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Fluid Drive Fluid


James_Douglas
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For anyone running a fluid coupling...

After a year of research as well as an analysis of some original MOPAR fluid drive fluid, I have developed a modern specification of what should be used in the fluid couplings.

The original fluid was a pure-base mineral oil with a Saybolt Viscosity of between 100 and 150. The fluid had a Viscosity Index of greater than 80.

The fluid had anti-foaming and anti-oxidation additives. It specifically did NOT have any seal swelling agents as these can attack the carbon-graphite seal and the copper in the bellows. This last specification eliminates all modern transmission fluids.

After finding several formulas to convert Saybolt Viscosities to Kinematic Viscosities, it appears that the best match to the original specification is a ISO 22 or a ISO 32 oil.

However, the ISO 22 is just below 100 Saybolt and the ISO 32 is much higher than 100 Saybolt.

Based on a period Lubrication Industry article on fluid couplings that had the following admonishment:

Contrary to popular supposition any attempt to use a higher viscosity fluid would actually reduce the torque transmitting ability of the coupling since torque-transmission is dependent upon a high circulation of fluid between the impeller and runner and is not caused by any viscous drag between the two.

Based on the research, and discussions with several lubrication engineers, I think that an ISO 22 hydraulic oil with the proper additives and VI above 80 is a documented replacement for the original MOPAR fluid drive fluid.

Anyone who wants a copy of my documentation, please drop me a email and I will send you a copy.

Best, James

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What is the viscosity difference from the tractor oil that they sell at NAPA and what you are stating here? I drive my Dodge all over and I guess with out knowing the difference in power from that oil you stated would there be a big difference?

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Jon,

Working and transferring maximum torque are two different things. What I have tried to do is to ignore all the common myths and have run down original technical data. Other oils may work, but how would anyone know how well unless one changed the oils and ran some real well done field tests.

Send me an email and I will send you a copy of my research.

The NAPA AW 32 oil should work fine. The R&O oil is of lessor quality and has an inferior VI. So when it gets hot, say in heavy stop and go traffic it will not transmit as much power.

I am looking at some ISO 22 hydraulic oils with a very high VI. Once I run it for a month, I can comment directly.

A ISO 22 oil is about 102 in the Sybolt viscosity SSU.

A ISO 32 oil is about 149 in the Sybolt viscosity SSU.

The documents I have suggest a oil between 100 and 150 SSU for fluid couplings. The sample of original MOPAR fluid I sent out came back at SAE 10 weight. SAE 10 is about 125 SSU.

Since it is fluid velocity that makes the power transfer and not the "thickness" of the fluid the lighter fluid will transfer more power.

Best, James

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Jon,

Since I have never talked with anyone that has run ISO 22 oil, I don't know what the difference will be.

I did play with some different oils a couple of years ago in the same car and I could feel the difference in zero to 20 MPH acceleration.

One last thing. After talking with several lubrication engineers, it is apparent that the organic additives for anti-foaming and such breakdown after about 5 to 7 years. So, to maintain maximum power transfer and "pamper" the carbon-graphite seal the fluid should be changes about every 5 years.

If you want to go into a LOT of detail, email me you number and I will give you a call and go into the absurd detail I have dug up.

Best, James

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Thanks for the info and it will becoming up in another year or so sence I first started to drive my Dodge. So I won't go and change the oil out till then. I will be waiting for some discussion from you on your oil testing program in the future on this site. Thanks for all the research-Jon:)

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  • 2 years later...

Well I really don't know the difference between any of the fluid drive oils but the tractor oil seems to be ok. As far as power it seems to give me enough get and go in second gear for starting power and for when I'm not in a hurry high gear does just fine.

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  • 3 weeks later...

Folks, I have a '50 Meadowbrook with I think the original fluid drive fluid. It does not leak and I think it may transfer more torque than it should since I read that the max rpm the engine should be able to run if I hit the throttle with the brakes on is 1800 rpm or so, yet mine will not spin over 700 in this condition.

Also, if I set the idle to 500 rpm in neutral and later engage 1st and let go of the clutch but don't allow the car to move, the engine gets draggec down to 350 rpm, shaking a lot.

Any thoughts?

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This may sound ignorant but why not mix 50-50 the ISO-22 and ISO-32 to get that maximum Rating. That is to say correct 100-150 SSU thing from 102 or 149 to say 125. Ah - What the hell do I know, I'm just guessing.

I mean this can or cannot be made into Rocket Science but Horse Sense tell me to find the same Brands in those 2 Viscosities - mix them and have optimal coverage.

Tom Skinner

Huntersville, NC

Edited by Tom Skinner
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My thought is that its got nothing to do with the coupling fluid, but rather the engine needs a tune up or better compression.

