I am installing R134 in an old AC system on a 73. What weight PAG oil is recommended with the stock A6 compressor and R134?

 

I'm sorry, but I have to ask: Why would you change over to 134a if you have a completely stock R-12 system? That system is not calibrated for 134a and will not work nearly as well as the stock system with R-12 refill.

If I wanted to change to 134a, I would want to upgrade/change the system as needed to support using that newer refrigerant.

 

GM A6 compressor is usually PAG 150 but you need to follow the recommendations of the compressor rebuild/ manufacture

Neal

 

The system has been open empty and open several years. I am replacing the VIR with the replacement as the factory unit is old and corroded.

 

7T1vette please explain "not calibrated" as r134a is a direct replacement for r12 and is more efficient. I do agree that to take full advantage of r134a you need to change to a parallel flow condenser, a new condenser is way cheaper than purchasing r12.

Neal

 

If you are replacing the VIR with an orifice tube/safety switch meant for 134a, it will work. If I went that far into it, I would (and I did) convert over to a much more efficient Sanden compressor. But, I understand that different folks have different priorities. If originality is a big deal, maybe doing what you are doing makes sense. VIR wouldn't be original, though.

Best to you with your project.

 

R134a is a replacement, but not a direct replacement. The C3 cooling system was designed with very little reserve capacity and R134a is about 15% more inefficient than R12 by volume. Changing the Condenser to a parallel flow will help.......but it will never cool quite the same a an R12 system.
I use R12 in old cars when I can if some components of the system are retained and in Corvette's.....but for something like a 72' C20....R134a is fine but only if the whole system is changed out......I WILL NOT flush and convert a system anymore because I cannot guarantee it will not have issues.

Jebby

 

Just a question for the experts: If you maintain the correct high side and low side why is one compressor cool better? Now lets leave weight out because it's maybe 15 lbs as the old A6 is about 37 lbs. Just curious.

Dom

 

Each compressor has its own "capacity". There are multiple sizes of Sanden compressors. I selected the largest size (and the largest condenser) when I put A/C in my '65 Grand Prix.

 

Jebbysan, r134a is more efficient than r12 and is very close to the same cooling characteristics (pressure/temp relationship). As it is more efficient it requires less refrigerant (volume) to meet the requirements for the r12 system as listed on the placard.

The pressure / temp relationship is close to the same between r12 & 134a, within 2* at system operating pressure at the evap coil (at 28 to 30 psi) but the head pressure is going to be higher at the same suction pressure which is why I said to go with the parallel flow condenser to take full advantage of the swap.

R134a will absorb more heat at the evaporator given the same conditions when compared to r12,

Neal

 

Dom, you really have to look at the ci of the compressors, they are not all the same

the system don't care what compressor you have as long as they pump the same volume at a given rpm then there will be no difference in capacity.

Neal

 

Neal,
I'm not a system man is why I was asking and your explination is a good one. Being that there is not a system by-pass pressure valve that I know of, you would think the high side would run away with high RPM. A regulating valve like ones used in hydraulics. engine oil pump.

Dom

 

with a fixed orifice the suction pressure also drops and causes the compressor to cycle at high compressor speeds

with OD trans and highway RPMs as low as they are the compressor speeds have come down over the years

there are variable orifice tubes that work fairly well to help with evap coil temps/pressures which can vary a good bit caused by changing RPMs of the enigne

Neal

 

Neal,
Great I will try to digest that. As I said I am not a system person. I appreciate your system knowledge. The A6 should not cycle the compressor speeds or it will ruin the clutch and spit oil from the front seal. It was not designed to cycle at high RPM speed, or other.
We would just turned it off when evap was frozen and let the ice drip. Also no recirculate on early A/C models. All old A/c was outside air and not recycled air.
Help me with this! I only do A6 compressors. With all respect, the older systems were human operated, and the new systems are human proof.I would like your advise on that as we have problems with old compressor clutch's failing and compressors loosing front seals because of miss-alignment of the clutch causing miss alignment of the ceramic seal when the clutch is out out of center when engaged.
Need to pass help for C-2 guys!! engaging clutch @ high RPM will cause shaft to miss-align and cause oil @ gas to escape. We need to think back to the 60's.Your expertise is welcome. Modern switches take care of drivers that do not know the system now. These are C2 guys that have been driving A/C cars automated and may not read the manual.
A/C has improved over the years, but the old systems need manual use.
Help!!

Dom

 

While the clutch does not interact with the front seal on the A6 it does and can exert forces on the front crank bearing which if those forces are substantial can cause enough rotational movement to cause premature seal failure. Another bearing to look at closely is the belt pulley bearing, if it has any play it will exert off center forces on the crank through the clutch.

Generally there is some form of evap coil temp control built into the system which will either raise the pressure in the coil (POA valve, etc...) or will cycle the compressor clutch (to prevent the coil from freezing).

If the coil is freezing then there are other issues that need to be fixed to prevent it.

The A6 compressor was used into the early 80's which by that time most all vehicles had been converted to fixed orifice and clutch cycling so the design of the A6 included clutch cycling and the ability to operate at or near 6500 rpm's without overheating.

Also remember that when the refrigerant flow increases the evap temp will drop as this temp drops it will also cool the discharge temp which will cause a drop in the discharge pressure of the compressor (this is if the system is operating correctly and there are no other issues).

Neal

getting a little off topic of the original question, sorry

 

After doing some more research....you are correct......but ONLY if you run a different condenser, which to my knowledge...there is not a drop in parallel flow condenser available for C3's, but I could be wrong. So in effect, if you just do the conversion with no other changes, it is less efficient, but the refrigerant, pound for pound is actually more efficient.
Learn something everyday......thanks

Jebby