AC Orifice Tube

[DMCJosh85]

I'll be converting my system to R134a soon enough and over the pat few weeks I've ordered everything I will need but contrary to what I thought I had on hand I'm short a new orifice tube. I understand its a common GM part so I shouldn't have any trouble finding it at my local auto-parts store, but what i'm hoping to learn here is does anyone know the correct part number for this that might be carried at somewhere like oreilys, autozone or even amazon.

Additionally I've read up a little about Variable Orifice Tubes that some some have claimed to have gone with for R134a systems but I cant find much in terms of how well these worked out and if anyone had a part number or source for that as well I'd be curious to give it a go myself.

Im not really wanting to put in an order with a vendor over this small part, I appreciate anyone having any help to offer on this matter.

[dn010]

MT0097-1 for standard or MT0629 for VOV (variable) at Autozone or one that fits an 84 Corvette 5.7 at other stores. I've used both types and with the VOV I would get a weird hissing gurgling noise from the orifice area every time I shut off the AC. I went back to standard and the noise went away. Otherwise, the VOV did work well and I didn't have any other issues with it.

[WHO1DMC]

T38623 is what I used! I did have to trim the little nubs off the backside to get a good fit. I think I paid 2.50 for it. Some people have had problems or no benefit with a variable.



Dave B.

[DMCJosh85]

MT0097-1 for standard or MT0629 for VOV (variable) at Autozone or one that fits an 84 Corvette 5.7 at other stores. I've used both types and with the VOV I would get a weird hissing gurgling noise from the orifice area every time I shut off the AC. I went back to standard and the noise went away. Otherwise, the VOV did work well and I didn't have any other issues with it.

Interesting, and thank you for the information. I'm changing my compressor to a Sanden SD7H15 model 4708. I wonder if it will result in a different outcome.

Much appreciate the help gentlemen.

[Helirich]

Can't say anything about the Delorean 134A, but on an old Ford truck converted to 134A, I tried both variable and standard orface tubes. Couldn't tell any differnce. Maybe, just maybe, the standard worked better. (Was cooler)

[DMCJosh85]

The more it seems I look into the VOV I get the idea that its not the way to go. Higher cost, uncertain benefits/gains, possible noise issues, etc... Perhaps the fixed valve is the winner here. I'll keep it simple.

[Bitsyncmaster]

The problem with the VOV is if your tube has a bend where it sits the VOV will not work.

[81dmc]

Supposedly, corvette owners have found that R134a doesn’t get very cold with the standard White Orifice Tube (1.8mm). When I had my car running in 2018, I also found that out. VOV didn’t help me either.

The corvette guys found that downsizing to the 1.6mm Ford orifice tube gives better performance and R-12 like high/low pressures.

So I purchased an orifice tube that is 1.6mm from a 2000 Ford F-250, and I will test how cold it gets once I get my engine back together. Part number 38635.

[Ron]

Supposedly, corvette owners have found that R134a doesn’t get very cold with the standard White Orifice Tube (1.8mm). When I had my car running in 2018, I also found that out. VOV didn’t help me either.

The corvette guys found that downsizing to the 1.6mm Ford orifice tube gives better performance and R-12 like high/low pressures.

So I purchased an orifice tube that is 1.6mm from a 2000 Ford F-250, and I will test how cold it gets once I get my engine back together. Part number 38635.Something don't sound right to me here??
Using your numbers, you would be decreasing the orifice diameter which would mean the refrigerant would flow through slower. That would mean longer cycle times, which would increase high side pressure. I always heard that people used VOVs on a D to lower the pressure because 134a has higher pressure than R12 (for 70°F and greater, which I know is true). Seems you would be making a bad situation worse. What am I missing?

[81dmc]

Something don't sound right to me here??
Using your numbers, you would be decreasing the orifice diameter which would mean the refrigerant would flow through slower. That would mean longer cycle times, which would increase high side pressure. I always heard that people used VOVs on a D to lower the pressure because 134a has higher pressure than R12 (for 70°F and greater, which I know is true). Seems you would be making a bad situation worse. What am I missing?

I thought the same thing, but with an orifice tube being $3 and easy to replace, I’m up for an experiment.

I’ve never had good ac performance in my car with all new components. I've tried everything but a different sized OT.

