Automotive Air Conditioning Information Forum

On my 79 DeVille with an LS I'm not getting enough airflow over the stock condenser and it seems like it is not good at getting rid of heat. I'm running a V7 variable compressor and orrofice tube. The system cools great at speed, but high side pressure is still on the high side at 195ish on a 75 deg day.
At standstill it quickly goes to 240 even with all 4 fans on high. All this is with R12.

I was thinking of adding a small parallel flow condenser after the old school tube and fin condenser. There is plenty of room of to the side.
I'm somewhat concerned if there are going to be issues with liquid flowing into the second condenser and if a small ish parallel flow condenser will be a flow restriction for a large compressor like the V7

On the surface, one might think that your inquiries are fairly simple. Speaking as a non-MVAC engineer, I believe they're more complex.

As for your question about adding a second condenser, that scenario has been discussed on this Forum within the past couple of years. If I recall correctly, doing so was not recommended. What was recommended was to install the largest single parallel-flow condenser that you can fit.

Your '79 DeVille originally had an A6 compressor, possibly a POA valve, and a TXV—correct? And now you have a kind of hybrid system with a V7 variable-displacement compressor, a white 0.072 Orifice Tube (?), an accumulator as opposed to a R/D(?), and still running R-12.

As originally conceived, an OT system (that has no feedback loop like a TXV system does) was a cost-savings compromise that wastes cooling capacity in the accumulator, which stores any excess liquid refrigerant before being boiled off by engine compartment heat, thus protecting the compressor from "trying" to compress the liquid.

In an OT system, liquid refrigerant is delivered to the OT, and the orifice is sized to provide the correct refrigerant flow for the maximum cooling load.

At low cooling demand, the V7 reduces its displacement and hence, its refrigerant flow to the OT, and all is well and good. However, under maximum cooling load (at low speeds or at a stand-still), the V7 also increases refrigerant flow to its maximum. But the OT's small, fixed orifice size restricts the refrigerant while increasing the high-side pressure.

Installing a smaller-orifice OT will increase the high-side pressure, but it will also reduce the amount of heat transfer in the evaporator, thus reducing overall cooling performance.

In my humble opinion, to improve low-speed or stand-still cooling performance given your existing compressor set-up will require a two-pronged approach: Replace the condenser with a parallel-flow type (go as big as you can fit), and replace the OT with a TXV.

As far as I can tell almost all variable compressors use an orrofice tube so there are not 2 things trying to control the system. Since 77 there is no POA valve, so no control issues from that. The system has an accumulator on the low side but no receiver drier on the high side, just like factory.
I used a .072 tube to get colder temps and slightly reduce condenser load. Return gas from the EVAP is just around freezing with low airflow after a highway drive. The variable compressor itself works great.

Do you by any chance have a link or a general time frame when the dual condenser question was discussed?

The search function on this board is quite good, try it. The issue has been discussed several times

I'm not an MVAC historian, but if I recall correctly, GM developed and began installing OT's in their vehicles primarily as a cost-cutting measure, and sometime in the 2000's, began phasing-in mechanical (as opposed to electronic) Variable Displacement Compressors (VDC's). They also "played around with" Variable Orifice Valves (VOV's) for a while, which proved to be a very poor substitute for TXV's. Apparently, GM dropped that venture and went with just the fixed-orifice OT's.

GM also eliminated Suction Throttling Valves (STV's) in the 1960's, Valves In Receiving (VIR) units in the 1970's, and Pilot Operated Absolute (POA) valves around the same timeframe. Then Ford, Chrysler (Stellantis), and some other manufacturers picked up on OT's and began installing them.

To my knowledge, Toyota, BMW, Nissan and a few others have never used OT's, favoring TXV's instead, which have an extra feedback loop that OT's lack, as I mentioned. In my opinion, TXV's are superior to OT's in almost every way, but the tradeoff is that TXV's cost more. Nevertheless, TXV's superiority is why I recommended retrofitting a TXV in place of your OT.

Arguably, Toyota currently has the best MVAC systems of any manufacturer, they've used TXV's in conjunction with electronic VDC's for quite a while, and haven't had issues with too many control points (that I'm aware of).

V8Power wrote: Thu Jul 17, 2025 5:34 am The system cools great at speed, but high side pressure is still on the high side at 195ish on a 75 deg day. At standstill it quickly goes to 240 even with all 4 fans on high. All this is with R12.

Primarily, it sounds like a condensing issue. To verify, mist the condenser with a garden hose while watching the high-side pressures. If HS pressures drop, then the water-mist test would seem to confirm it.

Replacing the stock condenser with a parallel-flow type will probably help it some. Also, ensure that air can't get around the condenser instead of going through it. If there's a gap between the condenser and radiator, you can fill it using gray foam insulation from your local home center. Also ensure that all baffles and/or fan shrouds are in place.

I don't have a link pointing to where a dual-condenser setup has been discussed, but try the suggestion by tbirdtbird and see what turns up.

If GM Tech happens to see this thread, maybe he can provide additional background about GM's development of the OT.