When the whole R-134a situation was on the horizon and the upgrading to it was being discussed, I consulted with a friend of mine (an ex-Chrysler parts man) that worked for an a/c parts manufacturer. He said that the Chrysler RV-2 compressor (and I suspect the earlier RV compressor it evolved from, plus the other compressors used by the main stream American manufacturers in the middle '60s or so) were plenty stout to handle the added pressures of the new refrigerant. No need to change them for some of the newer Sankyo or similar compressors.
I concur that most everybody's factory a/c system on the bigger passenger vehicles, from about '65 on, were designed with enough capacity to keep everything nice and frosty in the hottest southwestern USA weather. I recall that one maker designed their systems to handle a 110 degree F day in Phoenix, sitting idling in traffic.
Although the compressor designs did improve, the key to the added cooling capacity was the huge condensors (with more fin/inch than the earlier models) they were using. If you look at some of the later '50s condensors, they don't have nearly as many fins/inch and resultant heat exchanging capacity as the mid-'60s condensors did.
One thing that has been noted about R134a conversions is that there is sufficient air flow across the condensor at low speeds. If the pressures go higher than normal, just like the R-12 systems, cooling performance deteriorates until sufficient air flow is restored (by vehicle forward movement or an extra electric fan kicking on).
In about '73 time frame, Chrysler issued a service bulletin regarding a/c performance. In certain conditions, it was possible for the hot underhood air to "recycle" and go back through the condensor and radiator. When that happened, the head pressure went sky high and the a/c hoses had popped. The cure was to put thin, die cut rubber panels in front of the condensor (on the sides between the headlight supports and on the bottom between the grille and the lower condensor; there was already a "yoke seal" that sealed between the hood and the core support) such that only "normal" air could enter the grille area. They also added a "Thermostatic Ignition Control" valve via a new thermostat housing. When the engine got too hot at idle, it'd move the vacuum advance vacuum from ported (no vac advance at idle) to full manifold vacuum (so the engine rpm would increase and the added fan speed would help cool the motor and the condensor down some).
One other thing my friend noted was that all of the old oil needed to be removed from the system. It would react with the new R134a (and it's different oil), congeal, and find the lowest spot in the system to settle out. That usually was the bottom of the existing condensor, further reducing it's heat exchanging capacity. I knew then that modern chemistry would result in more universal and more tolerant oils, but I would suspect that the best thing to do would be to, as he suggested, flush the system via the recycling machine with liquid freon to get all of the old oil out, then refill everything with the appropriate freon and oil. I know others have reported good results without going that far, which is good.
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