R-134a vs. R-12 capacities

I am retrofitting my '74 Charger with factory air to R134. I have two cans in the system in the car, and it seems to blow cool at idle. I haven't taken it on the road to see if it will blow colder when moving.

I am concerned about adding too much 134, as I've read that only 60-80% of R-12 capacity is required before efficiency starts to drop. So how many cans of 134 should this system take? Anyone have any experiences or thoughts? Thanks!

use chart

No more than 80% of the R-12 capacity. As mentioned by the previous post there is a conversion chart online that will give you pressure readings for different temperatures. Keep in mind the R-12 systems can not handle high pressures.

Please share your results with us. I am about to be doing the same thing on a couple of new builds.

Thanks to all so far! I just checked FSM, which states that both '74 E & B body take 45 oz. of 12. So, 3, 12 oz. cans of 134 = 36 oz. which is 80% of 45 oz.

I know it is also important to keep an eye on the system pressure, but 3 cans of 134 would get you 80% of the R12 specs. Sound right as a rule of thumb?

80% of R12 capacity won't get you R134 pressures

http://www.gridgit.com/postpic/2010/01/conversion-r12-to-r134a-pressure-chart_265274.jpg

"80% of R12 capacity won't get you R134 pressures"

True. That 80% capacity is just a "do not exceed" benchmark so that when charging an R-12 system with R-134 there is no risk of over pressurizing the system.

That's why converted systems don't work very well either

You need an R134a specific condenser. Also the converted systems work much better with good air flow across the condenser. Electric fans help to keep the high side pressure down when sitting in traffic. Expansion/H valves are different too.

OK, so at about 90 degrees outside with about 60% humidity I'm getting about 55 psi on the low side. I didn't even hook up high side. It's comfortable in the car but I wouldn't say it's freezing me out. This was also at idle, so I imagine it will help when more air is going across that condenser. This is with 24 oz. of 134 in the system.

Just wanted to post my findings. Any advice and thoughts welcome!

I like to shoot for 38 degrees at the center vent

Hook up your high side and see what your reading. It's important to know what the high side reading is.

So I checked high side and it's about 200 psi. If I give it a little throttle past idle, the low dips to around 50 psi and high sticks at around 200 psi. I have no ideas what this means and still don't know if, or how much more 134 I need...

340clone I removed a complete AC system from a 73 Charger and installed it in a 72 Charger non factory air car some years ago. I did convert to R134A successfully. System worked well.
Look up December 1997 Mopar Muscle.
I removed the EPR valve and spring which is located behind the suction fitting and replaced it with an electrical cycle.
I pretty much followed the instruction in the Mopar Muscle article.
I did not have to replace the condenser or add an electric fan.

The pressures look ok. What is the temperature at the a/c vents?
Keep in mind on these R-12 to R-134 conversions you just don't get that freezing you out of the car effect like you get with the old R-12 gases.
How does the a/c system feel when driving the car? If you have a fan, face it to the grill so cool air is blown against the condenser. Sit in the car and feel how cold it is. Also, check the pressure while you do this and record the pressures. See what the differences are.
Forgot to ask; was the system evacuated before charging with freon?

I have changed many newer (1980 and 1995) cars over to 134 and as far as I remember all of them worked fine. I changed my 86 Aries K- car I used to have and I did not do much to it to change it over. At first they said you had to change a lot and now today you can just empty the R12 and pull the system down and add the right oil and then add the 134. That's about all the kits give you today. Heck when they first started to change some we got a 5 page manual on all the things they said to change including the compressor and drier or suction accumulator. I have used the kits of today and changed no hard parts at all and had the systems work fine. Course I have not don't any 60's or 70's cars that used the EPR valve and V comp. One thing I also notice is the 134 will get pretty cold but it usually takes longer to get real cold the the R12 system did. Ron

I couldn't get an accurate reading with my IR thermometer, and I don't have a vent thermometer so I'm not sure wnat vent temp is.

Haven't driven it yet but feels comfy, but not freezing cold at idle. (Being close to 95 degrees here in Louisiana doesn't help much!) Will need to drive it, but good idea about the fan. I will also try that.

System was rebuilt about 10 years ago but never filled. I sold the car then, and recently bought back from guy I sold it to. He never filled either. Rebuilt compressor, new evap, new hoses, bought new filter/drier this week. Condenser was not replaced, but removed and thoroughly flushed with lacquer thinner. I probably should have gone back with R-12, but since I didn't have any and since I thought I had a good clean slate I'd go with R-134. I do have some Freeze-12, but I hear it's mostly R-134 anyway and like R-12 it's so expensive if you ever need to add cans...

After reading about EPR valves, etc. I'm now wondering if I'm going to have problems since mine was not removed? What exactly will happen if EPR remains?

Also wondering now if I should evacuate system again and drop in Freeze-12? Would I have to flush all or part of system? BTW, I filled the system with about 7-8 oz. of Ester oil before I added the R-134. They say that Ester is compatible with either 12 or 134 oils. So what about the oil that was in the 2 cans of 134? Would this cause a problem if I went back to Freeze-12? Or, maybe I should just let it fly as is?

Thanks for any and all help! Sorry for all of the questions. Not really well versed in auto a/c and no one wants to fool with anything this old around here.

Drive the car around and see how the cooling feels with the R-134 and go from there.
As far as the Freeze 12; yes, I would vacuum the system before filling.
Check and see if the oils in Freeze 12 are compatible with the ester oil you put in when you were doing the conversion to R-134. If not, flushing needs to be done.
At this point before considering the use of Freeze 12, ask others who have used it. There are a/c forum boards that you can get good info from.
It's alot better than trying this and trying that in order to get the desired results with you a/c system.

The EPR valve is the evaporator pressure regulator valve. Basically it controls the evaporator pressures so it don't freeze up. In AC systems the pressures work right with the temps of it. Lower pressure in the AC system means lower (colder) temps and vice versa. In other words if the evap press gets to low for to long and hangs below about 28 psi the evap can start freezing up on the fins. Keep the evap press about 32 and slightly above and you get cold temps and no freezing evap.
The Fords and GM cars used the same type of valve that they called the Suction Throttle Valve. Most all newer cars have gone away from that system and have gone to cycling compressors to control evap freezing which will turn the comp off if the evap press gets to low and the newest cars may also use variable displacement compressors to control evap freeze up. Some also went away from the exspansion valves to Orfice tubes to control Freon flow to the evap and control the high to low pressure sides. I will admit that all the cars I have switched from R12 to R134 were a bit newer then the cars that used the EPR valve as most or all the ones I have changed were cycling systems that cycled the comp on and off to control evap pressures. So I cant tell you how the R134 acts in that system but some of the others on here seem to have changed that older system and might help you. Good luck with it. Ron

I agree-good plan. I have yet to actually drive it so it may do alright and not require any further action.

The Freeze-12 use has been long debated. I have heard good things and bad. I have actually added it to R-12 in a 1981 GM system with good results, but I can't say if that was a fluke or not.

The Freeze-12 is compatible with the Ester oil but my concern is whatever oil would be left over from the R134. I'm not sure if it is PAG and if so, how much would you get into the system with two cans of refrigerant? Ester is compatible with both PAG and mineral oils, but not sure about the outcome if the Ester had a little of both mixed in?

Thanks! I had forgotten about the suction throttle valve in the GM's. I think you are right that most of the cars that were meant to be retrofitted were a little newer than what I am dealing with.