AC Overhaul-advice needed!

[SWFL_NSX]

<!--?xml version="1.0" encoding="UTF-8"?-->The AC compressor in my 1992 has a moaning sound that seems to be getting worse-the system is performing well after a recent recharge, but I’d rather replace the compressor now than waiting for it to fail. I’ve already replaced the tensioner pulley bearing.

1. The current system is still r12, and I’d like to keep it that way. If I was going to switch to r134 I’d use the post 1998, p9k compressor as per Kaz in this thread, but I’m not sure what to order for the r12 system-the older compressor that was used for r12, or the newer, more efficient p9k compressor that was used for r134. What is the best compressor if I'm keeping the r12?
2. I’ll replace the receiver drier and any seals that I open. Keeping in mind that the system is working and I’m sticking with r12, am I Ok not replacing the expansion valve? I’d rather not get into pulling the dashboard unless necessary, and Kaz mentioned that the threads on the evaporator sometimes strip when pulling the expansion valve.

Love to hear anybody put in the .02 here!

 

[Jinks]

The expansion valve will work with either refrigerant you choose. I wouldn't mess with it if it's functional. It might be a good idea to open up the fan assembly and clean the evaporator and fan blade. Super easy job.
I cannot comment on which compressor to use though.

Just make sure you pull a good evacuation prior to charging with refrigerant. If you're changing the compressor I would just run the R134A. But I put nothing but R12 in mine. If I had to change the compressor though, I'd just do the R134 with fresh oil. My .02

 

[Kaz-kzukNA1]

I'm afraid normally, the new compressor is pre-filled with the specified amount/type of oil for the specific make/model of the car so if you are staying with R12, you must find the R12 compressor.

The compressor oil (ND-OIL8) inside P9K one is not compatible with R12 so you can't use it unless you are happy to overhaul the brand new compressor to wash off all the fresh oil.
Since you are in US, try contacting Denso North America or Denso Auto Parts for R12 compressor.
You can visit densoautoparts.com website and use 'find my part' feature but as you are looking for R12 spec compressor, best if you double check the parts # for R12 by contacting someone directly there.
This is because from time to time, they could be showing the superseded parts # that could be only for R134a since the R12 OEM compressor is discontinued.


If your local regulation allows you to re-charge the system with R12, then you may be able to find R12 compressor overhaul service with core exchange basis so that you don't have to wait for your own one to be serviced.
The base compressor design is 10PA15C and shared with many other make/model with just different mounting/pulley/CL design.
If your existing CL is still healthy, you can just re-use them.
Some people in Japan are using this type of service because R12 is much more efficient (cooling performance VS power consumption) than R134a although R12 is so expensive and hard to get these days.


In some countries, re-charging the system with R12 (regardless of whether it's fresh or recovered one) is illegal so no other option but to convert the system to later refrigerant system.
There are so called R12 substitute refrigerant on the market but I don't know its performance/efficiency plus how pure it is so may as well convert it to R134a instead of using such thing.

You mentioned that you had to re-charge the system and the compressor is making moaning sound.
Depending on how far you lost the refrigerant and how long you kept the compressor running under low refrigerant state, you may have splashed metal flakes inside the system.
Compressor oil travels through the system via refrigerant so if there is not enough refrigerant, the oil can't get back to compressor resulting in compressor pistons scratching the cyl wall splashing the metal flakes and eventually, results in seized compressor.
I always recommend the owners to disable the compressor if they lost the refrigerant too much.

I disabled my compressor when I noticed the accelerated refrigerant leakage due to puncture at the evaporator.
You can find my A/C refresh blog by searching for the key word 'ac refresh site:nsxcb.co.uk' or please browse through this category; http://www.nsxcb.co.uk/blog.php?1811&blogcategoryid=42


Personally, I would flush the system before installing the new parts especially if you are installing the new compressor.
There are sophisticated AC service machine that can flush the entire system by connecting to the compressor hoses but not sure how effective it is.
Our NSX is using the classic condenser design so I prefer the ancient method of manually flushing each sections by disconnecting each pipes/parts.

