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Transmission and Differential Temperature Limits

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As part of a larger aero mod, I have installed a full flat underbody on my track car, a BMW 135i. One of the biggest challenges introduced by this is heat management, as the exhaust is fully enclosed for the length of the car. I do have NACA inlets providing some ambient air to the transmission and differential, but the bulk of the air passing down the length of the underbody has passed through the radiator and over the engine, so is pre-warmed. The air passing over the exhaust pipes is mostly segregated from that flowing over the transmission and differential. I started out with temperature witness labels to keep an eye on things and saw that the differential temperature exceeded the maximum temperature on my labels (160°C). Seeing that, I added thermocouples measuring the surface temperature of both the diff and transmission, I improved the air flow, and I switched from 75W90 to 75W120 in the diff. The thermocouples let me see that under track conditions the transmission and differential temperatues rise continuously, with any asymptote being rather too distant. Essentially my sessions are limited by how hot I am willing to let the differential fluid get. I have since put temperature sensors into both the differential and transmission that measure the actual fluid temperature (which I expect to be a bit hotter than the case tempratures I measured previously. This brings me to my question - what is a reasonble upper limit for GL-5 differential fluid and GL-4 transmission fluid? I do note that the ECU starts engaging in engine protection strategies above 150°C crankcase oil temperature. Absent other data, that seems like a reasonable maximum for other lubricants, but I don't want to call that my normal. I am inclined to try to keep these temperatures below 120°C but that is really just pulled out of a hat. Datasheets for my fluids don't really offer any useful insight. Another way to ask the question would be, at what point do you think a differential cooler is needed in a track car?

Details of the aero mods can be found at https://www.1addicts.com/forums/showthread.php?t=1713247

My thoughts, for consideration rather than a 'definitive' coment.

The manufacturer of the transmission fluid you're using will have a maximum temperature in the specifications. However, that isn't quite the whole story, as the oil is critical for removing heat from the pinion and ring-gear - if you are having to replace then frequently* and/or there is any discolouration, it suggests a problem with overheating of the oil there and the lubrication breaking down.

A major influence on the heat buildup is the turbulence, churning, of the oils by the gears. The OEM oil levels in both is a reflection of the exp4ected use of the vehicle, with a lot of slow running, and so is a little high to ensure the oil gets splashed around inside the housing. You can try lowering the oil levels a little, as the gears will be providing plenty of splash. On that, turbulence will be putting more heat into the oil with the higher viscosity oil, you may not get the benefit you expect?

All that said, I would expect you WILL need to run coolers for both the gearbox and the differential housing - you can buy oil radiators with integrated fans, so at least you have some flexability on their location. The cooler oil will also slow down, or stop, the seals hardening, breaking down, and leaking. As a aside, are there any indications of the CVs running hot?

* many years ago I had a friend who was getting 3 events out of a CWP, and when he changed to AMSOIL that problem went away.

You may wish to read some of these articles - https://www.amsoil.com/newsstand/drivetrain-lubricants/articles/

Thanks for the input Gord. After that intial 160°C+ episode with the diff, the oil got dark and stinky immediately, which motivated the change to 75W140. Since then my temperature label has not gone above about 138°C (thanks some airflow mods). The fluid is currently nice and amber. Transmission on the other hand is black, but I had not looked at it in a while, thinking it wasn't an issue.

Regarding the CVs, the LH inboard shrivelled up like a dried apple. It is very close to the exhaust. I have replaced the boot, relubricated the joint, and installed a 1/16" diameter rollpin under the small end of the boot to allow the boot to vent. The old boot was not brittle. I think it had just overpressurised due to heat and vented out, and then collapsed when the internal air cooled and the boot could not vent back in. Winter here still so I haven't tried this configuration yet. Exhaust is wrapped and has some stainless shields in the vicinity to minimize radiant heating of the boot, but that was not enough to prevent the dried apple thing.

There are still some air flow management improvements I can make so that is plan A. I am still hoping to avoid adding coolers, but data will utimately dictate if they are needed.

I had a quick look through your build post James, really nice attention to detail. Easy to see 1000's of well-spent hours have been sunk into your build so far. Interesting to see such a well-documented development process too, well done.

The specification of different oils with change the maximum safe oil temperature, this is really only something that can come from the oil manufacturer. I would say if the oil manufacturer can't/won't give you a max operating temp then I'd be looking for a new oil supplier. Some great comments from Gord in there as well on some of the other practicalities involved with levels etc.

In the absence of any data, my mental trans/diff oil temp limit is the 130C deg mark. Again, this is just my guideline when I have nothing else to work on, so it's not gospel!

Looking at your application and particularly with the addition of a sealed floor, based on my experience I can not see a way where transmission and diff coolers can be avoided. It does add some complexity and weight, but in my experience, I am struggling to see how this can be avoided for your case.


Thanks for the compliment and the suggestions, Tim.

I am using Red Line synthetic fluids in both the diff and transmission so I gave them a call to discuss the temperature issue. They were very helpful and indicated that both the fluids will still work well above 150C, but that seal hardening will become an issue above 150C. For the time being, I have set an alert at 120C and an alarm at 140C. I will keep an eye on the temperatures once I can get to the track again and then proceed based on what I find. I am massaging the air flow around the transmission to better segregate the exhaust cooling and transmission cooling air streams. I think that will be enough to get the transmission temperatures under control. The diff is a bigger challenge and may indeed need a cooler.

I'm sure what you learn with the ongoing development of your car would be of interest to Racecraft members. If you feel inclined to, posting some updates (cooling-related or not) to the build thread part of the forum would be great.

Keep us in the loop with how your cooling project develops either way

Perhaps an inlet at another point on the vehicle, bypassing the engine bay and directly feeding cooler air to the gearbox?

Really nice work there James! I hope it pays of also in lap times!

Would there be any sense in combining the NACA rear your diff, with an outlet next to the exhaust, over the diffuser?

Sorry for the small hi-jack, but what are the common solution for plumbing trans coolers?

My first idea would be (ideally) dry-breaks on the fill and drain plugs, to avoid the necessity to modify the housings, but i have no experience on the topic.

Hi Armaki, plumbing in inlets and outlets into the fill and drain ports can be an option. However, you do have to be careful you're not just making a small cycle where the return to the gearbox gets mostly sucked straight back into the pump inlet. You can imagine how this would happen if the fill and drain ports were very close together, you would have this part of the fluid cycling through the cooler, with the main bulk of the oil in contact with the gearset not getting much chance to cycle through the cooler.

In this case, you're much better to modify the housing to cool the bulk of the oil in the transmission. So, it really depends on the application.

Hi Tim, good point. I guess some kind of splash structure, or a kind of jet on the return plug would help, as long as the cooled oil is not guided on a specific gear, to avoid "cold spots".

But then that might be more complex than just a hole on the housing, far from the drain plug. Maybe the other side of the pan, to avoid direct interference of the cooled oil feed with the moving gears.

James, I had a random look around the web and came across this:


Definently not exactly what you are looking for, but I guess a GTR might have similar issues, being aero-enhanched from stock. The gearbox is also in the rear.

Litchfield are no strangers and they have also some options for various BMWs.

Yea Armaki, there's always more than meets the eye when you start digging into this stuff! From memory, you will find some aftermarket transmission companies spraying the return oil onto the gears themselves with a specific pattern. Although I'm sure there is quite a lot of work that goes into controlling the return so that you don't unnecessarily agitate the fluid which would just cause power losses.

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