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85 Suburban with vortec 350, what compressor?

fix_it on Sun June 16, 2013 2:04 AM User is offline

Year: 1985
Make: GMC
Model: Suburban
Engine Size: 350
Refrigerant Type: r134a
Country of Origin: United States

Hello. I have been reading this forum for years. Actually, at one time I was signed up and received advice on several of my vehicles. I have not been here in a long time and could not remember my user ID, so I am singed up again.

Anyhow, on to the question. I have a 1985 GMC Suburban. Right now it is still has the R4 compressor. I installed a new 6mm condenser, severe duty fan clutch, an auxiliary pusher fan, sealed the air path through the radiator/condenser, added a vacuum operated heater cutoff valve, and retrofitted to r134a. It works pretty well. After it cools down I can get vent temps in the mid 30s. It just takes it a while (20 minutes or so) to get there, as this is a huge truck to cool with just dash air. Overall, I think it is working as well as can be expected. I am considering replacing the engine with one from a 1996 or so truck. This will include the serpentine belt system which comes with an HT6 compressor. My question is in regard to the HT6 displacement vs the R4. It seems that the HT6 was used on that vintage Suburban with rear AC, so it would seem that it has acceptable capacity. However, as far as I can tell the HT6 is 10 ci, while the R4 is 11 ci. The V7 is more of a match for the R4, as it is also about 11 ci. Will this make a difference?

If I were to stay with the HT6, the mounting is easier. That is what the bracket is made for. The lines connect to the top of the HT6 versus the back of the R4. May need to modify the line set. HT6 is a fixed displacement, therefore the system stays CCOT, and the low pressure switch determines vent temps.

If I use a V7, I will have to modify the bracket. Not sure yet on the extent of that, as I haven't actually had the two compressors side by side, only seen pictures. However, the lines connect to the rear of the V7 and may not require modifying the line set. The V7 is a variable displacement compressor. I assume I would lower the adjustment on the cycling switch to around 10 to 15 psi to become a low refrigerant cutoff. Then the control valve would determine the vent temps. This would be more in line with the old POA systems. Those worked really well. However, the lowest psi control valve is 37 psi. I have read that they have been consolidated and are all about 41 psi. I run my cycle switch in the 20's to get mid 30 to low 40 vent temps. I am worried that with 41 psi low, I might be in the 50s at the vent.

So does the HT6 have enough displacement for this huge truck? If not, then what, if anything, can/should be done to lower the set point on the V7?

GM Tech on Sun June 16, 2013 7:33 AM User is offline

YES - it is used on the later model Burbans--so why the question?

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......

fix_it on Sun June 16, 2013 10:35 AM User is offline

Probably just over thinking it. I had read that the HT6 was smaller in displacement than th R4. Just didn't want to create any problems.


mikeinohio on Fri June 28, 2013 7:07 AM User is offline

Compressor displacement often times has little to do with how much refrigerant a compressor will actually compress in a given situation. You also have to consider the rotational speed and volumetric efficiency. The R4 displaces 11 cubic inches per revolution of the compressor and the HT6 displaces 10 cubic inches per revolution of the compressor, but the rotational speed of the compressors will vary depending in the size of the compressor pulley in relation to the engine pulley. The trend in newer vehicles seems to be to drive a smaller lighter compressor at a higher rpm. So it is likely that the HT6 will pump more refrigerant simply because it rotates faster in relation to the crankshaft than the R4. The other issue is volumetric efficiency. Volumetric efficiency is the ratio in a percent between what the compressor displacement is and how much it actually pumps. It is never 100% because there is always some volume in the cylinder even when the piston is at top dead center of the compression stroke. The trend in newer compressors is tighter tolerances and a resulting higher volumetric efficiency. Say for example the R4 compressor has an 80% volumetric efficiency. That would mean that is can actually compress 8.8 cubic inches of refrigerant per revolution. On the other hand if the HT6 has has a 90% volumetric efficiency, that means it pumps 9 cubic inches of refrigerant per revolution.

In your situation, I think I would just install the HT6 compressor. As long as you properly service the system and don't cut any corners, I think you will find out that your system performs better with the HT6 compressor than what it ever did with the R4 compressor. Particularly at idle and stop and go traffic. Also it should be more durable. The R4 compressor was designed in the late 70's or early 80's with R12 in mind. Sometimes they don't do so well with the higher head pressures of R134a. The HT6 compressor is a more modern design and should hold up better to the higher head pressures.

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