Originally Posted By: dnewton3
As to the OPs topic, one should never use clutch FM additive on a Torsen (gear type) LSD. They rely on very critical clearances and friction coefficients to (quite literally) bind and release, based upon surface to surface contact of the gear teeth relative to the gear bores and helical cut depending upon torque direction and reaction. These are typically "tuned" at the factory for their bias by manipulating the clearances, the gear angle, the number of gears, etc. FM only serves to undesirably alter that engineering work by messing with the coefficient of friction.
Off-topic ...
Originally Posted By: ironman_gq
GM G80 does not require a limited slip additive either, it does have clutches.
That is only 1/2 correct.
The G-80 is only an RPO code for a traction control device. In their truck line, it has been the code for the Eaton M-Locker, seen here:
http://www.eaton.com/Eaton/ProductsServices/Automotive/DifferentialsLockingDifferentials/PCT_314766
This is called at times the "Gov Locker" as well.
This unit most certianly has clutches; that is true. But to say it does not need FM is false. And here's why ...
The M-locker is a glorified clutch driven, variable bias limited slip unit. It is not a "locker" in the true sense that I believe a "locker" to work. The M-locker operates as a limited slip device when under low bias demand, and then the ramps provide much greater clamping force for more clutch friction upon certain conditions. This this is ALWASY a limited slip clutch, but at times it "locks" the pack so tight that it cannot slip any more. Folks - there is a limited slip clutch inside and it most assuredly needs FM for the same reason that all others do; to resist chatter and nuisance locking events.
There is a GM statement out from years ago that one should never add FM (friction modifier) to the differential. But that is NOT because it does not require it; it actually does. That statment exists for two reasons:
1) the GM fluids already have FM in them
2) too much FM can be as detrimental as not enough, but for different reasons
So GM says not to "add" FM, as in "Please don't put more into our fluids; we already put it in there ..." This is the correct way to interpret their statement. The incorrect way to read it is "Don't add FM; it doesn't need any at all." That is completely wrong.
Here is a long-lived quote from an Eaton Engineer on the topic; you'll note his statment includes the topic of FM in the GM spec'd fluids (I will underline for emphasis):
"Automatic Locking Differential Lubricants. [Note from Michael Asmussen, Torque Control Products Division of Eaton Corp.]
We recommend the following lubrications for our locking differentials:
1)Texaco 2276; Synthetic 75 W90; Gm Part # 9986115
2)Texaco 9622; Mineral based 80W90; GM Part # 9985290
3)Texaco 2080; Synthetic 75W140 (heavy duty applications); GM part # 9985991
Note - All of the above lubes are preblended with friction modifier. No additional modifiers are necessary or recommended. As far as other lubes are concerned, any standard GL 5 lube will work, but the units perform optimally with the three listed above.
Amsoil also has a statement out about it that clarifies the fact that GMs fluids (both FF and OTC) include the FM:
http://www.amsoil.com/techservicesbullet...%20revision.pdf
The reason owners manuals, and Motor Information Systems both print the statement, “Do not add friction Modifier” is that GM specification #9986115, part #12378557 (USA) or #88901362 (Canada) have the limited slip friction characteristics already in the formula, (Reference 5) and the addition of friction modifiers are not needed."
Do not get wrapped up in the axle (play on words here ...
) being discussed in the Amsoil document. Focus on the GM fluid spec #9986115, which is the fluid used in MANY GM applications, including the ones for the G-80 "M-Locker" apps.
Not that this disucssion has anything to do with the OPs topic, but I wanted to set the record straight on the G-80 topic.
Per GM truck TSB......
Condition/Concern:
Customers may comment on a chatter or shudder from the rear axle on vehicles equipped with an Eaton locking differential. This condition would be noticed in parking lot maneuvers, turning, or during locking differential engagement and may be more noticeable when the differential is hot. Locking differential clutch chatter may be causing this concern.
If differential clutch chatter is encountered in a vehicle equipped with an Eaton locking differential, flush, drain and refill the differential with new synthetic fluid, use the fluid part number listed in SI for the vehicle currently being worked on. If the condition returns, replace the rear differential clutch plates and refill the differential with new synthetic fluid, use the fluid part number listed in SI for the vehicle currently being worked on.
Important: Installing any friction modifier causes the clutch pack in the locking differential to slip and miss engagement. A fluid flush can often remove debris that is a contributor to clutch chatter. Adding friction modifier additive can cause a loss of locking differential functionality.
Chatter is essentially the buildup and release of energy between the differential clutch packs during operation. This phenomenon often occurs when torque is transferred between the clutches or when rotational speeds change. During the stick-slip phase, energy is built up to a point where contact between the clutch plates change between static friction (stick) and dynamic friction (slip), resulting in noticeable vibrations. It is this vibration that causes an audible chatter or ratcheting to be heard within the differential. While certainly an annoying characteristic, there is typically no mechanical damage occurring to the differential. It is most often notice in parking lots or slow driving while turning where the outside wheel turns faster than the inside wheel.
In an effort to control this chatter, friction modifiers are often used. These additives result in a smoother transition between the two states of friction: static and dynamic. Static friction occurs when the clutches physically lock together, while dynamic friction occurs as the clutches are slipping. ergo Limited Slip Differential
