1966-1972 POA for a/c systems conversion to R134

[Scot Minesinger]

I have converted over many cars to 134.  usually I replace the hoses, expansion valve, POA, and dryer (at condenser).  Then flush out evaporator, compressor and condenser with a/c pro - all per directions.  The POA is from USA Parts all set up for R134, and looks a little different than factory R12 unit.  I have seen other conversions where they just change the low and high side fittings.  At GN the judges dinged me for using this R134 POA.  I have been helping judges at AACA and GN before and they never took off for R134 conversions.  I objected and the judges raised the hood of a car that the fittings were just changed, complete with original hoses as a defense.  I still think that they are wrong, but you can't always fight city hall.

Question, can the original POA really be used for R134?  The judges said I need to send it to Cold Air Products to be recalibrated.  I would rather do that myself.  How do you calibrate it for R134?

["Cadillac Kid" Greg Surfas 15364]

Scott,
The P.O.A. is designed to maintain a minimum evaporator pressure of 28 to 32 pounds.  The purpose of that pressure is to eoliminate icing the evaporator coil.  28- 32 pounds with R-12 will give you an evaporator temperature of from 30 to 34 degrees.  Allowing for the temperature differential that is necessary for heat transfer, this amounts to a minimum air temperature of 34 to 38 degrees.  Above the frost point.
With R-134a. the same pressure range results in an evaporator temperature of from 32 to 37 degrees.  There is no reason that a "stock" POA will not work and be able to provide maximum cooling (taking into account the 20% loss due to loss of compressor efficiency using R-134a in an R-12 compressor) at summertime design conditions.  The POA comes into play at reduced loads, typically non summer conditions, and a 2-3 degree increase in evaporator (and off evaporator air) temperature will not be apparent.
Calibration of the POA requires adjusting the stem on the spring shaft as seen from the evaporator inlet side of the POA.  There are countless cars using standard(ly) adjusted POA's, but you can, if you set up a test gage "system" get a couple of pounds of adjustment.  I personally wouldn't think it worth the effort.
Do you, by the way (I did not notice last week) have the replacement ()for the POA that has a compressor cycling switch, or is it an actual modified POA?
Greg Surfas

[Scot Minesinger]

Greg,

Greg,

Nice to meet you at GN.  Thanks again for starter assistance, made it home without any trouble.

The POA on my red caddy is the stock factory unit.  The car was converted over already before purchase in 2005 and did cool well.  I changed out the hoses, condenser, and dryer for cosmetics.  The car that I ask about actually won first place in 1967-1970 class primary and I restored the climate control, cooling system, vacuum hoses, charging, power steering, cosmetics, clock, and finished body to frame bushing installation.  If I'm going to lose, best to lose to a car that I worked on.  I used the revised POA with cycling switch on this car.  The owner told me about this judging and I guess we should put the original valve back in the car.

Thanks,

Scot

["Cadillac Kid" Greg Surfas 15364]

Scott,
If the car is to be a "show car" then authenticity is paramount.  Also if the car is meant to be driven frequently, the A-6 compressors do not hold up under conditions where you cycle them for temperature control.  When for conversations sake you are cruising down the road at 70 MPH and the compressor has cycled off, switching it on will be in a non (pressures) balanced condition, not with the pressures equalized when started up from an extended (20 minutes +) rest.  You are also taking a stationary compressor from rest to (approximately) 4000 RPM. It was not designed to do that.  I personally believe that is why AC Delco has ceased making new compressors and is ceaseing re-manufacturing of re-builts.  Too many were being cycled and the results has been a very significant failure rate.
Good meeting you and look forward to next year.  We will be down in your neck of the woods the last week in August (this year). Have a time share in Massanutten, and doing a couple of "Book signings" (Barbara not me) in Charlottesville and Richmond.
Keep the faith.
Greg