Oil coolers design and function

[booster]

After a long drawn out process in dealing with our Chevy four speed transmission running hotter than I liked (lots of discussion about it on the forum) I probably became hyper-sensitized to the cooler makers exaggerated claims, ratings based on who knows what real data, and confusing descriptions of their "benefits" over other brands and styles.

I am now looking at a plain old add on cooler for my 1996 Roadmaster so am looking at the lower end of the product lines compared to the large, more race capacity coolers I was for the van.


I have always been a fan of the the thermostatically controlled bypasses for trans coolers, as IMO things like transmission oil just plain work the best between certain limits of temperature, as do the clutches and bearings etc in the trans. It appears lots of the newest transmissions have internal temp control now, as people report seeing relatively high and very consistent temps in them.


Looking at coolers I mainly go by the BTU removal rating, although they never seem to mention the conditions or any standard they rated too. Size and design will mostly track the btu rating between brands, too.


What I am finding is not any of the above different than in the past, but what I am seeing is many more of the coolers claiming to have "automatic internal thermal bypass" built into them to aid in warmup time and cold weather oil flow. Great, I thought, but when I looked at what it really is I think it is not very great, and costs you hot cooling capacity compared to using an external bypass on same size and style cooler.


What the "thermal" internal bypass does is just bypass some of the oil through lower restriction tubes through the cooler so it goes straight through with minimal cooling but easier flow in the cold. They claim that as the oil warms up the flow "automatically" shifts to the small tubes that go through the efficient cooling area. This simple doesn't make sense based on how fluid flow works. The flow will split between all the tubes, all the time, based on the resistance to flow of the tube and the viscosity of the oil, AFAIK. So you get enough cold flow through the bypass tubes and some flow through the cooling tubes when cold and save the trans from oil starvation in cold weather or an overpressure. When the oil heats up you would get more flow in the bypass tubes from reduced viscosity and also more flow in the cooling tubes so, yes, the cooling capacity of warm oil is higher than it is cold, but you are still wasting cooling capacity by sending more oil through the bypass tubes when hot. This means to get the same trans cooling, you would need a bigger cooler and also more transmission oil flow to compensate for the bypass.


It all got pretty clear to me when I looked at the Tru-Cool coolers. They were one of the early pushers of the internal bypass as I remember, and still are. But, they also have a line of coolers called Max that come with an external thermal bypass and they have significantly higher cooling vs area. My guess is that to get the better cooling they got rid of the internal bypasson those coolers, but I haven't heard back on my inquiry to them yet.


I think the bottom line on all this is that manufactured have figured out that big coolers and cold climates were giving starvation when cold issues and they are putting in the bypasses to address that issue. This is good, but it should be stated that it costs you cooling capacity at any given size cooler. I do believe the cold flow needs to be addressed, but an external thermal bypass is a much better choice, IMO. This is true for both standalone coolers and if you run through the regular radiator cooler after and addon cooler as we are talking about oil starvation when cold, although the oil will run warmer as the engine warms up. The external thermal units seem to hold about a 15* range when using the higher than necessary flow capacity of the run of the mill engine oil bypass units. I have read dozens of rave reviews for the internal bypass units because they the hold the temps down very low in cooler weather in the 100*F range at freezing for instance, but they run warmer in warm weather and not much different than before the cooler was put in. Most them are plumbed to the radiator so that makes sense. It also points out that the addon cooler is not thermally controlling anything in reality.



An external bypass can be installed to bypass both the addon cooler and radiator cooler for much faster warmup in most cases as the water temp will lag some, although it will vary by engine and vehicle. It can also just bypass the addon cooler, which puts you back to be very much like a stock setup when cold, with the radiator affecting the oil returned temps. I prefer bypassing both, but the plumbing is often easier just to put the bypass right at the cooler. I would guess you get a bit more oil flow cold with both bypassed.



