I was going through all of the fluids on my truck the other day and when I was picking up oil for the rearend, I bought a tube of K&W additive for the oil. It was called posi-trac additive or something. Well after I was through I got to looking at the book and it called for 8 oz. and my tube was only 7 oz. Is that enough to make a difference? And is there a difference between the additives for Fords and Chevys? Because I have been noticing this slight vibration only when turning at idling speed. It seems to do it more in reverse but it did it in forward today. I thought it was something up front, but I'm beginning to think it's from the rear. So did I get the right oil additive and the right (or close enough) amount?

Thanks,

 

For limited slip differentials, it's usually recommended to drive the vehicle in several tight figure of 8 patterns after changing the lube to get the new stuff worked in.

If it continues to chatter after that, then I'd say you would need additional external friction modifier added. Just enough though, too much will render the limited slip useless, or ineffective.

Most synthetic gear lubes on the market now, already contain the friction modifiers in their formulation. This is one area where I feel synthetics are of benefit, compared to the crankcase.

 

Also, some of the conventional gear oils have the friction modifier in them already!!!

 

If a differential gear oil says it is "for" limited slip differentials, does that mean it already has the additive, or just that it is compatible with limited slip differentials?

 

I have a 94 F250 heavy duty 460 E4OD 4WD etc. My manual calls out for 16oz of additive if I recall. I always add 2 8oz tubes of Sat Lubes version of this additive. The tube says it meets spec for the Ford and Chevy Specification(called out by number on the tube).

On another website I visit, there is a lot of discussion about how much additive to add. In general #1, follow the manual recommendation; 2) If using synthetic, we usually get better results with 2 tubes; 3) If it still chatters add a bit more additive; 4) Do some figure 8s to spread it around.

I read that GM and Ford differentails work in differnt ways and the additive modifies the friction up on one and down on the other, strange.

Good Luck,

Jim Henderson

 

Correct, if the back of the bottle says it's for limited slip differentials that means it has teh additive in it!!

 

The shop that rebiuilt my 8.8 prefers the GM additive. Mine works great without chatter on the GM stuff. They can't be that much different because Dana axles are used in all makes and they don't specify a different additive for Dana axles.

Jim

 

With all due respect, are you sure?

Maybe I've been wasting money by using the additive, however, to my recollection, my manuals have always specified that an additive be added to the specified gear lube when used in a limited slip unit. Never has this been qualified in any way (such as: "unless already present").

Also, my bottle of Valvoline Durablend (synthetic blend) gear oil says it is for "all hypoid differentials (conventional and limited slip) and..." If limited slip requires different frictional characteristics, could it work for both without an additive? Or maybe the additive is necessary for LS, but doesn't hurt in an open diff?

Also, I'm interested in the idea that too much additive could prevent the limited slip from functioning as such. Do I properly infer then that we should use as little as possible unless chattering occurs?

 

I really thought that my book specified only 8 oz. but I could be wrong. I need to check that because it could be the problem. I used K&W, so I dont even know if it is for GM or Ford or both. Threw away the tube. I think my Valvoline oil said the same thing: for conventional and limited slip axles. I'll have to check around on this one I guess.

Thanks,

Edit: The only thing I can find is that they recommend 8 oz. friction modifier. And the oil does say it is for all hypoid applications (conventional or limited slip). Now I just need to find exactly what that K&W stuff is made for.

 

The amount of friction modifier needed I believe is related to how worn the clutch packs are on LSDs. It seems, the more wear, the more modifier is needed. Some have reported having to add friction modifier to synthetic lubes, that already contained the additives, to stop chatter.

I'd add the modifier by trial, a few ounces at a time until chatter is gone.
You'll know if there's too much. It'll act like an open differential on slippery surfaces.

 

Well, that doesn't seem right. If the additive makes the clutches less "grabby" would you need more of it as they wear? I would think that they'd slip more as they wear out, but I must admit, I don't really know!

 

[QUOTE]Originally posted by medic641
[B] It seems, the more wear, the more modifier is needed.
Yes, I'm wrong here. It should be the more wear, LESS modifier is needed.

Friction modifiers will eliminate noise, but can sacrifice performance.

 

I use the same stuff in my front diff and also my 3.73 LS rearend. I had to add about 2-3 ounces more of the friction modifier to stop some chattering I had going around corners. Leads me to believe that if there is an additive already in there, it's probably not a real heavy dose.