[Helirich]

Something don't sound right to me here??
Using your numbers, you would be decreasing the orifice diameter which would mean the refrigerant would flow through slower. That would mean longer cycle times, which would increase high side pressure. I always heard that people used VOVs on a D to lower the pressure because 134a has higher pressure than R12 (for 70°F and greater, which I know is true). Seems you would be making a bad situation worse. What am I missing?

The smaller diameter would make for higher "high side" pressures and lower "low side" pressures. But I would think it would make for shorter cycles because the low pressure switch would keep tripping. Not sure how it would effect on proformance.

I will say that it's been my experince that converting r12 to 134a just never works as good. New cars are great, but the conversions suck. That's why I'm keeping mine R12.

[Bitsyncmaster]

I thought the same thing, but with an orifice tube being $3 and easy to replace, I’m up for an experiment.

I’ve never had good ac performance in my car with all new components. I've tried everything but a different sized OT.

+1

I like owners experimenting. You never know what you find thinking outside the box.

I never had good AC performance until I replaced the condensation drain with a real elbow. The stock drain is pinched and can not keep up with humidity. When your box fills with water the evaporator does not work well.

[81dmc]

Actually I take it back. First I’m trying the 1.7mm Blue Orifice Tube p/n 38621 from a 2004 Ford F-150. If it’s not enough, then I’ll try the Red 1.6mm.


Got too much to do with the car. Getting ahead of myself lol.

[DMC-81]

The problem with the VOV is if your tube has a bend where it sits the VOV will not work.

+1. When i was refurbing my system, I bought a variable orifice valve to try and it broke due to the bend in the pipe. I installed the regular version and it works great (at least for R12).

Here's my thread which contains part numbers and process tips for A/C refurbishment..

http://dmctalk.org/showthread.php?15632-Refurbing-a-Very-Early-A-C-System&styleid=1

[Helirich]

Actually I take it back. First I’m trying the 1.7mm Blue Orifice Tube p/n 38621 from a 2004 Ford F-150. If it’s not enough, then I’ll try the Red 1.6mm.


Got too much to do with the car. Getting ahead of myself lol.

Please let us know how it works out. Never know if I will be forced to convert to 134.

[Ron]

I thought the same thing, but with an orifice tube being $3 and easy to replace, I’m up for an experiment.

I’ve never had good ac performance in my car with all new components. I've tried everything but a different sized OT.The consensus has been, if you live in the extremest of climates, don't convert due to performance. And since I've never had a problem getting 134a conversions to do ok here (but not as well as R12), I haven't played with a VOV on a D-134a.
Your results will be interesting!


The smaller diameter would make for higher "high side" pressures and lower "low side" pressures. But I would think it would make for shorter cycles because the low pressure switch would keep tripping. Not sure how it would effect on proformance.

I will say that it's been my experince that converting r12 to 134a just never works as good. New cars are great, but the conversions suck. That's why I'm keeping mine R12.After thinking about this a bit, I believe we were both wrong. A cycle would be from cut off to cut off, i.e. OFF time + ON time). The cycle switch will obviously operate in the same range (~ON 50 - OFF 30 psi), so the low side would too. What we need to know is if there is a difference in the time it takes for R12 and 134a to drop from 50 to 30 psi, while the compressor is OFF. According to the WSM chart, at 100°F ambient, the clutch should cycle OFF when the Low side is ~30 psi and the high side will be ~200 psi. Now it gets complicated. The pressure-temp characteristics of R12 and 134a are very close within the low side's 30-50 psi range. (33°F -> ~30.1 psi for R12 & 134a) So I would assume the percent of charge in the high side would be very close also. But with the 100°F ambient, would the 134a also be 200 psi at cut off? (A Fluke chart shows 62°F -> 60.1 psi for R12 and 134a, with 134a pressures raising faster; R12 @ 200 psi -> 138°F. At 138°F, 134a will have 223 psi.) This would mean the high side would have higher pressures, in its operating range. The higher pressure would mean the flow through the orifice would increase, to a point anyway. But then again, 134a retrofit's call for a 10% reduction in the charge. ...too many variables and things I'm sure I haven't thought of to nail down for a system that nobody has figured out how to make work as well as stock...YET :smile:

[81dmc]

The consensus has been, if you live in the extremest of climates, don't convert due to performance. And since I've never had a problem getting 134a conversions to do ok here (but not as well as R12), I haven't played with a VOV on a D-134a.
Your results will be interesting!

