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orfice tube size - Whats the difference

Jon21 on Fri August 07, 2009 1:23 AM User is offline

I saw this table on the internet: Whats the difference in orfice tube sizes? If mine is GM white can I put in a Ford RED one to make it cool better. Is there any considerations I should be thinking of, like less refergerent/oil to the compressor with a smaller tube? Is there any systems that this wont work on such as Variable Displacement Compressors?
What is the difference between GM white (0.072) and GM black (0.072)?


Chrysler Red .062

Ford Blue .067

Ford Orange .057

Ford Red .062

Ford Green .052

Ford Brown .047

GM White .072

GM Black .072

Volvo Red .062

GM Tech on Fri August 07, 2009 8:30 AM User is offline

GM Black OT are condenser outlet, or any where else where the OT is in the upstream of the OT connection--the Whites are all in the downstream flow of the OT connection--Blacks are built backwards- with the "tangs" on the exit end of OT- whites are most common- used in most GM vehicles- Blacks are used in most full size GM trucks- at condenser outlet.

So you wish to redesign your system- get better cooling by changing the original design-- obviously GM didn't wish to make the coolest possible system- and still stay away from the penalities of side effects.

Why not just assure that all components are performing like they should and keep it OEM? Air flow comes to mind-- dirt and debris over your heat exchangers can drop efficiencies dramatically-- why not clean them first.

If you live in a dry climate, you may get away with the Red OT-- but the vehical had to be optimized to live in all climates--

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......

Jon21 on Fri August 07, 2009 11:47 AM User is offline

Thanks GM Tech!!

I am not trying to imply that I am know better than the good Harrison engineers; I am just trying to understand the tricks people do like put a ford OT in a GM car.

Can you help me to understand the theory of operation on a smaller orifice tube? So the smaller OT means less liquid refrigerant into the evaporator, thereby a lower pressure in the evaporator, and in-turn lower tempature. What I don't quite understand is why the controls devices, cycling clutch or variable displacement valve would just keep the evaporator temp/pressure at its control point and this "trick" would be completely ineffective. Does this sound right?


bohica2xo on Fri August 07, 2009 2:22 PM User is offline

The common recomendation to use a ford "blue" tube on a GM conversion is an attempt to compensate for the 134a in an R12 system.

The difference in area between the .067 & .072 bore tubes is not a lot. R134a has a slight heat carrying advantage, so the theory is that the blue tube is a better match for an R12 evaporator running 134a.

CCOT systems are a compromise anyway, with flooded evaporators during part of the cycle. Sometimes when converting a system to a different refrigerant, other adjustments must be made. The blue tube seems to help in some circumstances. Any conversion is outside of the OEM design, and as such is an ongoing experiment...


"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.

NickD on Fri August 07, 2009 5:06 PM User is offline

Switching orifice sizes along with converting R-12 systems to R-134a is very vehicle dependent with different results. Reducing the orifice size does two things, raises the low side pressure and reduces the flow at any given pressure. Reducing the orifice size in say an R-12 Cadillac prevents the service AC soon lamp from igniting, and when it does that, it kicks off the compressor, whereas the smaller orifice increases the low side pressure to prevent this. Overall cooling is less than with R-12, but far better than having no cooling at all.

What vehicle do you have in mind?

Jon21 on Sun August 09, 2009 4:02 PM User is offline

Thanks to all who responded. Baised on the posts; I decided not to change the orfice tube and to leave the design alone. My car was designed for R-134 and is not a converson. This is for 99 a Buick Lesabre with V5. The 50 degree vent temps bother me but I have been told that this is with in factory spec.

bohica2xo on Sun August 09, 2009 4:40 PM User is offline

Your Buick would not respond to orifice changes - the evaporator pressure / temperature (and the 50f vents) are controlled by the valve in the compressor.

You could change the V5's control valve to reduce vent temperatures.


"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.

NickD on Mon August 10, 2009 12:00 PM User is offline

What I noticed with my V-5, the hotter the ambient, the colder the vent temperatures, not much difference if I switch on the AC at say 60*F, but really noticable when it's above 90*F outside, like ice. Didn't bother recording this, as long as it's comfortable. Also varies with blower speed and whether or not in recir mode.

Are you uncomfortable in your car? Have a friend that drives a refrigerated truck and hauls frozen meat, can get one of those. Keeps it -11*F.

Jon21 on Mon August 10, 2009 10:54 PM User is offline

Ok; I finally took some pressures. If you could tell me what you think I would appreciate it.

Outside air is 80 Degrees F with a relative humidity of 84%

A/C Set on low fan max(recirculate)
Idle: 27 low 175 high
1500 RPM: 27 low 175 high
Vent temp 41F

A/C Set on high fan normal(outside air)
Idle: 55 low 240 high
1500 RPM: 48 low 300 high
Vent Temp 61F

Is 300 PSI dangerous? I am positive system has proper 32oz charge installed into a vacuum. I checked the condenser fans and both are running and do ramp up into high gear with the A/C on. condenser looks clean.


NickD on Tue August 11, 2009 7:27 AM User is offline

Standard way to take vent temperatures is to close all vents except one center one, and measure the temperature at that single open one.

bohica2xo on Tue August 11, 2009 3:17 PM User is offline

Is 300 psi dangerous? Not really, but in this case it will be damaging to your bank account eventually.

