Typical rear differential temperatures

[dparm]

What are the average operational temperatures for rear differentials?

Comparing some 75w140s for my upcoming rear diff change and I've noticed that some are noticeably thinner than others at lower temps (i.e. on the low-end of the 75w spec) -- as much as 10%. I'm trying to discern if it's going to be a big deal to run one that's a tad thinner at those lower temps than the OEM Castrol Syntrax LS. In particular, the two I'm eyeing that are thinner are Renewable Lube's fluid and Torco SGO. They all seem to be about the same at 100C.

An interesting one is Gulf Competition LS which is nearly 20% thinner down low (KV @ 40 = 140) but still the same at 100C. The VI is a shocking 220.

Car in question is my stock 2011 BMW M3, which has a GKN Visco-Lok LSD in the rear diff. No cooler. Obviously it is street-driven, but I do take it to the track.

 

[turtlevette]

One of my theories is the larger the ratio the hotter it'll run. I have a 4.56/1 in my vette and it runs 210 at 60 mph. Measured with an it gun.

 

[dparm]

Originally Posted By: turtlevette
One of my theories is the larger the ratio the hotter it'll run. I have a 4.56/1 in my vette and it runs 210 at 60 mph. Measured with an it gun.




Interesting theory.

The FD is 3.154:1. If it matters, my 7th gear is 1:1.

 

[mjoekingz28]

Originally Posted By: turtlevette
One of my theories is the larger the ratio the hotter it'll run. I have a 4.56/1 in my vette and it runs 210 at 60 mph. Measured with an it gun.




How is it as hot as engine oil even if it doesn't have to deal with the hotness of combustion?

 

[MolaKule]

Tooth contact pressure, contact friction, and churning of the oil raises temps.

HP is energy and some of that energy gets subtracted by the losses above. That energy has to go somewhere and it goes into raising oil temps.

 

[Kestas]

Rule of thumb is 4% loss of energy through gears. I can only imagine that energy is mostly converted to heat.

 

[dparm]

Just confirmed there is no rear diff temperature sensor so it's unlikely I'll ever have any hard data.

 

[barryh]

I have a shaft driven BMW motorcycle where it's very easy to take some measure of the temperature by putting a hand on the casing. It runs vastly cooler than an engine or even a gearbox. I'd estimate that it's never been above 50 Deg C. It's a palloid spiral bevel gear set rather than an offset hypoid so it will be a bit more efficient than a car diff.

I'm a fan of thinner gear oils and use 75W90 rather than the recommended 90W or 80W90 and the difference in oil drag is quite dramatic when spinning the rear wheel by hand at low temperatures.

Ordinary cars like my W204 have quite thin 75W85 diff oils these days presumably to improve economy. On a recent BMW M3 I'd be inclined to go by the book though especially for track use.

 

[meep]

in general terms, I've read it is true that a higher ratio raises gear temps. I read the same in a hard copy publication on it not long ago. They noted that OTR towing practice is to use lower ratios to reduce temps. Diffs are not granted controlled cooling systems like transmissions are. They mentioned that if one was setting up a truck for towing, it was far more recommended to instrument the axle with heat monitoring than the trans.

The 4.10 rear in the tundra is hand-touchable after driving empty. After towing, it >radiates< heat-- I've never measured it. I'd wager it approaches 180 or more.

 

[dparm]

Originally Posted By: barryh

Ordinary cars like my W204 have quite thin 75W85 diff oils these days presumably to improve economy. On a recent BMW M3 I'd be inclined to go by the book though especially for track use.


The manufacturer spec is 75w140, but my point is that the range of KV for that given weight is huge -- it varies by up to 20%.

FF is Castrol Syntrax LS:
KV @ 40C = 175
KV @ 100C = 244.7
VI = 174


Other 75w140 fluids I am considering:

Gulf Competition LS
KV @ 40C = 140
KV @ 100C = 26
VI = 220

Renewable Lube
KV @ 40C = 153.9
KV @ 100C = 25.1
VI = 198

Torco SGO
KV @ 40C = 154.5
KV @ 100C = 25.51
VI =200

 

[barryh]

Originally Posted By: dparm

The manufacturer spec is 75w140, but my point is that the range of KV for that given weight is huge -- it varies by up to 20%.


I faced the same problem when looking for 75W Manual Transmission oil where for the locally available choices the KV @ 40C ranged from 32.2 to 40.8 Looking at the data sheets for the very few MB approved oils it was obvious they were targeting towards the thinner end of the grade.

Does BMW provide a list of approved oils that could be used as the bench mark or do their recommended oils span the whole 20% range ?