I refuse to agree that R12 is the only option for the DeLorean. Others said the same of my W126 Mercedes, and I have converted that R12 system with no issues. Difference is, that system uses a block expansion valve. But even then, I had bad performance until I charged it using superheat and supercool methods for optimum performance. Results were chilling.

IMO, all 134 conversions should be majority new parts with parallel flow condenser and PAG oil. If you don’t do that, it isn’t a good conversion.


Anyways, once my car is driveable, there’s going to be a lot of experimenting and I’ll be sure to post the results in a new thread.

[Ron]

I refuse to agree that R12 is the only option for the DeLorean.
Who here said that :confused:


Others said the same of my W126 Mercedes, and I have converted that R12 system with no issues. Difference is, that system uses a block expansion valve. But even then, I had bad performance until I charged it using superheat and supercool methods for optimum performance. Results were chilling.

IMO, all 134 conversions should be majority new parts with parallel flow condenser and PAG oil. If you don’t do that, it isn’t a good conversion.


Anyways, once my car is driveable, there’s going to be a lot of experimenting and I’ll be sure to post the results in a new thread.I agree. With an expansion valve you can constantly feed the evaporator with what it needs, rather than bounce across an "acceptable range".
(But, you can do the same with R12 -- No matter how you tweak it, R12 will out perform 134a in the exact same system, due to 134a's higher discharge pressures.)

...and definitely use PAG/134a .

That's a lot of part$ (especially with EEVs), may very well be worth it in FL/Vegas/etc. But, I personally don't see spending the time and money in this area, not while I'm sitting on a Snap-On R12 reclaim machine and 4 tanks of R12. :smile:

Please post the process rather than just the results!!!

[Helirich]

not while I'm sitting on a Snap-On R12 reclaim machine and 4 tanks of R12. :smile:
!

I used my robinair reclaim machine. I don't recall any perticular freon. It seemed to work. Did I screw it up for later?

[David T]

Unless you have to remove the orifice tube, either because it is broken or plugged up, leave it alone. It is in a place where the pipe is bent and it can be difficult to remove without damaging that VERY thin aluminum pipe. Once you damage that pipe you are replacing the evaporator coil and it is NOT a fun job. I do not understand why some people feel they have to "convert". R-12 is readily available, it is not that expensive, and you don't need much.

[Ron]

I used my robinair reclaim machine. I don't recall any perticular freon. It seemed to work. Did I screw it up for later?For the unit, you'd have to look at its manual. Some have oil-less compressors and made to recover several types of refrigerant, liquid/gas, etc. Mine is what they called a "station". It's older than dirt and I don't think it would like 134a because it has mineral oil.
I don't think one time would ruin it, but you might have mixed refrigerants...depending on what, if anything, was already in it.

[Helirich]

For the unit, you'd have to look at its manual. Some have oil-less compressors and made to recover several types of refrigerant, liquid/gas, etc. Mine is what they called a "station". It's older than dirt and I don't think it would like 134a because it has mineral oil.
I don't think one time would ruin it, but you might have mixed refrigerants...depending on what, if anything, was already in it.

My reclaim machine doesn't hold anything. It just pumps the freon out of the system into a tank. The tank I had previously vacuumed out, so there should be no mixing.

[Ron]

My reclaim machine doesn't hold anything. It just pumps the freon out of the system into a tank. The tank I had previously vacuumed out, so there should be no mixing.
Ah!...you don't know what the PO had in it???

[Helirich]

Ah...you don't know what the PO had in it???

Not following you. If you mean the tank, I bought it new and used it once to hold R22. That was put back in the fridge and whatever was left should be gone when I vacuum it out.

If you mean the system, it should be R12, but I guess the PO could have mixed it. If it was converted, they should have changed the fittings.

[Ron]

Unless you have to remove the orifice tube, either because it is broken or plugged up, leave it alone. It is in a place where the pipe is bent and it can be difficult to remove without damaging that VERY thin aluminum pipe. Once you damage that pipe you are replacing the evaporator coil and it is NOT a fun job. I do not understand why some people feel they have to "convert". R-12 is readily available, it is not that expensive, and you don't need much.We ought to call it the Pipe Of Regret.

[Ron]

Not following you. If you mean the tank, I bought it new and used it once to hold R22. That was put back in the fridge and whatever was left should be gone when I vacuum it out.

If you mean the system, it should be R12, but I guess the PO could have mixed it. If it was converted, they should have changed the fittings.??? I must have misunderstood what you were asking here:


...Did I screw it up for later?