You have a serious condensor issue. If the fans are working & the air path is clean then you have blocked passages.

240 psi @ 80f ambient would be a 3:1 ratio. Anything past 2.7:1 is cause for concern, and you are at 3.75:1 - OUCH.

2.2:1 is in line with a factory 134a system.

I would be inclined to tape the gauges to the windshield, and go for a drive. If you see the same kind of pressures with a 60mph air blast through the condensor, then the problem is on the refrigerant side of the condensor. If pressures are normal, it is an airflow problem.


"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.

Jon21 on Sat August 15, 2009 8:08 AM User is offline

Thanks bohica2xo!!

Well, I finally got the gauages taped to the windshield. The results are:

Outside Temp: 82F
At road Speed 35 MPH:
A/C Set on high fan normal(outside air)
34 low 220 high
Vent Temp 51F

At freeway Speed 60 MPH:
A/C Set on high fan normal(outside air)
30 low 195 high
Vent Temp 50F

The condesnser looks clean however it has many bent fins from years of road debree. Also many of the bent fins look corroded and deteriated. No leaks though as I don't see any dye with the black light. Also, my system had a system death kit (leak seal) installed years ago before I knew any better. When I replaced the compressor I flushed everything, especially the condenser, out with solvent baised AC flush and high pressure air.

I guess my sort term solution will be just to turn the fan to low when I am stoped and then to turn it back up when I get moving.

Nick, I would agree when you that my system has worked better in the past durring the hotter tempatures. 90 degrees works much better then 80 degrees.

NickD on Sat August 15, 2009 8:48 AM User is offline

I always seem to find a lot of debris between the condenser and the radiator. Fins can be straighten out, have tools for this, but my finger nails also do the trick.

Air conditioning can also be phrased as conditioned air, but either way, air is in there, can't conditioned the air unless you have it. Need very good air flow. Another sore spot is the evaporator, while they call it a blower motor with a squirrel cage blower wheel, also serves very well as a leaf chopper, all that debris piles up on the surface of the evaporator. Worse part of AC maintenance is all that clean up work, feel more like a janitor than an AC mechanic.

You cannot properly optimize your charge levels with pressures until you do all that clean up work first. Crazy dirt is everywhere, even in my computer, time to clean that up again. And those bathroom vent fans also need it. While on a long boring drive, to entertain myself, counted every device I own that contains a fan, Hit a number of something like 93 different items. Where there is a fan, there is dirt, and where there is dirt, there is dirt buildup. It's an endless job and that fan becomes useless if it cannot blow air.

bohica2xo on Sat August 15, 2009 7:45 PM User is offline


Well it looks like an air side issue from those numbers. All sorts of things can contribute to this - missing air seals / dams, dirt, debris, worn or failed electric fans... the list is long.

Start with the simple stuff. Like Nick says, dirt is everywhere. If you have small insects where you live, they are a big problem that is hard to spot. A condensor face that looks clean can be plugged up with gnats & other small bugs deep in the fins.

I like to use a strong light source inside the engine bay - at night. Put your droplight in the air path, and have a look at the front of the car. I have found all sorts of things in the air path, including a dead mouse between the radiator & condensor. If you find a "bird's nest" between the radiator & condensor, you will need to pull the radiator.

To clean the fins I use a strong surfactant like Simple Green or Zep Orange. Start with a cold condensor. Spray it full strength deep into the fins, and wait for 5 minutes before flushing with water. Avoid using a high pressure washer, you can easily flatten all of the fins - and then you need all of Nick's fingernails for a few days. It may take a couple of applications to get all of the bugs out, but either product will dissolve them.

Even here in the desert I find bugs in my condensors. A little bit of oil on a condensor is also a problem. A few drops of spilled ATF will wick into the fins & hold a big wad of dust in place.


"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.

NickD on Tue August 18, 2009 9:42 AM User is offline

Sometimes you get the strange idea that the way they make vehicles, they should never leave the showroom.

When I have to take HY 173 south, through a major wetland area in Wisconsin, and the WI DNR insist that these wetlands are good. Maybe good for generating zillions of mosquitos and other flying insects, not really an expert on this subject as to what else they are good for. But can spend hours cleaning my condenser/radiator, and one short 30 minute trip, it's all plugged up again.

It's bad enough cleaning off all those dead dried up insects in the front of the car, condenser is miserable, has to be a better way. While I read that vehicle manufacturers do not recommend putting a screen in front of the condenser, I said screw you. Got a fine mesh fiberglass screen with tie wraps and put it in anyway. I put it well in front of the condenser and angled the top down. Most of the insects that hit it, especially the larger ones hit it and drop off. I can tap the screen from the top and get most of the smaller ones off, occasionally use a garden hose to get the rest off, with the screen angled down, doing this from the clean side of the screen. Never had engine overheating problems, and the AC has been working great.

But this is not recommended by the vehicle manufacturers. I gather their thinking is for you to have a plugged up condenser so your high side skyrockets, overloads your compressor and quickly wears it out so you will have to buy a replacement. Or better yet, they are not thinking, period.

Knew rich people that would trade off a vehicle once the gas tank was empty, never have been that rich, but maybe they want you to trade if off when your condenser is plugged.

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