If you look at the state of the oil from your existing compressor, you may be able to tell the contamination level.

You must replace the drier/filter whenever disturbing the pipe connection.
It doesn't have to be OEM but just pay attention to the cyl diameter. You can always use tyrap, if required.


As part of warranty purpose, the compressor manufacture may force you to replace both the drier and the exp valve (and normally recommended any way since there must be a good reason for replacing the compressor) when installing new compressor but because of its position and risk of seized fitting, as Jinks mentioned, I would first try without replacing it.
If blockage happens then you can first try replacing the valve by removing the blower motor fan unit under the bonnet and without taking out the entire dashboard.


It's your choice and you may have other reasons to stay with R12 but please balance the requirement and future service difficulty (regulation change, parts availability, cost, etc) as Jinks mentioned, converting to R134a could be better option if replacing the compressor although you need to decide how far you want to refresh the system.


Kaz

 

[Jinks]

[MENTION=25737]Kaz-kzukNA1[/MENTION] Please never leave the nsx community.:wink:

 

[VancouverBCnsx]

Great write-up! The compressor on my 91 seized recently so the info in your post is very useful.
Marc

 

[Wavey1]

@Kaz-kzukNA1 Please never leave the nsx community.:wink:

+100

 

[goldNSX]

There are so called R12 substitute refrigerant on the market but I don't know its performance/efficiency plus how pure it is so may as well convert it to R134a instead of using such thing.

R413a

You mentioned that you had to re-charge the system and the compressor is making moaning sound.
Depending on how far you lost the refrigerant and how long you kept the compressor running under low refrigerant state, you may have splashed metal flakes inside the system.
Compressor oil travels through the system via refrigerant so if there is not enough refrigerant, the oil can't get back to compressor resulting in compressor pistons scratching the cyl wall splashing the metal flakes and eventually, results in seized compressor.
I always recommend the owners to disable the compressor if they lost the refrigerant too much.

There's a switch in the system which shuts the A/C compressor down if it's low on freon. From my experience since day I have the car with a leaking A/C system it's around 400 gramms when the system shuts down. My A/C compressor lived for 20 years until I replaced it before it blew up. So low on freon is not an absolute no-go IMO. I'm still losing freon, maybe 50 gramms per year and I know where the small leak is but I'm not crazy enough to take the dash yet. The critical point is if you lose freon you also lose oil but you don't know by how much. I guess that the freon finds it's way much better out of the system than larger oil molecules.

 

[Jinks]

You'll also have high superheat going back to the compressor and it will not cool properly when running low on refrigerant and also be prone to seizing.
I just bought one of the coolest hvac tools today. Wireless pressure gauges and temp clamps. You use an app and it does all the superheat and subcooling calculations for you. Very cool tool! You need SAE style adapters for cars though.

 

[goldNSX]

As far as I'd love to DIY the A/C system too the tools with a reasonable price only do part of the job. What do you do if you have to open the A/C system? R134a free in the athmosphere? In Europe, that's not allowed. We have to gather it back in the charging station.

 

[Jinks]

Here in the states auto parts stores sell R134 to anyone. You could just buy a small recovery unit and a recovery tank for a few hundred bucks. Might be a lower priced option as I am not sure how expensive the automotive charging stations are.

 

[SWFL_NSX]

I'm afraid normally, the new compressor is pre-filled with the specified amount/type of oil for the specific make/model of the car so if you are staying with R12, you must find the R12 compressor.

Ah! Thanks for this Kaz, I will just order the compressor for 1992-r12 vehicles.

I am in the USA and have the EPA certification to purchase r12. My local shop will evacuate all the old r12 from the system before I open the system. My research indicates that the different r12 replacements (other than r134) are not a good idea, and in my part of the country it is VERY hot and humid for 4 months of the year.

Additional question-

How do I know how much oil to put back into the system?

I am replacing the compressor, the drier/accumulator, and flushing the hoses from the compressor to the drier/accumulator.

Thanks again. Your blog has been very helpful in researching this project.

