cold). So far, pretty typical of how systems work today. But enter the POS, I mean POA valve. The POA valve, the Pilot Operated Absolute valve is a vaporized gas valve.
The POA valve is connected between the evaporator outlet and the inlet to the compressor. It's function is to regulate the pressure inside the evaporator regardless of inlet temperature, pressure, air flow over the evaporator, ambient temperature or any other influence. In other words, the POA is the key to getting the temperature of your evaporator as low as possible without freezing up. So far, I haven't had any problem with the evaporator freezing up ;-).
According to most everyone that has anything to do with classic automotive air conditioning systems, the POA valve is not adjustable. At the same time, the classic air conditioning people tell you to send the POA to them and they can make it work with R-134. Seem confusing? That's because the device actually is adjustable, just not adjustable after it is installed on the car. And because it regulates the pressure inside the evaporator, it has everything to do with how cold your vent temps get - if the pressure in the evaporator is too high, the refrigerant never "un-compresses" to a gas which must happen to make "cold".. So... If you lower the evaporator pressure, you stand a really good chance of making colder air. The problem is that the system must be opened up (drained) in order to make the adjustment.
Typically, the POA regulating pressure is set too high, keeping way too much liquid inside the evaporator effectively making your evaporator smaller (it's half full of liquid!!). The manufacturer errs on the side of too high because too low causes the system to freeze up. Either way, you don't get diddly for cold air. An adjustment of 1/8th to 1/4th turn on the POA adjuster makes a change in vent temps of up to 20 degrees! With this being the case, it's not hard to see why the "factory" setting might need a tweak since it is a mass produced part used on many different cars.
Before knowing any better, I sent my POA to one of the industry "go-to" businesses for this service. They didn't really get it quite right but I didn't know it at the time. I have since found out that you can adjust the POA with a relatively easy to build bench rig.
So here's the final solution to getting your AC as cold as possible: You have got to first see what vent temps you have, then adjust your POA and refrigerant charge based on how much you need vent temps to go down.. Assuming that all of the other components in your system are in good working order, adjusting the POA and charge is the key. You screw the adjuster "in" (down) for lower pressure.
Here is a picture of a test rig. There is an adjustable screw with a jamb nut deep inside the valve. The ratchet with the extension is in place to adjust this screw. The low pressure gauge is hooked to the side of the POA to monitor the regulated set point. Shop air is applied to the valve inlet (in this picture, the air goes through the TXV and evaporator first). As the POA is adjusted, you are adjusting the pressure inside the evaporator. The target pressure is right around 28 PSI. Some report that 27 or even 26 works in their system. 29 or higher will give you really poor performance with R-134.