Our vans tend to need as much trans cooling, with least air blocked to the radiator and AC condenser as we can get, so it seems like it just wouldn't be a good idea to leave capacity lower to get the ineffective for temp control but effective for addressing cold oil starvation when you can keep the capacity and control temp better with a regular cooler and external thermal bypass.


Any and all input welcomed and appreciated as this is kind of a new thing, I think to throw into the selection process.


Of course if you live in the deep south where it rarely even freezes, you will not have any issues with cold flow with any system. But you will probably need more cooling in the summer time.

[peteco]

The video on this Hayden page shows a thermal bypass gate that allows flow through the large tube (with lower restriction), then closes to force flow also through the smaller tubes. I wouldn't think this would add too much volume to the cooler.

https://www.haydenauto.com/en/produc...y-pass-coolers

Also on the Hayden site:

https://www.haydenauto.com/en/techni...on-oil-coolers

Should the cooler be installed before or after the radiator?
Answer: We recommend installing the auxiliary cooler after the radiator to return the coolest fluid directly to the transmission. Installing the cooler before the radiator will still provide additional cooling and may be necessary in some difficult-to-access applications.

I installed my tran cooler after the radiator cooling as directed by this and many other sites. However, some sites recommend installing the tran cooler before the radiator. Since I am mostly in moderate climates (rarely below 20-deg) I wasn’t too concerned about overcooling. My tran temps usually run in the 150-deg range. Before installing the tran cooler the tran temp was typically 190-deg, but would go way up in tough climbs when the engine temp went up as the two mutually feed each other. Since adding the tran cooler I have had high engine temps during a climb (220-deg) but the tran stayed below 200-deg.

On this same site the concern about overcooling is stated:

Can you over cool the transmission fluid?
Answer: Transmissions are not highly sensitive to cool operating temperatures. However, in sub-freezing (20-30°F) weather conditions, transmission fluid can actually gel up in an external cooler and cease to flow, causing damage. Use of the radiator cooler actually helps warm the fluid under these conditions. It is critical in extreme cold conditions to use the original equipment cooler in series with an auxiliary or by-pass cooler and allow the vehicle to warm up before driving.

The implication is the radiator heats up fast enough such that gelling is sufficiently avoided. If I were in a colder climate (like booster) I would probably install the bypass, and may yet install one. I wonder what Hayden’s recommendation setup is for below 20-deg???

[booster]

Originally Posted by peteco

The video on this Hayden page shows a thermal bypass gate that allows flow through the large tube (with lower restriction), then closes to force flow also through the smaller tubes. I wouldn't think this would add too much volume to the cooler.

https://www.haydenauto.com/en/produc...y-pass-coolers

Also on the Hayden site:

https://www.haydenauto.com/en/techni...on-oil-coolers

Should the cooler be installed before or after the radiator?
Answer: We recommend installing the auxiliary cooler after the radiator to return the coolest fluid directly to the transmission. Installing the cooler before the radiator will still provide additional cooling and may be necessary in some difficult-to-access applications.

I installed my tran cooler after the radiator cooling as directed by this and many other sites. However, some sites recommend installing the tran cooler before the radiator. Since I am mostly in moderate climates (rarely below 20-deg) I wasn’t too concerned about overcooling. My tran temps usually run in the 150-deg range. Before installing the tran cooler the tran temp was typically 190-deg, but would go way up in tough climbs when the engine temp went up as the two mutually feed each other. Since adding the tran cooler I have had high engine temps during a climb (220-deg) but the tran stayed below 200-deg.

On this same site the concern about overcooling is stated:

Can you over cool the transmission fluid?
Answer: Transmissions are not highly sensitive to cool operating temperatures. However, in sub-freezing (20-30°F) weather conditions, transmission fluid can actually gel up in an external cooler and cease to flow, causing damage. Use of the radiator cooler actually helps warm the fluid under these conditions. It is critical in extreme cold conditions to use the original equipment cooler in series with an auxiliary or by-pass cooler and allow the vehicle to warm up before driving.