EDIT===========================================================

Found the answer:

On page 22-65 of the 1991-1992 service manual it states how much refrigerant oil to add after replacing the condensor, evaporator, line/hose, receiver and compressor.

Be careful, some of these values are different for the r12 cars vs. the 134 cars!

Additionally, there are two copies of the service manual, this one in PDF format for 1991 and Oddomatic's fantastic searchable version that appears to be based on a 1996 manual. If you are servicing a 1991-1992 vehicle and want to keep the r12, I believe that the pdf version is correct version to use!

 

[Kaz-kzukNA1]

Hi, SWFL_NSX.
I'm also qualified for HVAC with my experience and qualification obtained through my Uni days but only in Japan and I don't know how to convert it for the F-gas certificate in Europe.

You already found the answer but if you are asking someone to evacuate the system, they may have the expensive HVAC service station that can measure how much oil and refrigerant recovered and also flush the entire system by connecting to the two compressor hoses. This may save your time and cost.

As you are EPA certified (I think Jinks is also certified and HVAC licensed based on what he posted above and I used to use portable recovery unit for different application), apology if I'm writing something you already knew but if you are replacing the compressor with a new one, you don't need to put any oil back in the system.
It's pre-filled with the required amount of oil (total system amount, not only for the compressor but including the amount that's very likely to be trapped at the pipe, condensor, evapo, drier) for the specific make/model .


If you prefer the old method of flushing it manually and use the gauge manifold like me instead of using the automatic HVAC service station, then you need to flush the system as much as possible.
As you know, you can't flush through the exp valve with manual method and thus, the entire evaporator can't be flushed while keeping it inside the car (dashboard out job) without removing the exp valve first.

I presume you are flushing through both the R & L condensers as it's before the drier/accumulator.

In ideal world, you want to disconnect the low pressure side after the evapo to flush the pipe between the evapo and the suction side of the compressor.
For the same reason, you want to disconnect the high side one between the exp valve and the drier.
Both are located at the front bulkhead but flushing may not be easy with the limited space.

The specified total amount of oil has some window and for NSX, you can go upto extra 20cc.
Although some oil is sitting inside the parts that you can't flush, if you manage to flush through the above pipes, you should be fine without removing any oil from the new compressor since you are changing majority of other parts.
Too much oil will disturb the efficiency of the system resulting in poor performance so if you can't flush some of the parts, play safe but consider the extra 20cc allowance and drain just a bit like 10cc.
However, as mentioned in my previous post, if your old compressor was making noise, there is a good chance that you already splashed the metal flake inside the system so flushing as much as possible is very important otherwise you could damage the new compressor within a few years or even much earlier.


If using R12 drier/filter/accumulator, it has huge capacity compared to the R134a one so you have fairly big window for the amount of refrigerant.
Just don't over charge above 950g (max limit, R12 only).

The operational pressure is much lower with R12 than that of R134a and the following info is based on R134a on NSX so not accurate for R12 but the basic tendency is the same.
The triple pressure switch will become active while you are charging less than 200g on hot day and once you charge about 500 - 600g, should be cold enough. From there, the cabin temperature won't drop dramatically so just carefully charge to the max limit and never overcharge it otherwise, you could puncture the evapo.
Because of the long-long pipe length on our NSX, even if you massively overcharge the system, you won't notice the eng power drop like you would if overcharged the a/c on ancient classic cars.
R12 is so much better on the efficiency so you won't notice the power drop any way.

Please check the performance against the graph on 91 service manual.
As the exp valve port size changes continuously and depending on the car make/model, compressor type, eng rpm, temperature, etc, the pressure readings would be different so there is no one single figure for good Hi/Low readings but good starting point.


Kaz

 

[SWFL_NSX]

Kaz,

Thank you for the response. When I say "EPA certified" this only means that i passed a simple test to purchase r-12, it does not indicate any level of knowledge about AC systems! You and Jinks are far more knowledgable.

I currently have the compressor and receiver/drier off, and have backflushed the system with mineral spirits and a compressor (120psi) through the pipe at the receiver, through both condensers and back through the compressor fitting near the wheel well. I didn't loosen any of the fittings b/c I was worried they may be seized as yours were.

