does any one know what the psi for low and high side would be for a 134 change over for a 72 chevelle thanks ed

NO ONE can tell you this unless you also include the ambient temperature at the time you're reading the gauges.

Ed,

here is a chart to use. You have to know the air temperature in front of your condensor. Then use that number to cross reference your psi for highside.

Here is an r134 chart:
http://www.efproducts.com/faqs.php?faq_id=53&category_id=12

Here is a link that has r134 and r12 pressures vs temp charts:

http://home.earthlink.net/~bob1.gardner/sitebuildercontent/sitebuilderfiles/3refrigerantpressuretemp.pdf

Hope this helps

thanks rod that's what i was looking for . seems the low side is a little high 70 and the high side is 275 this is at 95 deg vent is blowing 63 would like to see 55 out of vent . can i open the vale on the low side and discharge it till i reach 55 as per the chart on that web site ? they say its over charged . all parts are new and i have had this system working great w/ my sbc the bbc just cant get it right . i also put in 3 lbs of 134 would you say this is over charged they say 90 percent is good for 134 so really its 1/2 lbs under. would this make the difference:confused:

Im not a professional at it, but what I understand is to start at 80% of original ammount, then slowly add it till you get the correct pressures.

High Suction Pressure is caused by the following.

1. Loose expansion valve
2. Overcharged system
3. Expansion valve stuck open
4. Compressor reed valves
5. Leaking head gasket on compressor


Here is the repair for the respective problems.

1. Tighten valve
2. Remove some refrigerant
3. Replace expansion valve
4. Replace reed valves
5. Replace head gasket

I generally use 90 percent too. As a rule of thumb, the high side should be 100 plus the ambient, if its 90 outside, then 190 psi on the high side, and generally 25/30 on the low side with the A/C set on MAX at idle and the fan on low.

Was the system properly evacuated prior to charging?

Another thing that is often missed is airflow over the condenser. Your gauge readings seem to bear this out.
R134a conversions need as much air as possible flowing over the condenser because the old style serpentine unit can't quite remove enough heat.
While watching the gauges and with the engine at 1500 rpm, mist some water over the condenser with a garden hose. If the high side pressure drops considerably, you have an airflow issue.
Your BBC is generating more heat and it's larger bulk in the bay leaves less room for airflow under the hood. This will exacerbate a marginal airflow condition.

-Any time you do a conversion, you should replace the fan clutch with a new OEM unit (aftermarket replacements won't move enough air for good low speed performance).
-Make sure the fan shroud is intact and properly mounted.
-Seal up the gaps between the condenser and radiator

Once you address the airflow issues you will probably find that you are a bit undercharged also. Don't add more refrigerant until you have good airflow. You will raise the high side pressure and possibly damage the compressor.

Did you have the POA valve adjusted for R134a? When everything else is in order, having the POA adjusted for R134a can make a huge difference in the vent temps. (The adjustment needs to be made prior to installation).

Another thing that is often missed is airflow over the condenser. Your gauge readings seem to bear this out.
R134a conversions need as much air as possible flowing over the condenser because the old style serpentine unit can't quite remove enough heat.
While watching the gauges and with the engine at 1500 rpm, mist some water over the condenser with a garden hose. If the high side pressure drops considerably, you have an airflow issue.
Your BBC is generating more heat and it's larger bulk in the bay leaves less room for airflow under the hood. This will exacerbate a marginal airflow condition.

-Any time you do a conversion, you should replace the fan clutch with a new OEM unit (aftermarket replacements won't move enough air for good low speed performance).
-Make sure the fan shroud is intact and properly mounted.
-Seal up the gaps between the condenser and radiator

Once you address the airflow issues you will probably find that you are a bit undercharged also. Don't add more refrigerant until you have good airflow. You will raise the high side pressure and possibly damage the compressor.

Did you have the POA valve adjusted for R134a? When everything else is in order, having the POA adjusted for R134a can make a huge difference in the vent temps. (The adjustment needs to be made prior to installation).
yes all the above was done with the fan . no i didn't do the poa value you say this make a big difference ? i brang the car to work w/me sat and used are robin air charger on the car and recharged it ad went w/ 3.2 lbs was blowing 55 out of center vent until motor heat played a factor 62 deg i have other issues going on with it running warm been working on the hot problem a few night a week and then the front seal on the a/c pump start the leaking . maybe time to put it in the corner and worry about it later need a time out or shall i say the car needs a time out i thank every one who is trying to help the more in put some times works out i maybe just over looking the problem thats sitting in front of me

yes all the above was done with the fan . no i didn't do the poa value you say this make a big difference ? i brang the car to work w/me sat and used are robin air charger on the car and recharged it ad went w/ 3.2 lbs was blowing 55 out of center vent until motor heat played a factor 62 deg i have other issues going on with it running warm been working on the hot problem a few night a week and then the front seal on the a/c pump start the leaking . maybe time to put it in the corner and worry about it later need a time out or shall i say the car needs a time out i thank every one who is trying to help the more in put some times works out i maybe just over looking the problem thats sitting in front of me

Yes, adjusting the POA can be the difference in a conversion that works "OK" and one that works really well. That is, if everything else is up to par.

If the engine is running hot, this will reduce the AC performance. The radiator can actually "radiate"heat back into the condenser even though the fan is moving the air away. Get the hot condition fixed, then dialing in the AC will be easier.
You mention the vent temps going up as the engine warmed up. You may have some reheating going on through the evaporator case or from the heater core. Is there a shutoff valve in the heater hose? You might consider insulating the evaporator case under the hood if the engine sits closer to the case than the original. The exhaust manifold is pretty close back there even with an SBC.
Just some things to consider.
Good Luck