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Keep fixed displacement or go variable?

G-Body on Tue August 16, 2011 11:11 AM User is offline

Year: 1985
Make: Chevrolet
Model: Monte Carlo SS
Engine Size: 325
Refrigerant Type: 134a
Country of Origin: United States

I have a 1985 monte carlo ss that I am putting a 5.3L swap into. I have 3 different choices of A/C compressor that I could use.
The fixed displacement: 10s17f or 10s20f compressors (I think I would be better off with the 10s17f as it more closely matches the R4 compressor displacement)
The variable displacement HU6 compressor.

Are there any benefits or drawbacks to either? Is the HU6 a better compressor or should I keep it a clutch cycling system?

I already have a similar setup on my 85 el camino that I swapped a 6.0L into and used the 10s20f compressor, it seems to work well. I`m just trying to determine if switching to an HU6 compressor would make for a better system on the monte carlo.

JJM on Wed August 17, 2011 12:05 AM User is offline

I'm not the best one to ask since I prefer keeping things stock original. But if you're going to go with a different compressor and want fixed, you might as well get the extra displacement the 10s20f. Variable displacement compressors typically do a better job of cooling, since they never cycle, but the electrical requirements for such a system are a bit more complicated in the absence of simple cycling switch.


When considering your next auto A/C purchase, please consider the site that supports you:

G-Body on Wed August 17, 2011 1:20 AM User is offline

Maybe I will try a variable displacement compressor on this swap then. I was more worried about the variable displacement compressor being reliable, as GM started out using fixed displacement compressors, switched to some variable displacement compressors in the 90`s and then switched back to fixed displacement. I figured there must have been some issue/problem with them for GM to stop using the variable. The wiring side of it doesn`t worry me at all, its set up for the low pressure cycling switch right now, but I checked the diagrams for a HU6 truck and it looks like I just have to add a high pressure cutout switch. Thanks for the reply.

emsvitil on Wed August 17, 2011 2:36 AM User is offlineView users profile

GM probably went back to the fixed displacement because of cost............

Do you have something to control the variable displacement?


GM Tech on Wed August 17, 2011 8:51 AM User is offline

GM uses variables still on a bunch of its vehicles---the CVC is a variable.....other compressors are used based on $$$$$

The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

NickD on Wed August 17, 2011 1:22 PM User is offline

First issue is this, with engine swapping, will have to have your vehicle certified, fines can be stiff if you are caught. Second issue is to address your insurance company with these modifications. Not even getting into DOT safety standards for public highway issues. The freedom we use to have to modify our vehicles is history. A primary consideration before deciding what kind of a compressor to use.

"Engine Swaps

Rules regulating engine swapping are not make/model specific. The simple rule is that a vehicle must have all emissions components that were present when it was manufactured, which may include:

PCV - positive crankcase ventilation
ACL - air cleaner (thermostatic air cleaner)
AIS - secondary air injection
EGR - exhaust gas recirculation
EVAP - evaporative emission
CAT - catalytic convertor
SPK - spark control
FR - fillpipe restrictor
O2S - oxygen sensor

Read more information about emissions tampering regulations.

Reconstructed or Rebuilt Vehicles

All vehicles used on the public highways are required to meet all of the state equipment laws and requirements; therefore, reconstructed or rebuilt vehicles which are using the public highways are also expected to meet all of the state equipment laws and regulations. Reconstructed or rebuilt vehicles in many instances fail to meet state requirements and, therefore, are not legal for use on the public highways.

All reconstructed or rebuilt vehicles (sand or dune buggies or hot rods) must comply with inspection requirements for the class of motor vehicle it is being inspected as, such as car, truck, motorcycle, or motor-driven cycle.

Be sure to check reconstructed or rebuilt vehicles for all required items of inspection with particular attention to the lighting devices. Head lamps shall be of a type acceptable by the Department. No modifications are allowed that will change the original design or performance of any lamp. Only acceptable automobile head lamps may be used on cars and trucks. Either the 7-inch head lamps or both dual head lamps (type 1 and type 2) may be used. Only acceptable motorcycle head lamps may be used on motorcycles, motor-driven cycles, and mopeds.

All lighting devices must be of an acceptable type that meet Department standards and must comply with the mounting heights as specified in the inspection requirements.

The year model of a reconstructed vehicle will be the same year in which it was reconstructed and not the year of original manufacture. Therefore, the inspection requirements would be for the model year of the vehicle (same as the year of reconstruction) or the year model of the engine itself, whichever is the later model.

Motor vehicles used for competitive racing, such as modified stock cars, dragsters, and hot rods may be inspected. When such a vehicle is presented for inspection, all rules and regulations regarding the inspection of the vehicle will apply. This applies to brake requirements, exhaust systems, as well as any other item required in these provisions.

Kit Kars

Vehicles have to meet the emissions standards for the year the vehicle is assembled. Vehicle manufacturers have to certify that their vehicles meet EPA emissions standards. A lot of kit car manufacturers also comply with this requirement. If you purchase one of these kit cars, follow the instructions on assembly, including the emissions components. You should be able to pass an emissions test just like any other new car.

Just as the EPA does not allow an individual to reverse engineer a vehicle to defeat emission standards, they do not allow an individual to build a brand new "old" vehicle to bypass emissions standards. It is possible if you actually use old parts (like a 1965 engine, or complete 60s frame and powertrain) that the vehicle will be registered as that model year (replica), but this is a TxDOT issue. However it is registered, is how DPS inspection stations will test it."

G-Body on Wed August 17, 2011 3:33 PM User is offline

Originally posted by: emsvitilDo you have something to control the variable displacement?

I thought that the compressor controlled its own displacement via the temperature and or refriger ant pressure?
I have a HU6 compressor in the garage and the only electrical connections are for the clutch and a pressure sensor in the back.

NickD: Engine swap regs vary wildly from state to state. California is the toughest, but even they will allow engine swaps as long as you use an engine from the same class of vehicle, its newer than the vehicle you are putting it into and you utilize all of the emissions control devices from the new engine. Where I live there are no longer emissions tests or inspections for vehicles older than 1996. Thanks for the heads up though

NickD on Wed August 17, 2011 4:04 PM User is offline

That part is crazy, but so far, no customs to go through or emission inspections when crossing a state line. Can still buy a can of R-134a in Minnesota, Michigan, or Illinois without fear of getting stopped when coming back into Wisconsin. Can't buy cans in Wisconsin, only 30# containers and require a certificate to make that purchase. But I wonder how long this freedom will last.

In theory at least all you need to run a V-5 or a V-7 is switched 12V to the compressor clutch coil, along with a parallel flow condenser, accumulator and hoses with the proper orifice. If you have enough sense to switch it off if sub-freezing or high pressures that will cause belt squeal. Only control on these systems is a high side thermistor that when its resistance is high due to cold temperatures, or low due to high pressures, heat and pressure are directly related in AC, would be to kill the compressor clutch coil circuit.

This feeds into the PCM that you will not have depending on your swap, with additional functions to kill the compressor when accelerating or if the engine is overheating. But would guess that a high side dual function switch, say from an 85 Honda Accord will do the same thing for under and over pressures. It contacts would be in series with your AC climate control 12V to a clutch relay solenoid whose contacts would activate the clutch coil. This switch will have closed contacts if the pressures are between 40 and about 400 psi for compressor protection.

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