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Determining System Capacity on non-standard AC

TR 1 on Mon September 21, 2009 1:21 PM User is offline

Year: 1990
Make: Mazda
Model: RX7
Engine Size: 5.7
Refrigerant Type: 134A or R12?
Country of Origin: United States


My name is Mark, and I came across this forum looking for info concerning charging the AC on my 90 RX7. However it's a non-standard question in that my RX7 has an engine out of a 2002 Firebird WS6 as well as the associated compressor. This engine swap is becoming somewhat popular, and the common way to retain AC in your car post swap is to have custom lines made to join the GM compressor to the mazda high and low side lines. You then use the GM pressure switch and ecu to control the compressor via the stock GM ECU. Most seem to convert to 134a, though I have not heard much concerning flushing the existing oil out of the Mazda R-12 components.

So that's the background. I have a decent knowledge of auto AC systems, but I am still fairly new to working on them, so any advice/corrections to wrong assumptions would be appreciated.

Question 1: R12 or 134A? As I said most seem to switch to 134A, as using the GM ECU and pressure switches seems to be the most common, and this is the direction I am leaning. However, I do have tools, ect. for both types of systems, and if it is possible to use R12, and that would give me the best performance, I would look into that route as well.

Question 2: If 134A, should I flush out the oil in the Evap/Lines etc? I actually have a new, unused condenser, and well as a new drier I plan on using when I re-assemble the system. I had planned on flushing the remaining components, and adding new oil along with the rest. Is the flush necessary?

Question 3: Oil Capacity? I had planned to use the GM capacity simply since I am going with their compressor. Is this correct?

Question4: Refrigerant Capacity? I've been trying to read up on charging techniques on this site, and it seems the consensus is gauges are not the best option when charging 134a. Neither is using the sight glass. Since I do not really know what the "Factory" capacity would be on this setup, are their any other options? I had read about using the "Delta T" between the evap inlet and outlets to see when a system is fully charged. However, this is where I get fuzzy and was hoping for clarification. I may have read wrong, but I gathered this can only be used on "Orifice Tube" systems. I believe the Mazda has an expansion valve system. So does this mean I cannot use this method to determine if the system has a full charge?

TIA for any input advice, etc...


mk378 on Mon September 21, 2009 9:19 PM User is offline

That should be a V5 compressor. Being a variable compressor it doesn't need to cycle. It should stay on all the time the A/C button is pressed unless an abnormal condition (over pressure, under pressure) occurs. Keep the Mazda high/low switch and wire the compressor output of the Mazda system to the AC request input of the ECU. You don't need the evaporator temperature sensor any more because it's a variable compressor.

How exactly is the system set up? Is the only change to replace the compressor? Then it's still a standard TXV system (which is good). A TXV system does not have an accumulator or switch on the low side.

If the only difference is the compressor, refrigerant capacity should be very similar to the stock Mazda system. The oil capacity will be more because the V5 has an oil sump.

The 2002 compressor would come with PAG oil in it. PAG oil chemically reacts with R-12 to form sludge. You would need to remove every drop of oil before using R-12. Not worth it probably. It should be possible to upgrade the condenser if performance is inadequate. The pressure control valve in the compressor is of course calibrated for R-134a already.

Like any conversion, flush out as much of the mineral oil as you can. Don't try to flush through the TXV, it must be disconnected to flush the evaporator.

NickD on Tue September 22, 2009 9:56 AM User is offline

Can't blame you for dumbing the worlds greatest invention since the piston engine, the Wankel. Rotor seal problems, geometry limits compression ratio to a mere 7, combustion gases go into a wedge for reduced engine torque, loss of performance, and extremely poor fuel economy.

Feel dumping in a V-6 is an overkill, way too much front end weight for that little car, oversteering, lack of control around corners, should be a good rear tire squealer, and may even end up killing you. My option would have been a Honda Civic 4 banger, with all those hop up goodies. But that is your problem

With the AC system, you will have to decide to keep it stock or to switch over to the variable displacement compressor circuit, can't mix both, would add total confusion to the system as the both the TSV and the control valve would be trying to decide what the low side pressure would be doing. TXV depends on a receiver, control valve on an accumulator. Either go one way or the other, but not both.

mk378 on Tue September 22, 2009 10:35 AM User is offline

I disagree with Nick. Variable compressors work great with the rest of the system being standard TXV. That's how most of the later VW's are set up.

NickD on Tue September 22, 2009 2:53 PM User is offline

Don't disagree, but rather detail the system you have in mind keeping in mind he is dealing with a 90 TXV R-12 system.

Typical variable displacement system consists of the compressor feeding the condenser, to a fixed orifice, to the evaporator, then through an accumulator, back to the compressor, works super great. Normally thermistors are used for high and low side cutoff either due to excessive pressure on the high side, or low side pressure due to cold ambient temperatures, a standard dual function switch should work. His climate control would have to directly engage the clutch coil with a relay interface enabled by the dual function switch. Simple enough, blower and mode controls will remain the same.

jackhild on Sat September 26, 2009 10:15 AM User is offline

If you go with R134a, you are going to need a condenser upgrade. The stock RX7 condenser capacity is inadequate for 134a. It was just barely adequate for R12. In general you will not find successful R134a conversions in the 2nd generation RX7. Common issue is excessive highside pressures due to this inadequate condensing capacity. When the charge gets adjusted to control the pressure, the result is marginal performance.

You should look into the parallel flow condensers available on this site. If you do this, you will attain *excellent* results.

A less desirable, but easier path is using Freeze12. There are quite a few FC3S owners that have converted to Freeze12. The blend (80%-134a and 20%-R142b) helps control the highside pressure. It will also be completely compatible with the lubricant in your GM compressor. The downside is that very few professional shops will work on a system charged with alternative refrigerants.

Personally I would advise you to upgrade to the parallel-flow condenser, flush that GM compressor to remove the PAG, install Ester oil, a new drier and charge with pure R134a.

Note: I have a 90 FC vert with factory A/C charged with R12(4 years, no trouble). I bought it in 2005 with an inoperable AC. Converted to 134a with horrible results. Tried HC refrigerants, bad news. Tried Freeze12, worked OK. Charged with R12 and Ester with terrific results.

Good Luck!

Edited: Sat September 26, 2009 at 10:20 AM by jackhild

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