[Helirich]

Unless you have to remove the orifice tube, either because it is broken or plugged up, leave it alone. It is in a place where the pipe is bent and it can be difficult to remove without damaging that VERY thin aluminum pipe. Once you damage that pipe you are replacing the evaporator coil and it is NOT a fun job. I do not understand why some people feel they have to "convert". R-12 is readily available, it is not that expensive, and you don't need much.

On my Bronco, the orface had a bunch of junk on it. This was on a working AC. I only pulled it because it was easy and I had the system down for a condenser replacement. I haven't even looked at the orface location on the D yet. I guess it's no fun.

[Nicholas R]

I have experimented with the GM white orifice tube, the variable orifice tube, the Ford blue orifice tube, as well as the Ford Red. Blue is smaller than white, red is smaller than blue. I have noticed better performance with the blue over the white for sure. The red is installed now and I'll be charging my system the next month or so. I've used the variable tubes but not been a fan; personally I haven't seen the improvement with them. I know others have had different experiences thought with these.

Orifice diameters are as follows:
GM White .072
Ford Blue .067
Ford Red .062

With the orifice tubes, the smaller the orifice, the slower the release of refrigerant causing lower pressures on the low pressure side. The lower the pressure and more opportunity to expand, the colder the refrigerant will get. There is of course a point of diminishing returns where you either simply cant get enough refrigerant flowing, or, you allow it to continue to expand so much that you freeze up. There's no question in my mind though that the Ford blue performs better than the GM white. Search any 80's corvette site (DMC used the same accumulators, low pressure switches, orifice tubes, and functioned the same way cycling on and off) and you'll find countless threads on this topic. I'm still unsure where I'll land between red and blue. Ford orange, green, and brown get even smaller (down to .47) but I suspect that's far beyond the point of continued returns. I've rarely read of anyone using smaller than red.

[Ron]

Thanks!, Nick.

[81dmc]

I have experimented with the GM white orifice tube, the variable orifice tube, the Ford blue orifice tube, as well as the Ford Red. Blue is smaller than white, red is smaller than blue. I have noticed better performance with the blue over the white for sure. The red is installed now and I'll be charging my system the next month or so. I've used the variable tubes but not been a fan; personally I haven't seen the improvement with them. I know others have had different experiences thought with these.

Orifice diameters are as follows:
GM White .072
Ford Blue .067
Ford Red .062

With the orifice tubes, the smaller the orifice, the slower the release of refrigerant causing lower pressures on the low pressure side. The lower the pressure and more opportunity to expand, the colder the refrigerant will get. There is of course a point of diminishing returns where you either simply cant get enough refrigerant flowing, or, you allow it to continue to expand so much that you freeze up. There's no question in my mind though that the Ford blue performs better than the GM white. Search any 80's corvette site (DMC used the same accumulators, low pressure switches, orifice tubes, and functioned the same way cycling on and off) and you'll find countless threads on this topic. I'm still unsure where I'll land between red and blue. Ford orange, green, and brown get even smaller (down to .47) but I suspect that's far beyond the point of continued returns. I've rarely read of anyone using smaller than red.
Exactly what I read with the corvette guys. They have almost the same system as we do.


Let us know how the .062 Red OT works. I’m going to first use the .067 Blue, but if you have better luck with the red then I’ll go with that.

[81dmc]

Since I just finally got my car back on the road, I want to update this.

I installed the blue orifice tube and 2lbs of R134. I set my low pressure switch to cut off at 22-25psi. While it does cycle at highway speed, the AC is so cold, I often have to turn it down, and I haven't even tinted my windows yet. Additionally, in traffic at idle, I still get ice cold AC. It?s absolutely wonderful!

IIRC, high side pressures were about 175-250 at 80*F sitting still. When it's 90*F in summer, I will update this again.

[L3V3L1]

Unless you have to remove the orifice tube, either because it is broken or plugged up, leave it alone. It is in a place where the pipe is bent and it can be difficult to remove without damaging that VERY thin aluminum pipe. Once you damage that pipe you are replacing the evaporator coil and it is NOT a fun job. I do not understand why some people feel they have to "convert". R-12 is readily available, it is not that expensive, and you don't need much.
This is where my ac job ended in September, I pulled out the office removal tool, with no orifice attached, and a hole in the evaporator coil pipe. I think I'll have to throw in the towel and bring the car to to Grady, once the city stops salting the roads and have the evaporator replaced by him. 😢💸