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Inefficient R134a compared to R-12?

16 August 2022
I just bought a '91 with 83K and a sticker indicating a R134a conversion was completed. While conducting a PPI with the car sitting inside a service bay (i.e. in the shade in a conditioned space) I measured the temperature drop with an IR thermometer with the A/C running to be 30 degrees less than ambient at the center vents. I was told the HVAC unit had the electronics repaired, and it didn't seem to malfunction in any way. However, upon driving the car home in record MN heat (91F on Tuesday!), with the engine adding to the cabin heat load, as well as the solar gain of the black roof, the HVAC couldn't keep up at all. I was sweating and dehydrated while driving. My Honda engineer friend said R-12 is a better refrigerant, so the NSX system designed around R-12 may just be undersized running R134a. What say you? Is this normal performance or is my system ailing?
R134A has different characteristics and operating pressures. I think you may end up with slightly less weight of refrigerant in the conversion which may reduce heat transfer capacity in a system designed for R12 (what your friend was referring to).. I have a 2000 so I haven't had to deal with the refrigerant issue on the NSX; but, I did have another 1987 vintage vehicle that went through the conversion around 2000. There might have been a slight impairment in the air conditioning system; but, it was not really obvious.

Doing the conversion is not dumb ass simple if you do it correctly. There is some tweakng to do on the final set up for optimum operation which a lot of shops may not bother with. Some details here:

I have not been in a lot of NSXs; but, as a general observation, compared to other vehicles they do appear to be designed with excess AC capacity. If you put it on manual and max, mine can blow air that is cold enough to make your feet ache when it is 35 C outside. However, when I first got the car it was merely OK. While washing the car, I was alerted to a condenser problem by the stench of rehydrated dead grasshoppers. On checking, the condensers were about 30 - 40% plugged by dead bugs and a significant number of the remaining fins were squashed blocking air flow. After thoroughly wetting the grasshoppers to soften them I spent about 2 - 3 hours on my side using a fin comb to dig the grasshoppers out of the fins and straighten the bent fins. The low air entrance for the condensers make them extremely prone to accumulating junk. After the cleaning, I got a pair of Dali rock shields to prevent further accumulation on the fins. The shields are much easier to clean.

Check the condition of your condensers. If the fins are obstructed that will make it very hard for the system to reject heat to the outside when the ambient temperatures are high. Also, make sure that both condenser fans are operating and that the exit vents in the wheel wells are not obstructed. On a 1991, I would not be surprised if the fins are pretty banged up or one of the motors was dead. If the condensers are OK, take it to a good AC shop to make sure that the pressures are correct.
Thanks @Old Guy. I'll put it on my lift to check out the condensers from a better angle, but when I did my PPI they didn't stand out as excessively filthy or beat up.

I did check the sight glass with the A/C running and can easily see the "flow" by the window. I'm not sure whether this counts as "bubbles" or not--should I not be able to discern anything in the sight glass if it's properly full?
Since cleaning off my condensers 11 years ago I have not had to fiddle with my AC system. I have never checked the sight glass so cannot advise what mine looks like or what might be correct. I am not set up to fiddle with AC systems and if the condenser cleaning had no fixed my problems it would have been off to a specialist to get things fixed.

I just suggest checking those condensers because it is easy to do and after 31 years they tend to 'take it on the chin'. Confirming both fans are running is also easy to do. Owners have posted about poor AC performance because one of the fans is dead. It can be deceptive because with one fan running you still get the fan noise. An easy hand check for air flow through the wheel liner slits will confirm operation of both fans.
I've always considered the A/C condensers and the radiator to be consumable items with simply a longer change interval than other wear parts. Given how weak the yen is right now, it might be a good idea to order a set...