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v7 pressure control in hybrid system

byrden on Fri May 15, 2009 10:13 PM User is offline

Year: 1993
Make: Mazda
Model: RX-7
Engine Size: 5.7
Refrigerant Type: r134a
Ambient Temp: 70
Country of Origin: United States

I have an engine swap car. I'm using the mazda's r12 evaporator/txv, a V7 compressor from a 2001 Camaro, and an aftermarket 12x24 condenser. The mazda uses a thermoswitch to cycle the compressor which I have bypassed since the V7 is variable displacement.

I've noticed that even on cooler days the duct temperatures are not as cool as I'd like. Most of the time (when it's cool out) duct temperature stays around 50F, eventually creeps up to about 52F, then it will drop to about 45F then stabilize back at 50F. I took a drive with the gauges on to look at the pressure (70F ambient, humidity 70-80%, driving about 65mph, 1500rpm). The low side pressure stayed at about 27psig most of the time, sometimes it would go up to 29 psig, and would drop down to maybe 25-26 psig (which is when I get the 45F duct temperature). Having the fan speed either on low or high didnt make a difference. High side pressure was about 90-100psig.

Since I know the system will go down to 44-45F, I know it can cool which makes me think that the pressure control setpoint of the camaro's V7 compressor is not optimal for the new system. The camaro had an orifice tube/suction accumulator setup.

So, what should the low side pressure stabilize to in a low heat load, steady state condition? Right now it looks like it wants to keep the low side at about 27psig. Do I need to try a different control valve in the V7? Does anybody know where to find the specs on the available control valves for the V7?

Chick on Fri May 15, 2009 10:20 PM User is offlineView users profile

Looks like more of an undercharged situation, your high side should be more like 140 to 150 at 70 degrees, try adding a little more refrigerant and see if that helps..

-------------------------
Chick
Email: Chick

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Freedoms just another word for nothing left to lose

bohica2xo on Sat May 16, 2009 12:37 AM User is offline

I agree with Chick, you are likely undercharged.

The control valve data is HERE

Charge the system with a bit more 134a, and see if the low side pressure stabilizes first. V series compressors don't usually hunt that bad if the system is charged.

B.

-------------------------
"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

byrden on Sat May 16, 2009 10:12 PM User is offline

I was afraid to hear that (low charge). I can only charge the car on pressures since its no where near a factory system now. I charged the car on a low to mid 80's day and in order to get decent cooling I had to go to about 2.4 to 2.5 times ambient. The longer the car sits in one place (especially if the wind is still) pressures keep climbing. I'm sure there is alot of recirc and it gets worse the longer the car sits. The engine compartment is cramped and generates alot of heat. The fans are electric, and are working properly. I have even sealed up the radiator ducting and the gaps between the radiator and condenser. Spraying water on the condenser does lower the pressures a bit. I'm sure the car could use more condenser but it's the largest I can fit (12x24 parallel flow). Is there any hope to get decent cooling from this condenser? I can't immagine I can get much more airflow and there is very little space for any pusher fans. I'm thinking recirc is the issue, not airflow.

Back to the v7 pressure control, I checked and I have a "red" valve. According to the chart in the link above this is one of the higher pressure valves. What do the pressures in the table list? Is it suction pressure (in psia) or is it the pressure at full stroke (in psig)? Could the fact that the system has a txv and suction pressure control cause the excessive "hunting?"

According to the link the red valve is set at 44. Later in the thread it says gm consolidated the valves and is only using 1 valve set at 41, is this true? What is a typical r134 low pressure cutout switch pressure for a CCOT system?

Sorry about all the questions, but maybe this info can also help other crazy engine swappers.

Thanks!

Chick on Sat May 16, 2009 10:20 PM User is offlineView users profile

It really sounds like airflow, you might want to consider a stronger fan, like one off a Lincoln Mark?? or Mustang, not sure of the room you have to work with.. If you have a multi speed fan, make sure it switches to high when the ac is on, or pressures rise.. 2.5 times ambient is not uncommon for a retrofit to begin with..

-------------------------
Chick
Email: Chick

---------------------------------------------

Freedoms just another word for nothing left to lose

bohica2xo on Sun May 17, 2009 5:07 PM User is offline

That pressure number in the charts is a reference, based on the cartridge. The actual evaporator is directly related to the valve pressure, but is not the same. The occasional low pressure you see is most likely the setpoint of your control valve.

The TXV is not causing the hunting, having both pressure controls is a good thing.

134 conversions are ok as long as they stay below 3.0x ambient under most conditions - this is a common operating area. I have seen poor candidates that ran up to 3.5x ambient at times without major failures.

Engine swap cars are always a challenge too. Mist the condensor, add a couple of ounces. Tape the gauge set to the windshield, and go for a drive to see the real world operating range. If it creeps up to 3.2x in traffic, then you need to find a way to add condensor capacity or airflow.

Dodge used a small secondary condensor on some minivans with rear A/C. Basically it was a liquid aftercooler, plumbed in after the main condensor. It was about the same size as the RX7 OEM oil cooler, and might fit in that location...

B.

-------------------------
"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

byrden on Sun May 17, 2009 7:15 PM User is offline

Adding an aux aftercooler sounds like a great idea, and a great way to add condensing capacity on this car. The stock condenser size is probably barely adequate for R12 and because of the short and wide radiator, larger fans will be difficult. The way the radiator and condenser sits at an angle in the duct there is little room for pusher fans also. The oil cooler duct is currently empty, has it's own opening in the bumper and discharges out of the vent in fender behind the wheel. It would probably be less susceptible to recirc since it is fairly well isolated from the engine compartment.

So is the consensus that the suction pressure of 26 to 27 psig is where it should be? 27psig puts it about 32F for saturated vapor. Or should it be lower once you figure in the 3 to 5 degrees of superheat from the txv? This is ofcourse assuming that I can get enough condensing capacity to get subcooled liquid to the txv.

bohica2xo on Sun May 17, 2009 7:47 PM User is offline

That condensor was from a 96 to 97 Caravan. The info is HERE.

26psig is just fine. As close to freezing as you can get without building ice.

B.

-------------------------
"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

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