Where are you? If it gets hot-100 deg.+ or your AC duct temp goes up a bunch when sitting at lights,you will need an extra cooling fan on the condenser.134a runs at higher temp.and pressure,with the same condenser as r12.I've blown up enough systems,or had cooling problems.If your car runs hot,the AC system will die.Basically, if you can keep your high side pressures under 350-375# when idling for 10 minutes,and then holding it at 1500 RPM for 1 minute,when it's real hot out,you're all right.
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68 z28 68 rs 327 73 454 vette