Thicker gear oils and LSDs

[dparm]

My car will need its rear differential fluid changed soon. The car has a factory GKN Visco Lok LSD, which is a speed-sensing variable-lock LSD used in other cars like the Viper: http://www.gkn.com/driveline/our-solutio...co-lok-lsd.aspx

Originally Posted By: Unitrax website
"The Visco-Lok is a unique hermetically sealed speed-sensing limited-slip differential. The unit benefits through its high-torque progressive locking characteristics which provides maximum torque transfer in excess of skid torque.

The Visco-Lok operates through a self-contained silicone-fluid filled reservoir and shear pump that actuates a wet friction clutch pack. The pump generates pressure proportional to the speed difference between the wheels. The clutch pack actuates thereby transmitting torque to the appropriate wheel. The locking effect increases with the speed difference across the drive wheels.

This locking characteristic enables the Viper to accelerate well on surfaces with different friction conditions between the drive wheels. Additionally, GKN states that the Visco-Lok allows even better acceleration and less under-steer in cornering than compared to torque-sensing LSDs."




It's spec'd for a 75w140 GL5 and the factory fluid is Castrol SAF-XJ, now known as Castrol Syntrax Limited Slip. KV @ 40 = 175 and KV @ 100 = 24.7, with a VI of 174. For the first year, there was no FM and lots of people complained of chattering, so they made the FM standard fill going forward. There's conflicting info on whether or not they require it; BMW says in one document that the differential will fail without the FM, but I don't believe that (and they didn't use the FM at first anyway).

I am considering using a different fluid for cost and performance reasons. As with anything else, there is a huge range of viscosities that are considered 75w140. How will a fluid towards the thinner or thicker end of the 75w140 range affect the LSD operation? I am not necessarily interested in a fluid with the FM included, because it apparently decreases the effectiveness of the LSD (makes the surfaces more slippery).

The fluids I am considering are:

Renewable Lube (KV @ 40 = 153.9, KV @ 100 = 25.1, VI = 198, no FM)
Gulf Competition LS (KV @ 40 = 140, KV @ 100 = 26, VI = 220, has FM)



And just for fun, there is always Motul Nismo Competition Oil 2189E for $72/L (KV @ 40 = 202, KV @ 100 = 29.9, VI = 190).

 

[andyd]

On a trick piece of equipment such as this, I would use what the maker specced. Clutch chatter is annoying. I was amazed when Mobil 1 75 90 cured it with over 200k miles on it. It is the only synth I use. Whatever you get the writing on the jug will tell you whether it has FMs. If it says "for top off only" dont buy it

 

[Rand]

so the clutch pack is bathed in gear oil(how I read it) or the silicon fluid?

 

[dailydriver]

Originally Posted By: dparm
My car will need its rear differential fluid changed soon. The car has a factory GKN Visco Lok LSD, which is a speed-sensing variable-lock LSD used in other cars like the Viper: http://www.gkn.com/driveline/our-solutio...co-lok-lsd.aspx

Originally Posted By: Unitrax website
"The Visco-Lok is a unique hermetically sealed speed-sensing limited-slip differential. The unit benefits through its high-torque progressive locking characteristics which provides maximum torque transfer in excess of skid torque.

The Visco-Lok operates through a self-contained silicone-fluid filled reservoir and shear pump that actuates a wet friction clutch pack. The pump generates pressure proportional to the speed difference between the wheels. The clutch pack actuates thereby transmitting torque to the appropriate wheel. The locking effect increases with the speed difference across the drive wheels.

This locking characteristic enables the Viper to accelerate well on surfaces with different friction conditions between the drive wheels. Additionally, GKN states that the Visco-Lok allows even better acceleration and less under-steer in cornering than compared to torque-sensing LSDs."




