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#1073867 - 05/09/22 11:53 AM Questions About Charging new AC?????  
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88ssBrent Offline
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New serpentine ac system in car here is the link for those that are unfamiliar.

http://www.montecarloss.com/community/ubbthreads.php?ubb=showflat&Number=1073271#Post1073271

Ac system
Sanden 4864 flx7
89-92 camaro Original Air condenser 11-215p
89-92 camaro Original Air liquid line 13-281
89-92 camaro four seasons soft hoses 55451
Four seasons 38904 variable orfice tube
88 Monte Carlo Gm 151589 accumulator
88 Monte Carlo oe evaporator

As seen by the parts list my car pretty much has a 89-92 camaro ac system. Two differences is the Monte accumulator which I compared side to side with the camaro accumulator, same size different location of inlet and outlet . Other difference is the Monte evaporator, not sure how it compares in size to the camaro evaporator.

Stock 89-92 camaro holds 36 Oz. Of r12, 80% r134a charge should be 28.8 oz. I overshot a little and put 29.5 Oz of r134a in the system but I probably did lose some from bleeding air out of yellow charge hose. Probably not the best time to try tuning my ac but with long work hours you do it when time permits.
-73°
-52% relative humidity
- static pressure 72 psi
- ac on max
- blower on high
- 1200 r.p.m.
- radiator fans still running off low pressure switch
- fan in front of car blowing through grill

Here is what the gauges looked like under those conditions.

https://youtu.be/2Lqq4QGqcB4

Suction Line Temp - Gauge Temp = Superheat
Gauge Temp - Discharge Line Temp = Subcool

I borrowed a friends meter that reads ambient temp, relative humidity and has a temp pipe clamp. Where is the best place to read the suction and discharge line temps? What is opinions on the above video? What is ideal system setup (blower speed, state of ac controls, idle speed, doors opened or closed etc) when tuning the ac? When driving the car around 2000 r.p.m. center vent temp was between 47°-51° as compressor would cycle. Also as noted I'm using a variable orfice tube.

#1073868 - 05/09/22 02:01 PM Re: Questions About Charging new AC????? [Re: 88ssBrent]  
Joined: Apr 2011
Posts: 1,234
Hunter79764 Online content
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Hunter79764  Online Content
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Joined: Apr 2011
Posts: 1,234
Grand Prairie, Tx
Quick note, your subcool is measures from the liquid line (after the condenser), not the discharge (that feeds the condenser). If it was me, I'd read the temps at the liquid line immediately before the orifice tube, then read suction temps switched between two places, the first is feeding the accumulator, and the second is the portion feeding the compressor. You can shoot for nearly zero superheat leaving the condenser, then make sure you have plenty feeding the compressor and you should be safe. Superheat is essentially performance left on the table in order to protect the compressor. Subcool is making sure that you have a good column of liquid feeding the metering device for consistent operation.

My gut on the video is that the charge is low, but given the mild conditions, I'm not sure exactly how low. You could disconnect the low pressure switch, jumper it, and see what pressure it wants to settle in to and do your adjusting from there. That should also get a stable discharge pressure, which I think ought to be more around 150 where the video shows it struggling to get around 125 before the low switch kicks it off. Speaking of low pressure switch, make sure you plan on adjusting it for the right cut out pressure for you, google says 21 psi for 143a but you might adjust it to your liking or experience. You want to give it enough so that you don't freeze the evaporator if for no other reason than you will have to wait for it all to melt again if you do, but if you're actively working it I don't think you will have much issue.

Are you using individual cans of refrigerant, or do you have a drum of it? Either is fine, just wondering how much flexibility you have on tweaking charge. Hooking up the hoses again is a lot simpler when you don't have to worry about breaking open another 12oz can to get 2-3 oz into the system.

Last thought, your setup sounds good, the fan in front is going to lower pressures a little, so if you are on the low side of "normal", I'd consider it fine. It will be more realistic for driving performance, but you might want to do at least on sanity check without the fan to make sure discharge pressures don't get too high, especially on a much hotter day. (It was 98° here in Texas yesterday, just run down here real quick and you'll have some nice AC testing weather smile ). Also, I keep my windows rolled down and/or doors open so that the car doesn't get too cool. The down side of running on Max is that you can start to affect your return temperature, the upside is that you aren't sucking engine heat from the cowl with the hood open.


When I ran R&D tests for stationary systems, the basic logic was to run a few charges at a range of operating conditions, inside and out, then choose the fattest charge that kept the system happy (to allow for some moderate leakage over time). Then you would run "Abnormals" at the worst extremes plausible, like a really hot exterior temp with a very light load inside, then a very heavy load inside with a mild exterior temp. The problem with automotive is that you have less access to the data and about 3 times the number of variables. You also have about 3 times the window of "Good enough" generally, so that's nice. But basically, I'd get a charge that seems to work for a good low pressure reading and good vent temps (probably reading you superheat at the evaporator and shooting for a small number there). Then make sure your subcool and superheat at the compressor are adequate, then remove your front fan, rev to ~2000 rpm on a warm day and make sure your pressure isn't in danger of tripping or relieving itself (I'm not sure what type of relief you have or what the high pressure cutout is on your system). Then run your fan at idle on a cool-ish day and make sure your low pressure cycling witch is adjusted right. That's fairly overkill, as most folks (shops included) would stop at step 1, but if you want to check it all out, that's what I'd suggest as a full battery of tests.