The implication is the radiator heats up fast enough such that gelling is sufficiently avoided. If I were in a colder climate (like booster) I would probably install the bypass, and may yet install one. I wonder what Hayden’s recommendation setup is for below 20-deg???

I just spent some time bouncing around the Hayden site and it appears that the built in, probably bimetal, bypass blocker is only on one series of the coolers, so only about 3 coolers I think. Very few if any specs on any of them except for size and they all are large size but thin so probably quite low in heat removal. The price generally, sort of, predicts capacity between comparable quality brands and they are very inexpensive.



Their other coolers appear to be non bypass, and they did indicate the use of bypass thermostats for them.


The addon cooler before or after the radiator has been discussed a lot, in a lot of places, I think. For max cooling, you would want it after the radiator for sure, but in cold weather you would really not get the trans pan oil much warmer, I think. When it gets cold here, the cool end of the radiator is cold unless you put cardboard in front of it so trans fluid is going see lots of subfreezing air and take out whatever little heat the radiator put in. The other way would be marginally better I think, but not much unless you have a car that runs warm radiator all winter, which would be a big engine, I think. Full bypass thermostats probably would be a better option around here, but I do wonder about the approx 10% to the coolers they run minimum to keep the flow moving as that probably wouldn't be enough to warm the coolers and probably would cool the trans some below thermostat point. We have seen a bit of this with our bypass themostat in the van in 30* weather, and we see a bit cooler trans pan temps. I think some has to do with the fact that the thermostats tend to be the ones also used for engine oil cooling so probably capable of 5-10 times more flow than the trans needs. This makes the 10% to the coolers all the time more significant as you limit out the heat at some point after the stat closes as far as it will to the coolers. Tru-Cool had what looked like a smaller setup, but only in 180* so that might have better low temp performance, but I have never seen on in person or seen a diagram of how they are designed.


I think it is getting very hard to tell which coolers have the bypass. In all my looking at the B&M site I saw nothing on it, but saw it mentioned in one of the reviews online, so I looked further and it looks like it does but it is a viscosity type it appears. The Hayden may be the only one to do it mechanically, which I think would be better.


I did notice the Hayden site was still showing the decades old transmission life vs temp chart that shows double life at 175* than hotter than that. With the new fluids and designs, I think those days are long gone as we hear about the newer transmissions running over 200* all the time and appearing to be controlled there by the trans temp sensors. I think our 07 6.0 Chevy was the first year with DexVI in the vans so I have no idea if the guts of the old school 4L80e were changed to match or not. It is very possible, maybe likely, they did not change anything as they knew they were soon going to the 6 speed. Wife's very early 2009 Honda CRV came with their dino fluid in it, but I think the guts were for the new synthetic that showed up only months later. I changed out the fluid 4 times in succession to get nearly all synthetic and the shifting got hugely smoother.



I have a 175* thermostat in the van now and it runs in the 182 to 187* range most of the time on the Scangauge which reads trans internal temp, The pan runs up to 10* cooler depending on speed variations. On long flat runs, that pan and Scangauge are nearly the same temp.


I was kind of set on the B&M as it was a better size fit as the Tru-Cool is very long and narrow so tougher fit, I think, so still needing to reconsider what I will do. Whatever cooler it is I do plan on cooler after radiator, but with a bypass thermostat. Perhaps Setrab will have something that would fit but they use all AN fittings which a really expensive for them as they are thread specific to the cooler, also. I think the nice Earls are also all AN.


Interesting stuff, good find on the Hayden, very interesting. Thanks for for posting it.


Perhaps the best for anyone who has an external cooler in cold country. would be to strap a piece of 3/4" of the urethane foilboard insulation sheet to it in the winter to help it keep warm. It is highly unlikely you will every overheat the trans in below freezing temps, I think. The Buick is an odd case as it is mostly a summer car, but does get out in the winter if the roads are clear and we need to haul stuff. The clear and dry roads more often happen when it is too cold for salt to melt snow, so below zero is very possible for old Buick.

