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System Capacity...Different Model Compressor

69Sixpackbee on Fri November 14, 2008 2:56 PM User is offline

Year: 1972
Make: Chevy
Model: C-20 Pickup
Engine Size: 402
Refrigerant Type: 134A
Country of Origin: United States

My '72 Chevy with the factory "four-seasons" calls out for 3.25lbs of R12 with the original A-6 pump. I have swapped in a Sanden SD5H14 and have been using it very sucessfully now for the past year with my R134A conversion. I have a picollo condenser and a good clutch fan as well as an auxilliay pusher. I am going to completely evac. the system and recharge it since I need to get the POA down another 1PSI. I did not charge up by a known weight initially because I put it in at the 80% of the 3.25lbs (41.5 ozs) and it seemed to be too much so I kept backing it out until I got the best temps...Crude but it worked yet I did not know how much I took out because my scale on the charging station was broken and I was using a portable scale under the jug of 134A.

The question I have is: Does the original capacity of the system still hold the same amount of gas, theorhetically, even if the compressor has been swapped out to another type/size?


iceman2555 on Fri November 14, 2008 4:49 PM User is offlineView users profile

Dang computer...could not wait for my message....
Since both the condenser and compressor have been changed, the recharge rate may have changed also. The compressor volume should be app the same, however, the change in the condenser design would probably contribute the greatest change in recharge.
When 'playin' 'round' with retro's in the early days, we discovered that when changing the OE 'tube/fin' condensers to the more efficient parallel flow/pic flow units that in most cases the recharge rates could mimic OE specs.
One aspect of the pic condensers, is that these units were at times constructed using brass and copper instead of aluminum. This type condenser does not have the same heat transfer as the OE aluminum units. This, of course, may contribute to possible elevated high side pressures...and possible loss of cooling at low RPM operations. Just a thought.
Since the 'cooling' efficiency has been upgraded a suggestion would be to charge as close to OE specs as possible. This takes in to consideration that the remainder of the engine cooling system is functioning at its peak efficiency.
Charging until a 'suitable' temp has been reached is not a foundation for a successful A/C repair. Keep in mind that an 'under charged' system may actually indicate cooler vents than a properly recharged system. This is especially evident when the 'high rpm, closed door, max cool, med/low speed' performance test is used. Many years ago, we made a change to testing our repairs using a high heat saturation instead of attempting to 'see how low' we could make the temp gauge read. We were seeking a temp difference between amb and vent to determine performance.
But back to the issue at $.02 worth would be to arrive at a recharge rate as close to OE specs for the system as possible. A difference of 5-10% may drastically effect both system performance and system longevity. Too little refrigerant and the lubricant tends to 'puddle' in certain areas of the system and does not migrate as it should to keep the compressor lubed. The best aspect of that 'ole A6' was the ability of the compressor to maintain a certain amount of lube within the crankcase. Most modern compressors today lack that ability. The Sanden does maintain a 'slight' amount of lube within the case...but does rely on the refrigerant to maintain the proper lubricant migration.

The strongest reason for the people to retain the right to keep and bear arms is, as a last resort, to protect themselves against tyranny in government.
Thomas Jefferson

Edited: Fri November 14, 2008 at 5:14 PM by iceman2555

69Sixpackbee on Sat November 15, 2008 12:15 AM User is offline

Thanx for the reply!
I was toying around today and made some more preliminary testing. The ambient today was 96 degrees (6in in front of the condenser) as measured with my Fluke "K" thermocouple digital meter which, by the way, is he one I use for all of my readings. I had the engine idling, pulled the spade connector off of the fan connection and took the low-side reading right off of the tap on the POA. It was a rock-steady 26PSI so I guess I had pre-adjusted the POA correctly initially. I then reconnected the terminal, spun the fast idle up to 1700 RPM, opened the doors with the fan on "HI" in recirc mode and watched as the high-side climbed to 230PSI and pretty well stayed there. This coincides fairly well with the "ambient x 2.2" rule. The pusher came on (wired up to the trinary switch) and there was already alot of air coming across the condenser from the big 7-blade clutch fan anyway but the more the merrier. The center vent temp was 44-degrees. I closed up the cab and the pressure started to come down as expected. If the fan is on a lower speed I get the usual frosting of the back-half of the POA clear back to the connection on the compressor. It is pumping well and my low-speed performance rarely diminishes even on the hottest of days. But, I had to shut the truck off to go into the house for a while. When I came back out to go I started off and within 10-12 minutes the cab was very comfortable once again. I get the center vent temps to hover around 38-40 degrees regardless of fan speed so it looks like all is well. I just know, however, that when I first charged the system to 3lbs. (which is what I started with) it did not cool as well until I started to remove some of the charge and so I do not know exactly what I have at this point. I get some gas visibly "whizzing" past the sight glass currently and I know there is some speculation that this is "normal" with a 134A charge and a few others have disputed this and say "charge 'till the bubbles are gone" just like the R12 was. The rec/drier is physically smaller than the original unit and the condenser is very large---348sq/in. I guess because of the design it is probably, like you eluded to, that is is probably close to the same capacity as the original single-pass tube and fin unit was. I guess I am just being too anal but for my own edification I would like to know what the EXACT volume is so I can document it for future reference as well as to be able to have a "baseline" for the others who have a similar setup. This is how it looks:
Completely insulated and all new seals on the blend door so I get absolutely no "carryover" from the heater core side even when the "fresh" air lever is moved into place which then opens the water control valve on these old Chevy systems. I put all new parts on this system when I installed it, i.e., vacuum pods, seals, fresh air flapper (inside the cowl), water valve, TXV, hoses, "O" rings, trinary switch, rec/drier,...everything!

Tomorrow I am going to completely evacuate the system, pull a vacuum and start charging it up once again. I figure I am really close it's just that I don't know exactly what the final amount is...and THAT is what's bugging the hell outta me!

I will have quite a few more days of warm weather to play with..that is the ONLY thing nice about Southern California
I will post my results and many thanks for the helpful advice!


mk378 on Sat November 15, 2008 5:20 PM User is offline

My first impression would be not to mess with it. Trying to pusht he vent temps below 40 in a humid climate really risks evaporator freeze-up.

The obvious way to find out how much charge you have is to recover the charge and weigh the recovery tank. Then add a little because there is alway some left behind, as recovery machines don't pump to a full vacuum.

There is some lattitude in charging a TXV system. At first, excess refrigerant will fill up the receiver drier without affecting performance. This also allows some leakage to occur before performance decreases. When badly overcharged, the receiver becomes completely full and the condenser starts to fill up with liquid and performance suffers. Do beware of undercharging like Iceman said.

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