Shawn

'85 MC with budget 5.3L swap, TH350 with stock 2.14 rear end
It ain't much off the line, but it's nice on the highway
#1073869 - 05/09/22 06:53 PM Re: Questions About Charging new AC????? [Re: 88ssBrent]  
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88ssBrent Offline
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In case anyone is curious the r12 charge for the 89-92 camaro is 2.25 lbs and the charge decal on my 88 monte carlo calls for 3.25 lbs of r12.

Shawn, thank you for all the above information it's exactly what I was looking for. Hopefully towards the end of the week or weekend I will be able to get back with some numbers. When you say small number at evap for superheat what value would you think, 10 or less? This is my first dip into the ac world so I've been trying to study up a little on the internet which as you know the info can be good and bad. I'm using self sealing individual cans, luckily I have a couple extra so it's no big deal to add some if needed.

Last edited by 88ssBrent; 05/09/22 07:18 PM.
#1073870 - 05/09/22 08:06 PM Re: Questions About Charging new AC????? [Re: 88ssBrent]  
Joined: Apr 2011
Posts: 1,234
Hunter79764 Online content
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Hunter79764  Online Content
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Joined: Apr 2011
Posts: 1,234
Grand Prairie, Tx
Probably 2° at the evap would be fine, basically enough to know that it isn't completely liquid at your "normal" operating range but close to it. The closer it is to the saturation temperature, the more liquid is pulling heat out of the evaporator. You would want something more like 10° at the compressor inlet to make sure that you don't have any liquid getting past the accumulator and into the compressor. Compressors are made for gas, not liquid, and don't appreciate being fed the wrong phase for too long. They will take it for a while, but it causes extra wear.

Probably worth focusing on the pressures mostly, and use the superheat to make sure it is safe long term. Your setup is not that far off of a factory configuration, just far enough that factory charge numbers can't be trusted. For any given evaporator condition (charge level and superheat), your subcool will be primarily a function of your orifice tube size. There's not a whole lot to tweak unless you are way off, then you will need to swap to a different orifice (which shouldn't be the case, you have a variable one already). But since you have a variable tube, you might not see much variation on the subcool anyway. Pressures and vent temperature should be your primary charge determination tools, evaporator superheat should be an indicator as well. the rest of them are sanity checks for longterm operation.

Think of it as a confirmation step rather than design step. If you were in an extreme environment or had a wildly different arrangement, then the subcool would tell you if you needed to increase or decrease the orifice size. In rooftops, we would set charge based on Superheat and evaporator performance, then see what the superheat was. If too low, we would reduce the orifice size, too high, open it up. And all of that was done at a controlled setting, such that we knew what was "enough" there that we shouldn't have issues at the extremes, then we go to those extremes and verify. But for auto systems, it's all an educated guess and the only outputs that generally matters is vent temps, system function, and owner satisfaction rather than energy efficiency, warranty, sound, manufacturability, cost, data centers in Dubai, snowy rooftops in Canada, and everything else.


Shawn

'85 MC with budget 5.3L swap, TH350 with stock 2.14 rear end
It ain't much off the line, but it's nice on the highway
#1073880 - 05/10/22 01:19 AM Re: Questions About Charging new AC????? [Re: 88ssBrent]  
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86ttop Offline
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86ttop  Offline
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An 82-92 Camaro has an accumulator, so the charge would be 90%, the filter/drier on earlier systems gets the 80%


Leo Paugh
It's not an attitude, it's just the way I am.
POW*MIA
You are not forgotten
If things improve with age, I must be approaching magnificent. thumbs
#1073884 - 05/10/22 01:30 AM Re: Questions About Charging new AC????? [Re: 88ssBrent]  
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88ssBrent Offline
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Thank you for the tip Leo!

#1073887 - 05/10/22 08:28 AM Re: Questions About Charging new AC????? [Re: 88ssBrent]  
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FinallySS Offline
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FinallySS  Offline
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Nice Sunny Hawaii
Just my experience. I have replaced my R-12 system with R-134. Everything I have is stock, but brand new from hoses to compressor. I needed new heater core, evaporator and condenser anyway. Followed the sticker that was on the car and calculate the amount needed for my newly setup A/C system (was it about 80% of R12 system? Forgot what was the calculation but I got the formula from here).

I first thought I overcharged it because compressor kept cycling. My gauges confirmed this. So took it to a friend who does the AC service and took out some 134 out. Thinking it's going to be OK, I just left that as is. Sure, when the compressor was engaged, it was nice and cool but it still was cycling. Thinking "what the hell. If I overcharge it again, I can take it back to my friend to take some 134 out again", so I charged the system again, but this time, I went by what the gauges read. It took almost a pound more than the calculation that I came up with.

To this day, system is working great (however, thinking of changing it to Sunden compressor and also change the condenser to more modern design ones) and haven't touch it other than tightening the belt once in a while. Same with my DD Lincoln. I had to change the compressor so went into AC shop (my friend wasn't available that time due to our schedule) to discharge the system, changed the compressor (which is whole another story by itself) and rechanrge the system. I first followed the repair manual, charge it but compressor kept cycling. added little more by reading the gauges and now it's trouble free.

BTW, I live in middle of the Pacific, so we rarely go below 80, and during the day is almost never.


1986 Silver MCSS - Poly bushing and Hotchkis spring w/Bilstein, Posi w/3.73, MSD 6-AL, E-Brock 600cfm, Aluminum 2-row radiator.
1986 Black MCSS - R.I.P............Finally.

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