I will check compression, but I can tell you it is fully tuned, runs like a top, starts easily and does not smoke at all. I just think the fluid coupling just seems to drag the engine down a lot when I come to a stop without pushing the clutch and there is very little slip as I drive. I mean when I accelerate and then coast, my tach shows almost no change in rpm.

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I will check compression, but I can tell you it is fully tuned, runs like a top, starts easily and does not smoke at all. I just think the fluid coupling just seems to drag the engine down a lot when I come to a stop without pushing the clutch and there is very little slip as I drive. I mean when I accelerate and then coast, my tach shows almost no change in rpm.

My 49 does the same thing. Engine has about 10k on a rebuild and runs great. I just drive it like a regular clutch and forget about the fluid drive.

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My 49 does the same thing. Engine has about 10k on a rebuild and runs great. I just drive it like a regular clutch and forget about the fluid drive.

If you can drive it like a regular clutch, the Fluid Coupling is doing it's job. You are using it all the time so you can't forget about it.

Remember, Fluid Coupling is totally separate from the transmission, be it 3 speed standard or M6. Separate and distinct units.

The term "Fluid Drive" refers to the Fluid Coupling which transfers the power, via a liquid, to the transmission. Looks like a torque converter, and replaces the flywheel.

If you are having shifting problems but the car still drives by manual shifting, the problem is in the transmission and not the Fluid Drive.

Some folks think that Fluid Drive means their car will shift on it's own in certain situations. If it was equipped with an M6 transmission, it should.

BUT, some Fluid Drive cars had a regular old manual 3 speed transmission behind the Fluid Coupling, not an M6.

If your car has a gear when you pull the gearshift towards you and down from neutral...you have a regular 3 speed manual.

If it's an M6, there will be no gear in that spot.

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PatS,

Excellent observation about the Transmission!

I was under the impression this Thread was about the SSU rating of Fluid Drive Fluid though, and somehow was "transformed" - into a Thread about someone's Shifting difficulties. Either way I hope it helps people. LOL.

Tom Skinner

Huntersville, NC

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  • 6 months later...

Thank you guys but I should clarify something. It seems in my manual they have 2 references to fluid drives. One is a torque converter type device (which is what I have in this fork lift) They specifically call it a fluid drive.

Latter in the manual they have a fluid drive coupler/power take off which is not what I have. They are different devises but I am not sure we are all talking the same device.

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If you can drive it like a regular clutch, the Fluid Coupling is doing it's job. You are using it all the time so you can't forget about it.

Remember, Fluid Coupling is totally separate from the transmission, be it 3 speed standard or M6. Separate and distinct units.

The term "Fluid Drive" refers to the Fluid Coupling which transfers the power, via a liquid, to the transmission. Looks like a torque converter, and replaces the flywheel.

If you are having shifting problems but the car still drives by manual shifting, the problem is in the transmission and not the Fluid Drive.

Some folks think that Fluid Drive means their car will shift on it's own in certain situations. If it was equipped with an M6 transmission, it should.

BUT, some Fluid Drive cars had a regular old manual 3 speed transmission behind the Fluid Coupling, not an M6.

If your car has a gear when you pull the gearshift towards you and down from neutral...you have a regular 3 speed manual.

If it's an M6, there will be no gear in that spot.

Sorry I am having trouble reading this forum format and did not see this post.

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I'm wondering about using a blend as well... I work in the hydraulics supply industry and have stock in AW22,32,46 and 68 as well as HyGlide (universal tractor trans) oils. I have been assured by my distributors tech department that blending all weights of AW is completely safe and will simply effect the weight of the oil. Specifically that an equal blend of AW32 and AW68 will in fact create an AW46. That being said, a blend of AW22 and AW32 "should" create a hybrid weight with a rating of AW27 with a 123.5 in the Sybolt viscosity SSU. Correct?

By the way, we currently retail 5 gallon pails of all AW oils at $45 plus tax, although it can be had cheaper at the big box farm store chains.

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According to James Douglas information, the thinnest oil will work the best which seems logical to me.

The only drawback to too thin oil would be the possibility of leakage from the shaft seal. This could be determined only by experimenting. But if the oil is not too much thinner than original it should not leak. In other words if I had ISO22 on hand I would give it a try.

2 gallons should fill up a Fluid Drive unit.

Let us know how it works.

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  • 4 months later...

Hi, Mr James Douglas.

Would you be willing to send me the info you have on fluid couplings? I have a 1950 Meadowbrook with a simple fluid drive on a 3 speed and it does not leak but seems to drag the engine down quite a bit if I leave the clutch in when I stop. The regular idle is 500, but it takes it to 350-400. I think it has the original fluid, which could have gotten thick with age. I am afraid of changing because as of now it does not leak. Maybe if it ain't broke dont fix it?

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