Two questions, one regarding flushing, one regarding oil.

1. Should I worry about seized fittings on the high and low connections to the evap? If it is a worry, I'll probably keep them in place, if it isn't a worry, then i'll remove them and flush those sections.

1. When I removed the compressor, only about 5ml of oil came out, even though I tried the compressor in different positions and turned it by hand for about 3 mins. The oil is clear, with a greenish tinge from the dye, with no metal flakes. If I am unable to remove the high and low fittings on the evap (and flush whatever oil may be in those pipes), what is your recommendation for the amount of oil to remove from the compressor?

My current through would be to remove perhaps 20ml because of the oil that is still in those un-flushed sections.

Thanks again!
SWFL_NSX

 

[goldNSX]

1. When I removed the compressor, only about 5ml of oil came out

Your system was clearly low on oil. There should have been more in there. It's advisable to flush as much of the system as you can and fill it with the correct amount of new oil.

 

[SWFL_NSX]

goldNSX,

Thank you for the reply!

Agree that the system was low on oil. I've already flushed both condensers as well as the high pressure line from the drier to the compressor. Additionally, the compressor and the drier will be new, as will the oil. Further, this compressor was running well, and I could find no trace of contamination in the oil that I removed (although I was unable to capture the flushed fluid). The only thing that hasn't been flushed is the evap and the two lines that I asked Kaz about.

If the connections to the Evap are known to be prone to seizing (as are the condenser lines, and the expansion valve) the I may decide that flushing 75% of the system is far more than most AC shops would do for a non-seized compressor and begin re-assembly. After looking at the high/low lines I'm not sure I could flush them without completely removing them from the car-eager to hear what Kaz did when he overhauled his system.

 

[goldNSX]

With a new compressor you should be able to flush all components except the evaporator (and the compressor of course). I'd not mess with the valve and leave it as is.
120-140 cc oil for the complete system. So you might add around 110 cc. Not sure if it's a good idea to fill the compressor with 110 cc all at once. When I did mine, I've left 50 cc in the compressor and added the rest through the filling machine my A/C pro used.
R12 950 mg
R134a 850 mg

Just wondering: what fluid did you flush with and how did you dry the lines?

 

[SWFL_NSX]

120-140 cc oil for the complete system.

This is interesting! I haven't been able to find a place that explicitly states the amount of oil for the complete system. How do you know it's 120-140?

Just wondering: what fluid did you flush with and how did you dry the lines?

I'm using AC flush by FJC. It doesn't recommend anything to dry the lines with after, but I'm blowing air through until nothing comes out the other end.

 

[Jinks]

Yes guys, I am hvac certified but I am not super knowledgeable for car systems. My compressors are 8 cylinder beasts with open drive motors that run R404A refrigerant. I still learn from these great threads.

 

[goldNSX]

This is interesting! I haven't been able to find a place that explicitly states the amount of oil for the complete system. How do you know it's 120-140?

I can't state you a link to the source out of my head but I've been researching extensively years ago when I did mine. Also: the new compressor comes with the amount of oil for the complete system and that was 150 cc in my case. Maybe someone can confirm.

I'm using AC flush by FJC. It doesn't recommend anything to dry the lines with after, but I'm blowing air through until nothing comes out the other end.

You wan't to make sure that there's no excessive moisture in the lines before you close the system so the vaccuum can remove the rest of it completly. My A/C guy told me that he uses (inert?) gas to dry the lines after flushing them but he didn't say which gas.

Jinks: Back in the time when A/C was new to the car industry in Europe you rarely had a knowledgeable guy out there. Unlike USA or Asia: in Europe A/C in cars came widespread only 20 years ago. When the A/C maintenance stations become widespread in car shops you nearly had no one understanding what he was doing with all kind of false arguments and wild guessing of course. They were just able to do the standard procedure and even did it when the job was more sophisticated. The same guys tried to rip you off when the A/C went south arguing that they had to change the whole system for $$$$.
It took me more than 5 shops in Europe to find a really knowledgeable guy. All the infos I got from the US: this forum, Eric the car guy and even Scotty Kilmer.