[peteco]

Yes, if it gets real cold I can put a blocker in front of the cooler.

An advantage of the Hayden bypass cooler is it eliminates the external bypass device with its 4 connections and the potential for device failure, which has been reported.

More info on the Hayden cooler here:
https://www.haydenauto.com/media/547...oler_flyer.pdf

Prices pretty reasonable too (check Rockauto).

I have one of the Hayden integral fan/cooler units. Might change out the cooler for the bypass type when I get all my other projects done...

[booster]

Originally Posted by peteco

Yes, if it gets real cold I can put a blocker in front of the cooler.

An advantage of the Hayden bypass cooler is it eliminates the external bypass device with its 4 connections and the potential for device failure, which has been reported.

More info on the Hayden cooler here:
https://www.haydenauto.com/media/547...oler_flyer.pdf

Prices pretty reasonable too (check Rockauto).

I have one of the Hayden integral fan/cooler units. Might change out the cooler for the bypass type when I get all my other projects done...

I had run across most of what is on that flyer somewhere also, but it didn't have the GVWR rating. Of course as mentioned earlier we have no idea how that get that rating calculated.


If you assume, and it is a big assume, that they and Tru-Cool calculate the GVWR the same the Tru-Cool max is 24K and 92 square inches at the same 3/4" thick. The Hayden is about 125 square inches for 22K rating. I put almost no value in that rating, but it is interesting. The Tru-Cool does also give a 22K btu rating also which is quite high if accurate. I measured again, and I might be able to mount that cooler to the big X of round bar they have crossbucking the radiator opening. I can go anywhere on the radiator that way and it is wide enough for the 23" to make it. Still not sure which way to go.


Which size of the Hayden and model of the Hayden do you currently have, maybe it already has the shutoff bypass, but of course it may not have any bypass at all, too.

[peteco]

Originally Posted by booster

Which size of the Hayden and model of the Hayden do you currently have, maybe it already has the shutoff bypass, but of course it may not have any bypass at all, too.

I have the Hayden Rapid Cool 526. No bypass that I can tell.

https://www.haydenauto.com/media/550...flyer_dawg.pdf

[wny-pat]

I have a Long Tru-Cool LPD4454 on my Honda Odyssey. No bypass or thermostat. The fluid does not get warm enough in the winter - ambient temps under 20 f!!! It cools just fine in the summer.

[booster]

Originally Posted by wny-pat

I have a Long Tru-Cool LPD4454 on my Honda Odyssey. No bypass or thermostat. The fluid does not get warm enough in the winter - ambient temps under 20 f!!! It cools just fine in the summer.

That is one of them that has the built in cold flow bypass I mentioned.


How do you have it plumbed into the cooling system of the Honda?

[wny-pat]

Originally Posted by booster

That is one of them that has the built in cold flow bypass I mentioned.


How do you have it plumbed into the cooling system of the Honda?

It has designed in capillary action that won't pass transmission fluid thru the entire cooler until it gets hot enough and therefore thin enough to pass thru entire cooler.

[booster]

Originally Posted by wny-pat

It has designed in capillary action that won't pass transmission fluid thru the entire cooler until it gets hot enough and therefore thin enough to pass thru entire cooler.

I really doubt that capillary action would do that, but have no data on it. Basically, they really are addressing pressure and flow, not heat directly, but the warming lowers the viscosity so you get less back pressure and more flow.


The problem they addressed as far as I know was that the cooler resistance in cold weather would limit flow enough to starve the transmission of oil and wreck them. I know Chrysler had huge issues here in Minnesota over a decade ago and they traced it down the the external cooler they had on them.


