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help-classic car-with modern engine/compressor

vet652000 on Mon June 09, 2008 9:41 PM User is offline

Year: 1969
Make: chevrolet
Model: camaro
Engine Size: ls1
Refrigerant Type: 134a
Ambient Temp: 80
Pressure Low: 43
Pressure High: 250

i need some help/advise. i have a 69 camaro with factory air, but has a 2002 corvette ls1 and the compressor that came with motor. it has an aftermarket poa with pressure switch, original condensor and evaporator. i replaced drier with 134a unit as well as txv-1.5 ton 12 superheat-charged with 37 ounces of r134a, but vent temps are like 60 degrees. anyone have an idea what i can do to lower vent temp? is aftermarket poa adjustable like oem? any help would be appreciated.

vet652000 on Mon June 09, 2008 9:58 PM User is offline

a bit more info-original system specs call for 48 oz r12. also evac system after component replacement for about 4 hrs with high quality vacumn pump. pressures have me stumped-should i check low side after poa or were pressure switch is located? how much will air flow over evap affect pressures? i have 1969 riviera with same system but r12 and i do have more air flow out vents, but figured this was bigger car/bigger system?

mk378 on Tue June 10, 2008 8:57 AM User is offline

Should have a V7 compressor, that's a variable displacement. No need for any sort of POA or cycling switch, would work best to connect compressor directly to evaporator.

Make sure your TXV sensing bulb is securely attached to the evaporator outlet line and insulated from outside heat. 12 degree superheat is a lot for an auto application, most work with about 5 degrees. On a lower fan setting, the low side should drop and evaporator should get colder.

The V7is designed to regulate the low side to about 25 psi at the compressor over a wide range of conditions.

vet652000 on Tue June 10, 2008 11:28 AM User is offline

so if i understand what your saying, the compressor has some sort of pressure regulator built in? is your thought that i should eliminate the POA and and replace existing TXV with something around 5 superheat? i was wondering about the variable orfice that is used on some later model r134a sysytems as well. the other thought was to install the components of say a 2001 camaro/firebird except for evaporator and condensor(altho condensor wouldn't be a big deal) this would duplicate what the compressor would see in a factory setup except for evap size/configuration. or i could install an aftermarket evap setup like vintage air, but not sure what they use for metering. i do know some people have made this work with 1969 components on evap side with these new generation power plants and compressors, but i have not been able to find detailed info of exactly what they did, so thus my dillema.

mk378 on Tue June 10, 2008 11:45 AM User is offline

TXV with a variable compressor is a really good setup. The CCOT was invented so car makers could save money using a $1.00 tube instead of a $15.00 TXV. The variable OT never was good for much.

Also I see the high side is pretty high for the ambient, if that's because you're running an old tube and fin condenser a condenser upgrade would be worthwhile.

vet652000 on Tue June 10, 2008 12:04 PM User is offline

ok, so the plan would be eliminate the POA, and install new R134a condenser and keep TXV. what would i do with the bleed line from the original evap to POA? should i change the TXV to one that eliminates the cap tube to the POA? the TXV i have is 1.5 ton with 12 superheat, do you have a reccomendation as to the best TXV for this application for both tonnage and superheat. i really appreciate your help!! i am setup to be able to build new lines, so i can modify add as necessary. also were should i locate lowside tap for charging/pressure checking?

bohica2xo on Tue June 10, 2008 2:37 PM User is offline

That V7 is a great compressor.

Apparently you have an HVAC background. Generally MVAC does not specify "superheat" or "tons", since superheat is all over the place, and actual tonnage of the system is too.

Your V7 de-strokes the pistons to maintain evaporator pressure. It should hold a constant low side pressure, much like the POA did. If you have installed a "POA Eliminator" it should be a straight through tube & have a connection for the oil bleed line etc.

If you look at the above graph, the V7 is capable of about 48,000 btu/h - or 4 "tons" at maximum capacity. I would say that your 1.5 ton TXV is a severe handicap.

You need the biggest parallel flow condensor you can fit, and a good fan clutch to get all of the performance available.