 

[Kaz-kzukNA1]

Hi, SWFL_NSX.

For R12 NSX, the total system oil amount (compressor + pipes + drier + evapo + condenser) is 120cc +/-10cc when used with 10PA15C type compressor. I would stay with the Denso compressor.
R134a system requires larger amount but I'll leave it out of this thread to prevent confusion.

There was mistake in the original workshop manual for the specified total oil amount.
In fact, many R12 NSX left the factory with not enough compressor oil in the system.
Later, the manual was revised and the dealers were advised to add extra oil whenever the certain R12 NSX visited their place for servicing.
Should be the same in US.


Regarding your question;

1. If you look at the front bulkhead, the fitting for the hi/low pipe at the evapo is not like the coupling type used at other pipes and instead, it's just a push fit style held by the 12mm bolt.
It's the same style at the compressor service plate.
Therefore, no problem in removing these pipes.
Just be careful on re-installation as you can damage the new o-ring with the limited space and vision while you must attach the pipe square without catching the o-ring.

I don't know how the a/c pipes are routed on the LHD NSX but with the RHD model, at the time of flushing, one of the fairly big problem is, without removing the evapo out of the way, the pipe end (especially the low side one, thicker pipe) has very little gap against the evapo/bulkhead that you are going to struggle attaching the flushing nozzle or the drain tube.
If you are not removing the evapo, you will need to be creative.

Hi side pipe (drier - exp valve) is also in the same boat but this one is narrow but with bit of juggling, you can create enough gap.
Again, it could be not a problem on LHD model but I have very limited experience with the LHD A/C pipe routing.


2. Only 5ml left in the compressor……
You must have been running at super low refrigerant.

Unfortunately, the triple pressure switch won't prevent the compressor from running until the Hi pressure drops almost close to the empty status during the hot summer season.
One of the most common failure mode in Japan as the temperature stays above 35degC or even over 40degC for months and this will keep enough pressure to keep the compressor running until it seizes.
This is the reason why I always recommend the owner to disable the compressor once they no longer get any cold air out of the vent or not even bubbles were seen at the site glass.

With the layout of the NSX A/C system, under super low refrigerant, most of the oil has either leaked outside world or sitting at the Hi side section upto the drier.
You will see wet sticky oil marking at the compressor, evapo joint at the front bulk head and the fittings at the R&L condensers.

Also, when you evacuate the system, part of the oil is removed from the system together with the refrigerant and the total amount depends on how much refrigerant left at that time.

Therefore, ideally, you want to flush the above pipes in my previous post to prevent the guess work.

If you can get R12 spec Denso new compressor, it should be filled with about 130cc oil.
If you have clean/dry glass container, you can always measure it but please note that you won't be able to drain all of the oil from the compressor no matter how long you wait but close enough.

You may want doing this any way because as I mentioned above, Honda made mistake in specifying the total amount and original R12 OEM compressor didn't have enough oil.
By now, updated info should be used but OEM R12 compressor was discontinued long-long time ago so could be old batch left and still using the old info with the aftermarket Denso one.
Initially, Honda made mistake and set it at 60 - 100cc (80cc +/-20cc) when it should have been 120cc +/-10cc.

About 10cc will be staying inside the evapo but as in my previous post, there is fairly big window available so just aim for 110 - 130cc total system amount.


Please note that with the new compressor, you don't need to add extra oil.


Also, if you are adding the green dye, that's also part of non-refrigerant material in the system acting like the oil.
Very little amount for big R12 refrigerant capacity so can be ignored but don't put too much.


Kaz

 

[SWFL_NSX]

Update

On Sunday afternoon I buttoned the system up (installing the receiver drier last!), leak tested it under vacuum (gauges held steady after an hour), and pulled vacuum for 1 hour. After charging with 31 oz (872 grams) the gauges looked like this-

low side-53 psi (365 kPa)
high side- 220 psi (1516 kPa)

Ambient temperature from a weather station near my house was 87F (30C) and 68% humidity.