If you look at the pics on the Long Tru-Cool site, they show a larger top passage across the cooler which lets the thick fluid through when it is cold and viscous to prevent the damage. The other passages are smaller so they let through much less fluid. As the fluid warm up, I would expect the flow in all the passages to increase as I don't see how the low restriction, larger passage suddenly gets higher flow resistance without being closed off like the Hayden in Peteco's posts and link. The only way it could do much at all was if the inlet and outlet were both at the top of the cooler so it had an air pocket that would form when the fluid got hot so the fluid couldn't get to the top passage. Do they require on specific edge to be up in mounting?



As you have found it doesn't keep the flow shut limited enough to keep the oil very warm as it is still getting cooled by an amount even cold and thick in the auxiliary cooler. If it did shut off the flow to keep the fluid warmer, there would be no big passage and the flow issues would be back.


The reason I asked about how it was installed is that it makes a big difference if the fluid goes through the cooler or the radiator last, with radiator last keeping it warmer and cooler last making the fluid colder.


The thermostats can be put behind the radiator where it is warmer and bypass both the radiator and the aux cooler, so essentially no cooling takes place in either the aux cooler or radiator, except for the about 10% of flow that the thermostat lets go to the cooler to make sure the fluid is always circulating. The trans always gets full flow of fluid through the thermostat so isn't starved for lubrication.


All this said, as I mentioned earlier, the thermostats also let the fluid run cooler in cold weather than warm, and I think it is because the stats are large ones that are sized for engine oil cooling flow rates. Thus 10% of their capacity is a much larger % of the trans flow. I will be testing the smaller Long Tru-Cool stat that is designed for the lower flows of transmissions, so it might do better for holding up the temps. I have a non internally bypassed Tru-Cool cooler and stat ordered to put in the old Buick as part of the driveline rebuild and improvement winter project, so I will know more next summer and fall when it is back on the road.


We discussed earlier the idea of taping a piece of insulation or cardboard over the cooler to keep the cold are out, like you would partially cover a radiator (also a good idea in the very cold weather).

[wny-pat]

I'm going to have to believe that the companies that offer capillary action coolers have consulted with lubrication engineers to ensure that their product does not hurt vehicle components connected to their transmission coolers. They would have a very high liability problem if they didn't.

[booster]

Originally Posted by wny-pat

I'm going to have to believe that the companies that offer capillary action coolers have consulted with lubrication engineers to ensure that their product does not hurt vehicle components connected to their transmission coolers. They would have a very high liability problem if they didn't.

If you have any references to how they are using capillary action in oil coolers, I would really like to see it, as I just don't understand how it could apply without reducing capacity. My guess would be that if there is a tech paper on it somewhere it will mention and explain if they chose to give up a bit of capacity hot to gain better cold weather performance.


Capillary tubes are often defined as being accurately sized tubes designed to limit flow, which is certainly the function they are talking about in the Tru-Cool but the issue for me is that those limits are based on fixed parameters. In an oil cooler, if you limit flow in one or more of the cooler passages to make it flow very little in high viscosity conditions it certainly will start to flow or flow more when the viscosity drops, so all good there and is why there is a problem in oil coolers and cold weather. The tubes are normally as small as possible to increase the contact area between the tube and oil being cooled. The problem comes, for me, in the "bypass" tubes which would be large enough to flow the higher viscosity oil. These tubes won't get smaller when the viscosity drops, so they will also flow more oil at that point. Being larger tubes, and from the drawing on the Tru-cool site, going straight across the cooler, they will not be doing as much cooling per tube as the other, smaller tubes and will be flowing more oil per tube than the small ones. This will cost you cooling capacity compared to if the cooler was all smaller tubes, which is almost certainly why Tru-cool does not use the bypass setup for the Max-cool coolers as they want to keep the cooler capacity as high as possible vs the physical size of the cooler.