 

[dparm]

Originally Posted By: barryh
Originally Posted By: dparm

The manufacturer spec is 75w140, but my point is that the range of KV for that given weight is huge -- it varies by up to 20%.


I faced the same problem when looking for 75W Manual Transmission oil where for the locally available choices the KV @ 40C ranged from 32.2 to 40.8 Looking at the data sheets for the very few MB approved oils it was obvious they were targeting towards the thinner end of the grade.

Does BMW provide a list of approved oils that could be used as the bench mark or do their recommended oils span the whole 20% range ?





No. Since they are using a special version of Syntrax LS with extra FM, there's technically no equivalent fluid.

It should be noted that they only switched to that over the standard fluid (called SAF-XJ back then) because people complained about noisy differentials.

 

[dparm]

I actually did just learn something cool from one of the resident BMW electronics experts over at M3post: apparently there is no rear diff temp sensor, but the DSC (stability control) computer is able to calculate it based on various inputs! Neat stuff.

 

[mjoekingz28]

meep,

A question about temps rising. Do you think it is the load they are under AND speed?

Because, an off-road dumptruck pulling max GVW going 25mph isnt getting (it seems) a whole lot of cooling because of limited airflow. Then add the dust coating the pumpkin and that probably holds a lot of heat in.

Then an OTR truck is pulling heavy AND fast, but is on clean(er) pavement and is getting alot of airflow. So, maybe heatsinks and someway to direct airflow, that coupled with cool synthetics and it seems there shouldn't be a problem!

 

[Shannow]

Thread from a little while back on the selection process.

http://www.bobistheoilguy.com/forums/ubbthreads.php/topics/3910846/Re:_putting_synthetic_oil_in_a#Post3910846

If you can come to a stop, put your hand on the diff, and not pull it away in a couple of seconds, it's typically under 65C.

On my Nissan, the diff runs about 40C (70F) cooler than the tranny case on the highway (heat gun)

 

[zeng]

Originally Posted By: dparm
The manufacturer spec is 75w140, but my point is that the range of KV for that given weight is huge -- it varies by up to 20%.
FF is Castrol Syntrax LS:
KV @ 40C = 175
KV @ 100C = 244.7
VI = 174
Other 75w140 fluids I am considering:
Gulf Competition LS
KV @ 40C = 140
KV @ 100C = 26
VI = 220
Renewable Lube
KV @ 40C = 153.9
KV @ 100C = 25.1
VI = 198
Torco SGO
KV @ 40C = 154.5
KV @ 100C = 25.51
VI =200

Originally Posted By: dparm
Car in question is my stock 2011 BMW M3, which has a GKN Visco-Lok LSD in the rear diff. No cooler. Obviously it is street-driven, but I do take it to the track



As highlighted to you in the other thread, IF you are for components longevity .... pick one with the highest KV@40*C.

Otherwise, don't bother.

 

[meep]

Originally Posted By: mjoekingz28
meep,

A question about temps rising. Do you think it is the load they are under AND speed?

Because, an off-road dumptruck pulling max GVW going 25mph isnt getting (it seems) a whole lot of cooling because of limited airflow. Then add the dust coating the pumpkin and that probably holds a lot of heat in.

Then an OTR truck is pulling heavy AND fast, but is on clean(er) pavement and is getting alot of airflow. So, maybe heatsinks and someway to direct airflow, that coupled with cool synthetics and it seems there shouldn't be a problem!


Yes, exactly. It will be a function of the amount of work performed over time. Crude math might show it as:

(distance * load)/time

dump truck at 25mph has more time to dissipate the heat, or is doing less work over an equal period of time.

-m

 

[dparm]

Decided to stick with the FF Castrol Syntrax LS, but not the BMW version with extra FM. I'll see how it does and add more FM if it really is as noisy as some people complained.

 

[wrcsixeight]

I was considering using some arctic silver thermal epoxy to attach a K type thermocouple to my Diff and monitor temperatures via my Digital multimeter or a dedicated display.

I have one attached to my Alternator , and when charging a depleted battery, idling causes temperature to spike, and movement and more rpms have the temp plunge. I suspected this, but it was nice to confirm it.

K type thermocouple wires come in various lengths. Not sure where on the differential would be the best spot for a temperature reading, but actual data sure beats guessing or a butt dyno.

 

[philipp10]

Originally Posted By: Kestas
Rule of thumb is 4% loss of energy through gears. I can only imagine that energy is mostly converted to heat.


"mostly converted to heat"...ahh so the rest is converted to what, light....lol