Evap temperature measured from thermometer in the center vent was 67F (19C)

I plan on getting a wet bulb/dry bulb thermometer next weekend and conduct this test again to more accurately measure the temp/humidity inside my garage (I would bet it was a few degrees hotter after running the car for 20 mins to charge the system!).

It appears that my low pressure side is running slightly higher than expected (by maybe 15 psi, 90 kPa), but my high side is below the expected pressures by 43 psi, 300 kPa

Any thoughts on pressures/temps so far?

 

[crxguy52]

On Sunday afternoon I buttoned the system up (installing the receiver drier last!), leak tested it under vacuum (gauges held steady after an hour), and pulled vacuum for 1 hour. After charging with 31 oz (872 grams) the gauges looked like this-

low side-53 psi (365 kPa)
high side- 220 psi (1516 kPa)

Ambient temperature from a weather station near my house was 87F (30C) and 68% humidity.

Evap temperature measured from thermometer in the center vent was 67F (19C)

I plan on getting a wet bulb/dry bulb thermometer next weekend and conduct this test again to more accurately measure the temp/humidity inside my garage (I would bet it was a few degrees hotter after running the car for 20 mins to charge the system!).

It appears that my low pressure side is running slightly higher than expected (by maybe 15 psi, 90 kPa), but my high side is below the expected pressures by 43 psi, 300 kPa

Any thoughts on pressures/temps so far?

For what it's worth, I replaced the compressor\condenser\dryer\expansion valve on my 98 integra found that the delivery temperature on those carts was way off. When properly configured (temperature damper cable wasn't adjusted correctly initially) my AC blows 45 - 50°F air once it's down to temp, the chart in the service manual said it should be closer to 60°F. Different car, but might be relevant.

Thanks for posting and asking all these questions - I have a 91 and I'll probably end up having to fix the AC at some point as well. I assume you ended up staying with r-12 and buying a r-12 compressor?

 

[goldNSX]

Have a look at this: https://www.youtube.com/watch?v=r_em9lm69HQ

I can't help you on the pressures. I had 17 bar/<1.5 bar in my case but don't recall the temperature and humidity that day, certainly lower than in the US. And my car is 134a anyway.

 

[Kaz-kzukNA1]

Super busy this week and weekend so just a quick question and notes for now.

The pressure readings depend on many factors but the high side moves in the wider window in the order of 0.1MPa (about 15psi) whereas the low side is like 0.02MPa (about 3psi) so the first impression is, not efficient and while there is a room for improvement on the vent outlet temperature, not too bad.

Because the low side pressure (evap exit) stays high, so does the vent air temperature but cold enough for that high outside temperature.

Exp valve could be staying open bit wider than it should but at this stage, not enough to be replaced considering the repeat of multiple tasks.


1. What compressor did you use? Any adjustment to the amount of oil?
This question is not relevant to the pressure reading you are getting but just interested for the future reference as other owners may select the same aftermarket compressor.
As mentioned, the pressure readings would change depending on the make/model of the car, rpm, other components especially the compressor, environment, etc.
Another reason why I prefer staying with Denso compressor even for the aftermarket one as OEM is by them.


2. At what rpm did you take these readings? Any chance you were reading them at idle rpm?
You must apply full load to the system to measure the efficiency so has to be 1,500 - 2,000rpm.
The compressor pulley diameter has some effects but at these rpm, by not much.


3. With your readings, not related but do you see any bubbles at the site glass of the drier at idle rpm?
What about at 1,500rpm?

With a/c system design of NSX (small cabin, large total capacity condenser but separated in two plus long system length), site glass check is just a reference only but with R12, it's fairly reliable.


4. Amount of refrigerant; 31oz.
Is this the actual amount went inside the system or does this include the amount left inside the multiple hoses?
Bit short from the spec (32 - 33.5oz) but if you used similar capacity R12 drier as the OEM one, you should have enough reserve before allowing too much bubbles entering the exp valve resulting in low efficiency.

Please never overcharge it.


