In my stock differential I've gone from 75w90 which was in there when I first bought the car, to 75w110 in my first fluid change, to 75w140 in there now. While it would seem that 75w140 is overkill on a stock differential I have noticed improvements in the LSD action along the way through the different viscosity changes. I don't have any complaints with the 75w140, to my surprise my fuel economy hasn't suffered at all, and I'm going to get it analyzed at the end of this summer season. I made the switch to 75w140 after draining my last fill (75w110) and the fluid looked poorly, I sent in a sample of the 75w110 used gear oil for a used fluid analysis but the sample never made it to the laboratory (aaaaaargh !) and somehow got lost in the mail.

On to my question and findings. I have a long sweeping curve on a road very close to my home. I tend to change gears partway through the long sweeping curve rather than holding the revs at high rpms through the entire curve. ( I know, I know, don't lift throttle or change gears in corners, maintain balance and all that) , but I do it on this road only and I'm not driving the vehicle at high speeds or high cornering forces or in the rain and I'm usually in lower gears as it isn't a very high speed driving zone. What I've found is that the car feels much more stable in these conditions as I've increased the viscosity of the differential fluid. With the thinner fluids I would get more of a jittery feeling in the rear end under the same conditions on the same curve. The car never got anywhere near being loose or squirrely, but you could feel the transfer going on in the rear end as if the LSD was engaging/disengaging. As Ive gone up to thicker fluids the jitteriness has decreased substantially. With the 75w140 in it now the rear end is much more stable. It just feels like the LSD is being affected by thicker viscosity fluid, I don't know exactly how it might affect the LSD, but it does feel much smoother in engagement/disengagement of the LSD in turning situations.

Anyone have any opinions on how fluid viscosity may affect LSD operation on this vehicle ?. Please don't reply telling me not to shift in a curve as that isn't the question here, I'm just curious on knowing if fluid viscosity can affect the LSD engagement/disengagement. Thanks for any replies.

 

I doubt the oil viscosity has any impact on the gears of the Torsen unit. The spider gears physically displace to create the locking action.

 

I have read of people experiencing funky handling under power with fluid that's well overdue for a change.

 

the diff fluid does thin out due to shearing over time if it is left in place too long, maybe that is related to what I'm feeling with viscosity changes ?

 

The point is when there is little rev difference between the gears the oil needs to be able to let them slide without creating locking action.
This happens during cornering under throttle, the tighter the corner the more it happens.
So when the oil can not do this, the Torsen will lock more and makes the inner rear wheel spin faster creating understeer (IMO)
The inner rear wheel pushes the front out of the corner.
I've noticed this and posted about it a couple of times.
Also, I've asked Torsen about my experiences and they agreed that oil condition may change the way the LSD reacts, allthough they said it would not be noticable.
Now, in a car where one can notice 0.1 bar of difference in tire pressue I'm sure one will notice a LSD acting different too.
And, in my case a diff oil change changed the LSD behavior instantly.
Neutral handeling during cornering under throttle.

So: YES.
Diff oil condition (*) has an noticable affect on Torsen LSD operation.

(*) It may be to thin to begin with or it sheared down over time/miles, the E(xtreme) P(ressure) additives have sheared or they don't work very well.

 

 

Quote:

I doubt the oil viscosity has any impact on the gears of the Torsen unit. The spider gears physically displace to create the locking action.

This was my initial reaction but I ran Redline Heavy Shockproof for a year and it "seemed" like I got more inside rear wheel spin when I changed to Mobile 1 differential oil. So it's possible a heavier oil may slow or decrease the opening of the Torsen unit.

 

When the side gears - the big ones - spin at different revs the planet gears - small ones - and the special shape of the gear teeth create axial forces.
They push the side gears together and themselves against the housing, creating lock.
The oil needs to be able to reduce the friction enough to allow small rev differences to not lock during cornering under throttle.
The LSD doesn't know if the rev difference is because on wheel lost grip or because you're taking a corner.

And don't forget this is chewing up the diff oil every inch you drive.

 

With the thinner viscosity fluids it felt like the LSD was locking under throttle cornering, with the thicker fluid i don't get that sensation and it feels much different.

 

I just did a change from redline shockproof to mobil 75w140 recently also. I do not track my car, but i did notice while taking some turns that I pass by on a daily basis, the rear end does feel different with the new oil. It almost feels like the rear end wants to break loose a little more easily.....should this be the case?