It's spec'd for a 75w140 GL5 and the factory fluid is Castrol SAF-XJ, now known as Castrol Syntrax Limited Slip. KV @ 40 = 175 and KV @ 100 = 24.7, with a VI of 174. For the first year, there was no FM and lots of people complained of chattering, so they made the FM standard fill going forward. There's conflicting info on whether or not they require it; BMW says in one document that the differential will fail without the FM, but I don't believe that (and they didn't use the FM at first anyway).

I am considering using a different fluid for cost and performance reasons. As with anything else, there is a huge range of viscosities that are considered 75w140. How will a fluid towards the thinner or thicker end of the 75w140 range affect the LSD operation? I am not necessarily interested in a fluid with the FM included, because it apparently decreases the effectiveness of the LSD (makes the surfaces more slippery).

The fluids I am considering are:

Renewable Lube (KV @ 40 = 153.9, KV @ 100 = 25.1, VI = 198, no FM)
Gulf Competition LS (KV @ 40 = 140, KV @ 100 = 26, VI = 220, has FM)



And just for fun, there is always Motul Nismo Competition Oil 2189E for $72/L (KV @ 40 = 202, KV @ 100 = 29.9, VI = 190).

Where can you get the Gulf fluid in this country??

That Nismo Motul fluid is almost impossible to find (for ANY price).
I was told by Motul USA that it is only available at Nismo certified Nissan dealers, but EVERY one I ever called/stooped in to, not only had NO CLUE what I was asking about, they claimed that there was NO part number for that Motul fluid in the Nismo catalog.

 

[dparm]

Originally Posted By: Rand
so the clutch pack is bathed in gear oil(how I read it) or the silicon fluid?


The URL above has a technical document that reads:

Operating Principle
> A self-contained silicone-fluid filled reservoir
and shear pump generates a hydraulic
pressure proportional to a speed difference
across the axle
> The speed difference between the feed disc
and the fluid filled channel of the pump disc
and the resulting fluid shear forces moves the
fluid from the reservoir to the apply piston
> The hydraulic pressure creates an axial force
via the apply piston, compressing the clutch
pack
> The resulting locking torque through the clutch
pack transfers torque to the wheel with the
better traction
> A variable volume reservoir compensates for
the fluid thermal expansion



It also reads:

> The shear pump and silicone fluid reservoir are
self-contained and maintenance free
> The progressive locking characteristic is tuned
by specific silicone fluid viscosity and clutch
pack sizing



So, to answer your question, the silicone fluid is used only to adjust the variable-locking function.

dailydriver, there are actually a few sites that stock the Gulf fluid. This must be a recent thing because as of a year or two ago, they had no distributor in the US. The Nismo fluid is available from some GT-R performance shops, but it costs even more than my OEM differential fluid (already ridiculous at $50/L).

I guess I'll stick with a fluid with FM or buy the additive. Is deviating from the factory-fill viscosity by about 10-20% still relatively safe? Would I experience different LSD lock-up? Both fluids I'm considering are pretty stout so I'm not worried about them thinning out too much or breaking down.

I'm still not totally clear on how a thicker or thinner fluid might affect the LSD behavior.

 

[Ducman]

Seeing as the Limited slip action is externally actuated by a hydraulic mechanism with a silicon based fluid(probably a DOT 5 brake fluid).

I wouldn't think the actual viscosity of the diff oil, will make much difference to serviceability/longevity as long as it's the correct viscosity in accordance with the service manual.

Nearly all of the 75w-140 oils have much the same kinematic viscosity, only some have the friction modifier( Sturaco 7098) already added into it, at what is generally deemed to be the optimal treat rate(about 60 ml / 1 litre), with no further additive required.
However, it's not uncommon for an extra amount of FM to be added if diff chatter is problematic in certain circumstances, without resulting in too much change in effective limited slip action/operation.

I would suggest using whatever oil you find without the FM, and see how it goes.
Add the FM separately as required in the event of diff chatter occurring.
It can be added in increments of half a 100 ml bottle until chatter ceases to your satisfaction.
100 ml is usually the amount required. Hardly ever less.
Sometimes more is required, depending upon the diff capacity.

I hope this helps you to work out a course of action.