What I am not saying is that the passive bypass style cooler won't protect the trans in cold weather as it will do that. It will also help cool the trans in hot conditions. What I am saying, as in the original post, is that the bypass coolers cost you some cooling capacity compared to the same size, non passive bypass, cooler when hot. Looking at the Tru-cool drawing it appears the bypass tubes still will cool, but how much less and how much of the total number of tubes are that way are unknown, at least to me. In the drawing it is two out of six so 1/3 or the tubes.


Probably the only ones that really would be concerned would be those that need all cooling capacity they can get while blocking the least amount of air with the cooler. The other issue could be overcooling in cold weather as was asked about earlier as all of the bypass and such are passive so wouldn't be able to react to different vehicles that might flow different volumes of fluid or use fluids with different viscosity/temp characteristics.


Side note is the non bypass Tru-cool cooler for the Buick showed up yesterday and is pretty much as expected. It is an odd design in that it is a two pass, out and back, and does not have a big round bypass tube for the return to connection end. This means they have to use 1/2 the tubes for each direction. It also means this 8 row cooler that is 23" long is really about a 46" long 4 row cooler and that is a really long cooler for only 4 tubes. My guess is that it would absolutely be an issue in cold weather and is why they say it has to have a thermostatic bypass used with it. IIRC, when I was looking at coolers for the Chevy van a few years ago, they made this style with internal bypass, but could be mistaken on that. If they did, they probably found they still had to much cold flow resistance in even the, very long, bypass tube it would have and changed the unit to full cooling tubes and external bypass need in the cold.

[booster]

For reference, here is the illustration from the Long Tru-Cool website showing how the cooler works, and more importantly to me, the construction.


If accurate, I think it makes it a lot more clear what I have been saying, maybe

[booster]

I thought I would bring up this long dead thread now that a bit more information had surfaced on the subject.

On the Hayden coolers that claimed/implied that they had an internal thermal controlled bypass (and IIRC had an illustration of it) they are now claiming an new design with pressure controlled bypass (like many other brands do).

Hayden’s internal pressure by-pass design incorporates an integrated valve that eliminates the
need for external valves and additional plumbing at installation.

This is a significant change and my imply that the other system didn't hold up well or was really pressure all along.


IMO, the only way to control the cooler is with temp as pressure has big downsides in capacity loss or flow reduction.


External bypass thermostats have been considered the gold standard in transmission cooling but have had some issues claimed of reliability and temp control range problems.


I have had an external thermostat on our Roadtrek 4L80e for quite few years now with decent, not excellent temp control, and flawless reliability. It is the Improved Performance brand. Disclaimer is that the van does not see much for below freezing weather so don't know about how it would perform there.


I am now putting together a second version of trans cooling for my 96 Buick Roadmaster wagon, 5.7 V8, 4l60e transmission. I put it together with a Flocooler thermostat and cooler originally, but their barb fittings were not good enough to prevent leaks, so am changing to AN fittings and new components.


This time it will be all AN fitting hoses and fittings with the hard to find proper adapters to fit all the connection points. Cooler will be a Hayden non bypass AN fitting equipped one. All pretty normal stuff. The bypass thermostat, however, will be new to me. It will be a new to the market Improved Racing mini thermostatic controller which is much smaller in size and flow capacity compared to the old school engine oil thermostats that have been used on transmissions. The mini has a flow capacity of 5gpm vs 20 gpm for their standard oil cooler. Most transmissions flow 1-2 gpm as far as I know, so getting closer to the flow range needed should improve response time and accuracy of the the thermostat


I will be getting this new system in place in the next week or two, and it will be tested as soon as the roads clear up of salt in the spring here in Minnesota. I am hopeful it will give very good results, but testing actual real world is always needed.


Note, I do like Improved Performance stuff as I have had no issues with them, and the visual quality of the machining and fits is very good. I have no affiliation with them at all, I buy over the counter only. Testing will tell if this mini lives up to my hopes.


It this mini works out well, it could solve a lot of the issues with external stats ons coolers, I think.