"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

mk378 on Tue June 10, 2008 3:22 PM User is offline

GM's TXV originally specified for the Camaro will work. A R-12 TXV will work with R-134a. Most of the newly made replacements you buy are specifically rated for either refrigerant.

vet652000 on Tue June 10, 2008 8:56 PM User is offline

where can i buy a POA eliminator and will it have connections for both the evap bleed line and cap tube connection for the second capillary on TXV? i do have HVAC background, i'm retired business owner and now play with cars to satisfy my mechanical urges, but my auto a/c dqates bac to early 70's when i was a young buck working in a service station when they still had such things. i usually seem to be able to figure my way thru these things, but this one has me stumped. i did remove some freon and add some freon to play with charge and fine tuned for lowest evap exit temp, but the best i can do is a 50 degree vent temp driving down road, so i would like to get a bit better if i can. its great to have forums were guys can trade info and ideas, i sure appreciate your expertise in this area. i have a far amount of money invested in this car so i would like to make a/c as good as possible with the compressor and evap being my main limitations. if i'm going to have to evac system again to install POA eliminator, then i would also like to install the best choice for TXV at same time. i will also change the condensor while i'm at it and redo some of the lines for neatness/looks while i'm at it. i've been making my own lines for years, so thats easy, finding fittings locally is getting tougher tho. thanks for the help guys!!

vet652000 on Tue June 10, 2008 9:56 PM User is offline

i looked up a picture after "googling POA eliminator" and it looks like what i already have installed, is there any way to tell if i have an "eliminator" or a modern POA "replacement"?? or are they the same thing? i would just like to fix it right this time and not keep changing parts if at all possible. any thoughts on this and correct TXV??

mk378 on Tue June 10, 2008 11:53 PM User is offline

You want to end up with just a straight pipe from the evaporator to the compressor, no valve or restriction of any sort. Tee in the oil return line and the TXV pressure equalizing line, if it is that type of TXV.

bohica2xo on Wed June 11, 2008 1:42 AM User is offline

The so called "modern" or "updated" POA "replacment" is usually just a tube with places to connect the bleed line & equalization line - and perhaps an added cycling switch.

Your OEM TXV should have enough capacity to match the OEM evaporator.

If you want it colder, you will need to change the control valve in your compressor. With a 43 psi low side, it looks like you have one with a high setpoint. They are available in different settings:

Control Valve Thread

Pull out the old P/T chart, and decide how low you want to go...

Condensor capacity & airflow are very important in MVAC. 134a can actually move slightly more heat - if you have a way to get rid of it. Your high side says there is some work to do there.

Good luck with your project.


"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

vet652000 on Wed June 11, 2008 8:24 AM User is offline

i used a digital temp meter with 2 sensors and i see a 7-8 degree difference between the inlet and outlet temps of the POA valve. would this mean that this a metering type aftermarket or is this because of the bleed line from the evap?? i do have a pressure switch on it with the equalizer line from txv and bleed line from evap and low pressure port. i now plan on changing the condensor and would like to replace whatever else makes sense at same time. did the 2002 camaro have any metering device in it? if so where was it located? if i can get vent temps into 40's i would be happy yesterday with 73 degree ambient the pressures were 43/225 at idle and vent tempatures ranged from 50-54 on an approx 5 mile run with 6 stops and cruise to approx 60mph. i have large electric fan that is setup to run constant when a/c is on. where is the control valve located in compressor? how difficult to change? lots of ideas here but i'm still a bit confused about what TXV i really should have, is this going to a trial and error type setup? will TXV change my evap temp or is that a function of compressor control valve? or is my POA the issue? sorry for all the questions.

bohica2xo on Wed June 11, 2008 10:24 AM User is offline

POA valves have not been made for GM cars for a long time. If you bought a new part, it was either a NOS valve (and worth a small fortune) or a POA eliminator. No POA valve came with a pressure switch, so you have an eliminator.

The evaporator temperature is directly related to the low side pressure, which in this case is controlled by the valve in the compressor. The valve is located in the lower left corner of that drawing I posted. It is held in with a snap ring. You must recover the system before removing the snap ring - if you ever want to see the control valve again.

No trial & error on the TXV. The OEM unit was sized properly for your OEM evaporator. The V7 takes over the job of the POA valve in the original system. This is usually a very good conversion, with plenty of cooling power. The V7 duplicates the original A6 capacity, and gives you the efficiency of variable displacement.

Be sure to check for things like air door seals & operation in the 40 year old air handling equipment. Re-Heating from the heater core will reduce performance significantly.


"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

vet652000 on Wed June 11, 2008 10:17 PM User is offline

ok, i see where the control valve is at. i'l check with my friendly chevy dealer for a new pressure rated valve and install new condensor and see how we do. thanks guys for all your help!!!!!!!!!

Runboy on Fri June 27, 2008 10:47 AM User is offline

Someone had suggested connecting the compressor directly to the evaporator for better performance. Is this to say that the accumulator should be removed and replaced with a receiver dryer after the evaporator?
I find this post very interesting and informative,

89' F-350 Crew Cab 7.3L Banks Turbo

2000 E-series Condenser, R134 Converted with Orange Orifice Tube

mk378 on Fri June 27, 2008 11:11 AM User is offline

The POA system is a TXV system at it's core, it always had the receiver on the high side between the condenser and the TXV.

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