You can easily check the efficiency of the condenser by spraying water on both R & L sides.
As it says, it is to 'condense' the high pressure gas into liquid (saturation press/temp) so if lowering the condenser temperature increases the amount of gas back to liquid 'dramatically', you will know the weak point.
However, with the correct amount of refrigeration in the system and healthy compressor, if the condenser was not efficient, normally both the high and low side pressure stay high.


By the way, please check 2 x 10A fuse inside the pentagonal fuse box under the bonnet/hood for the R & L condenser fans.
Quite often, you will find one of them or both blown.


Kaz

 

[SWFL_NSX]

The pressure readings depend on many factors but the high side moves in the wider window in the order of 0.1MPa (about 15psi) whereas the low side is like 0.02MPa (about 3psi) so the first impression is, not efficient and while there is a room for improvement on the vent outlet temperature, not too bad.
Because the low side pressure (evap exit) stays high, so does the vent air temperature but cold enough for that high outside temperature.
Exp valve could be staying open bit wider than it should but at this stage, not enough to be replaced considering the repeat of multiple tasks.

This is my feeling as well. I am very happy with how quiet the new compressor is, and it feels cool enough in my daily driving. On the highway it will get down to 45-50F from the center vent. One other mystery-I am now getting some steam/condensation from the center vent when the compressor cycles off and i have the in-car temp set below 72-73. Unsure what this indicates.

1. What compressor did you use? Any adjustment to the amount of oil?
This question is not relevant to the pressure reading you are getting but just interested for the future reference as other owners may select the same aftermarket compressor.
As mentioned, the pressure readings would change depending on the make/model of the car, rpm, other components especially the compressor, environment, etc.
Another reason why I prefer staying with Denso compressor even for the aftermarket one as OEM is by them.

I used the Denso compressor that Rock Auto lists for the 1992 NSX-10PA15C, or Nippon Denso partnumber 4711424. Agree 100% on using ND parts. I removed 15 ml of oil from the new compressor to compensate for the amount still in the evap and the 2 lines that I didn't flush.

2. At what rpm did you take these readings? Any chance you were reading them at idle rpm?
You must apply full load to the system to measure the efficiency so has to be 1,500 - 2,000rpm.
The compressor pulley diameter has some effects but at these rpm, by not much.

Ah! I can't believe I didn't use the right RPM range! Yes, this test was performed at idle-I'll re-test this weekend and see what kind of difference that makes. The pulley diameter appears to be the same (new cl and pulley came with the new compressor).

3. With your readings, not related but do you see any bubbles at the site glass of the drier at idle rpm?
What about at 1,500rpm?

With a/c system design of NSX (small cabin, large total capacity condenser but separated in two plus long system length), site glass check is just a reference only but with R12, it's fairly reliable.

I didn't look at the sight glass, but will do so this weekend.

4. Amount of refrigerant; 31oz.
Is this the actual amount went inside the system or does this include the amount left inside the multiple hoses?
Bit short from the spec (32 - 33.5oz) but if you used similar capacity R12 drier as the OEM one, you should have enough reserve before allowing too much bubbles entering the exp valve resulting in low efficiency.

Please never overcharge it.

Hmmm...good point about the hoses, I hadn't considered. How much refrigerant would stay in the hoses, and would it have an effect?

You can easily check the efficiency of the condenser by spraying water on both R & L sides.
As it says, it is to 'condense' the high pressure gas into liquid (saturation press/temp) so if lowering the condenser temperature increases the amount of gas back to liquid 'dramatically', you will know the weak point.
However, with the correct amount of refrigeration in the system and healthy compressor, if the condenser was not efficient, normally both the high and low side pressure stay high.

This is my understanding of how poor condenser performance would effect the pressure readings as well. However, I am going to try to clean them this weekend before the test and see what (if any) difference it makes.

By the way, please check 2 x 10A fuse inside the pentagonal fuse box under the bonnet/hood for the R & L condenser fans.
Quite often, you will find one of them or both blown.

My last check was about 2 months ago, but I will check them again.

After this weekend I'll finish this thread with a set of "lessons learned" along the way, along with a parts list and resource links for those in the community that want to come after me.

Thanks Kaz!