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Old 01-18-2014, 04:47 PM
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Vintage Air specific technical question

Most cars with AC using 134A have a low side pressure of about 30PSI, which would put the evaporator at around 34 degrees (just above freezing)
Why does Vintage air recommend low side pressures of 6-18 PSI ?
I'm very confused by this, because theory would tell me that any moisture would ice up instantly and block my evaporator coil.
Thanks for any reply

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Old 01-18-2014, 09:13 PM
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The outside temp determines what the pressures should read. There is no one size fits all.
No!! The outside temp will determine the high side pressure. Anyone else??
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Old 01-18-2014, 09:34 PM
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Good one, Denny
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Old 01-19-2014, 04:58 AM
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Too bad that chart doesnt go down to freezing. I would be interested if the pressure gets down low enough so the compressor wont kick in, as thats the problem I am having at the moment....
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Old 01-19-2014, 09:35 AM
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Too bad that chart doesnt go down to freezing. I would be interested if the pressure gets down low enough so the compressor wont kick in, as thats the problem I am having at the moment....
When "off" the pressures equalize, so both the low and high sides are ~ 150#. It should always kick on, it just won't run long before it starts cycling. If yours is not even kicking on, you have other problems...

Russ
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Old 01-19-2014, 12:30 PM
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Originally Posted by S10xGN View Post
When "off" the pressures equalize, so both the low and high sides are ~ 150#. It should always kick on, it just won't run long before it starts cycling. If yours is not even kicking on, you have other problems...

Russ
Mine will kick on if I bypass the pressure switch at the dryer.......I figure either the switch is bad (6 months old) or there isnt enough pressure to trip it. I guess spring will tell.
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Old 01-19-2014, 02:50 PM
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I appreciate the chart you posted, as it has excellent information, but my original post was for someone to answer a specific "Vintage Air" question.
Why does Vintage recommend such low pressures on the suction side?

http://www.vintageair.com/DownloadsS...ng%20guide.pdf
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Old 01-19-2014, 03:59 PM
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Thanks for that response.
My car's AC works perfectly, and the pressures do fall within Vintage Air's specification. I have always been curious, and I'm asking because the basic theory that I have in automotive air conditioning tells me that such a low pressure would basically turn my AC into a freezer. My outlet temp when I checked last summer was about 40F.
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Old 01-20-2014, 07:12 AM
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the owner of Vintage Air (Jack Chisenhall) wrote a book about hot rod air,if I recall correctly he stated one cause of low side pressures being too low was the compressor was too big for the system,standard of the aftermarket has been the Sanden SD508 (5cyl 8ci) which maybe the reason we are seeing more Sanden SD7 (7176) being used

Other issues are not enough heat transfer at evap coils,which maybe the result of low ambient temps,restricted blower inlet or evap case outlets,expansion valve cap tube not attach to suction (wide open floods the evap) I'm sure there are more reasons but that is all I can think of right now.

Most hot rod a/c systems are the result of what will fit in the car and not what a engineer has designed to work properly,sometimes it amazes me that auto a/c works as well as it does considering everything it has to overcome,such as engine rpm,ambient temps, humidity,improper refrigerant charge,mismatched components,poor installation technique etc
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Old 01-21-2014, 07:03 AM
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Denny, want to say your advise is spot on,after a lifetime of air conditioning installation and repair,my shop bought a tubes n hoses machine which morphed in doing a lot of custom lines/hoses/ repairs/ installs for hot rodders and now this is all I do,much of what comes into my shop are cars that have beautiful body work /paint/interiors/running gear and suspension but the air conditioning and electrical in these cars indicate a lack of understanding in how these systems work, therefore it is refreshing to see someone who knows what their talking about and is willing to share accurate advise!
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