C4toC2

The AC is in and functioning. I set it up as stock as possible using the STV (I rebuilt it) and the original compressor.
I bought a "hose set" from corvette central. I took the evaporator box apart and cleaned out all the sticks and leaves.
Did some painting and put in new rubber insulation. Following the instruction in the 1965 manual, R134 compatible
oil went in the condenser, evaporator, and the 7 oz in the compressor. I evacuated it to 28 inches of mercury and let
it sit over night.

I put in about 47 oz of r134, it was about 88 degrees F out side, and I got about 42 degrees F at the outlet at 2000 rpm.
(This was measured by pointing my laser thermometer at the vent.)
High side pressure was 250 psi. Low side was 20 psi on the compressor side of the STV . So this was all looking
good based on my interpretation of specifications in the manual. I do believe that I need to tune my STV. Presently the adjustment
screw is all the way out and the bench test indicated it was set to regulate at 29 psi with vacuum applied. At sea level it is supposed to be
more like 31. (I have vacuum hooked up to the STV, I "think" that is supposed to be there for normal operation.)


Things I have noted -
1. It took a while to charge (2000 rpm) and even with a fan blowing at the front of the car, it got pretty hot - 220- 230 water temp.
2. That old compressor really takes a bunch of power to operate (I had heard that so no real surprise here). My idle is set at
850 RPM. When I pull the "cool ****" which turns on the compressor, the RPM drops at least 100 rpm - maybe 150.
(my tach needs work - another task for another time) Also, if I'm cruising down the road and turn on the AC, I can really feel
the extra load kick in.
3. Using the ac pushes the cooling system - at 75mph (3.36 rear end, 3000 rpm) on a 90 degree F day, slight up hill
the temp will run about 190 degrees F. If the ac is in use, the temp will increase about 10 degrees at speed - actually not bad.
However, when you slow do, with the engine hot the temp will jump to 220, and with that hot engine and the slow idle speed
it will further increase when you come to a stop - so - us it when you are moving - it needs air flow beyond what the fan can deliver.
4 The AC actually does a pretty good job of keeping the car cool. On this 90 degree day test drive, I was "cool" - My wife shut off her
vent - she was cold. The real tests of course is The Grape Vine in August.

Powershift

Some good info. Thanks for sharing.

1. The compressor requires about 12 horsepower to drive it. Yes, the RPM will drop about 150 RPM when is energized.

2. Your observations about engine temperature are what others have observed........and are typical.

3. The STV needs to be set to hold about 26-28 psig on the evaporator when using R134a. The 30-32 psig is for R12.

4. If you truly added 47 oz of R134a, you may be slightly overcharged on your system. It is probably okay, but just watch the HI pressure when the outside temperature gets to 100 F.

FWIW.

Larry

C4toC2

Thanks for tips, appreciated - I'm guessing 47 oz - it pretty much drained 4, 12oz cans and I figured some loss (an ounce or two) but did not have a way to measure it.

It is supposed to be 100 this weekend so I'm going to follow your advice and check it. I observed that as the temperature of the condenser goes up (lower air flow) the high side pressure goes up also, which makes perfect sense, so I'm assuming that I should watch the pressure with low air flow, 2000 rpm and make sure it does not exceed something like 300 psi (if I remember that was the spec for r12) .

Aaron

Powershift

If I remember correctly, the R134a charge is supposed to be about 80-90 percent of the R12 charge amount. This is due to a couple of factors. For a factory AC system that is about 44 oz of R134a (85%).

Did you check the drier sight glass?? It should be clear with no bubbles. At your 47 oz charge, bubbles would indicate air in the system. That will also cause the HI side to run higher than normal. It is easy to introduce air ito the system when using small, multiple cans of refrigerant.

Yes, stay below 300 psig on this system.

Larry

C4toC2

You were absolutely correct about the quantity and pressure, there was too much r134 in the system.
The high side was headed toward 350 psi at 98 degrees ambient and 2000 rpm – but the cooling was good.

I put a few ounces back into one on the canisters and dropped the high side pressure down to 300 – this is at 98 degrees ambient, 2000RPM and a fan running in front of the car. The cooling was still working fine. With the STV in the system, I believe the low side is regulated to 29 psi - I'm seeing about 20 psi into the compressor.

Which brings me to a question. The spec for high side pressure in the manual for r12 is 300 psi with this temperature (and some airflow). So - With r134, should I run the same pressure? Do you think this is “correct” If I pull a bit more r134 out of the system and still hold the same cooling ability?

Tiros

Since you already seem to have a gauge set, why not hook up a thermocouple and measure the Superheat / Subcooling? Without those measurements, you are just guessing. You can not set charge based on pressure alone.

You changed the drier right?

Powershift

Agreed...........but not everyone has the inclination or skill to do this and then make the adjustments that are indicated.

I recommended approx 300 psig maximum pressure based on the recommended working pressure and temperature of the hoses and other AC equipment. Exceeding this number on a regular basis will likely cause issues.

The goal is to get your maximum cooling and lowest temperatures inside the cabin without overheating your engine or the AC compressor/components or blowing the hoses apart.

Larry

GTOguy

When retrofitting an R12 system to 134, you should only use 75% capacity when recharging. This is industry standard. If it were my car, I'd evacuate it, and re-charge with the correct amount of 134. No sense risking blowing a compressor seal or worse.

Powershift

I have typically used around 80 percent.

When doing some research awhile back on this, I found that the liquid density (pounds/volume) of R134a is about 90 percent of that for R12. So to keep the same VOLUME of refrigerant in the system (and not overfill things) you MUST limit yourself to a max of 90 percent R134a compared to R12 by weight. Other differences in refrigeration properties allow even less R134a to be used. That is why a reduced amount of R134a is specified.

I have determined that at the 80 percent R134a charge amount (compared to R12), the drier sight glass in the C2 cars will not show bubbles if correctly charged to eliminate air and moisture.

Larry

EDIT: For clarification

C4toC2

Had a quick question - does it make any sense that the AC will actually work better at 2000 RPM, than at 3000 RPM. Common understanding would be more pressure at higher RPM. This is with R134, a STV set at 29 LBS.

Second quick question - smelling AC oil when I first turn on the AC, The unfortunate obvious answer is leak in the evaporator, which means it all needs to come apart. Could this be something else that my lack of experience has overlooked?

Powershift

The A6 compressor is factory rated at 2000 engine RPM. I don't know what its output is at higher engine RPMs, as I have not seen any performance curves. But I would think it may be equal or slightly better performance at 3000 RPM. At any higher RPM, it is just a guess...............the compressor is not really designed to run in that condition very long. Compressor efficiency MAY drop way off at the higher RPMs. I dunno. And if the evaporator pressure gets to low, the STV will also close off and add an additional variable and complexity into the mix.

But even if the discharge pressure does increase, the expansion valve (TXV) will simply pinch back or close off to maintain the proper refrigerant flow and evaporator superheat.

Don't try to out-think or redesign a system that has been successfully working for over 50 years. But if you MUST know more, than reading some automotive AC design manuals may help you.

If you suspect a leaking evaporator core, I suggest getting a TEK-MATE Electronic Refrigerant Detector (or equal/similar ) by buying or borrowing from an AC tech and do a check on the cabin vents. It will tell you if a leak exists. If a leak, you will also lose refrigerant and have to add back periodically. Is this the case??

I purchased one of the detectors years ago and it has proven valuable in AC work. It has helped me and many of my automotive friends.

Larry

GTOguy

Be advised that if you had a leaking evaporator core, your system would go completely flat in a short period of time. Most slow AC leaks are from hoses moving as the vehicle is operated (especially 1990-up FORD products!!)

C4toC2

This is on my 65 corvette with factory air and a STV valve. I did a little reading up on the AC - and my 350 psi on the high side was a bad thing. It seems my expansion valve was bad - it came that way new from the factory. I replace it and put 4 new cans of r134 in for a total of 48 oz. This is 4 ounces down from the 52 onces of R12 called for in the shop manual. Running 2000 rpm on an 80 degree day I have 205 psi on the high side, 29 PSI on the output of the evaporator and 20 psi at the input to the compressor. The air out of the passenger side vent is 40 degrees F. My thinking today is that this is all correct. The output of the evaporator is regulated to 29 PSI (I have STV adjusted all the way out for lowest pressure =29 psi) which I believe will correspond to 40 degrees for R134 - I need to go look that up still to verify. around 200 PSI on the high side what my AC book indicates for 80 degrees F. I assume that 20 psi on the suction side of the compressor is probably fine as long as the hight side is good and the STV is regulating. It all seems to work, so I'm call it done.

wmf62

sounds good...

Bill