I changed my rear diff oil over to Redline Lightweight Shockproof and noticed that during low-speed tight turns the rear diff really grinds/chatters a lot. This distrubed me a bit, so I threw in some Amsoil Series 2000 75W-90 last night and it's still doing it some, though it doesn't seem to be quite as bad.

I wonder if anyone else has changed the fluid and is experiencing this? Does anyone's stock car do this? I don't remember mine doing it ever before the fluid change... and it happens when I pull out of my driveway, so if it did I think I'd notice.

As a side note, I found this blurb on Amsoil's website:

I didn't do a break-in like this with either of the two lubes, though I did leave the Shockproof in for a few hundred miles of driving and it didn't get any better. Maybe I'll try this break-in with the new Amsoil and see if that helps out any.

 

Most OEM's require an additive to the rear diff fluid if it's equipped with a limited slip diff. This used to be derived from whale oil (show's how old I am), but PETA and the like put an end to that. Check with your dealer. Some OEM's blend their diff oil with the additive already in it, others have the additive separate. The fact that you are using synthetic, has no bearing on this, the additive is for the clutch plates in the LSD. You are hearing the chattering because they are failing to unlock. Hope this helps.

Adios,

Fusoguy

 

Actually this does have to do with synthetic. Synthetic is too slippery for the clutch plates. They can't get enough friction. Try the OEM fluid or KAAZ fluid.

 

Well both the Redline and Amsoil claim they're good for limited-slip units, which is why I went ahead and tried both. I did the figure 8 stuff a little while ago in the local mall parking lot. Everything seemed fine after that, but we'll see what happens after I've let the car sit for a little while.

If it's still jumpy then I'll look into going back OEM or buying some Redline friction modifier from the local performance shop.

 

Here's a post from another board that explains things a little better - especially relevant to Redline gear oil. Seems some (and I emphasize SOME) of the synthetics already have friction modifiers in them for use with LSD's. Others you have to add. Anyway, here's the quote...

<<I was looking at RedLine's site about to buy some of their LSD friction modifier in the hopes of making my LSD actually WORK worth a damn and noticed they have 75W90 and 75W90-NS oils. The regular 75W90 already has the LSD friction modifiers, the NS doesn't. The NS type says "recommended for manual transmissions and non-limited-slip transaxles that recommend 90 WT oils. Can be used in racing limited-slip units to increase lockup and reduce wheel spin" which I take to read that the friction modifiers actually make the LSD less effective.
Indeed, I just got off the phone with a tech at RedLine. Told him I was running Mobil 1 (which includes the modifier) w/ my LSD and I wasn't satisfied with the performance of the LSD and I was about to buy some of Redline's LSD friction modifier additive thinking it would make the LSD operate better until I read what they said about their NS-type oil.
He agreed and said I was going in the wrong direction. Adding the modifier only makes things more slippery and the clutches would be less likely to grab and kick the LSD into action. His advice was to go with an additive-free oil like their NS-type and add just enough of the additive to get rid of any chatter in turns.>>

Adios,

Fusoguy

 

Redline works fine for torsen and quaife diffs. When it comes to clutch pack diffs though it's simply too good. The plates can't generate enough friction to engage. Driving in a figure 8 won't do much for clutch packs. They wont' work until there's wheel slippage.

 

Izen: In this case fusoguy is correct. There is too much friction, not too little. If the fluid was too slippery and the clutches don't engage, then the diff effectively acts like an open diff. Open diffs have no problem in this situation, which is a slow, tight turn on completely dry pavement. However if there is too much friction, then the clutches are more likely to engage and will try to lock up when you don't want them to. The chattering/shaking is the clutches repeatedly breaking loose and locking back up.

I also have been exchanging emails with a tech at RedLine about all of this, which pretty much confirmed all of my suspicions.

The figure-8's seem to have done the trick, this morning the car didn't chatter when I turned out of my driveway. And it didn't do it last night leaving the parking garage at work, either. I believe figure-8's do exercise the clutches of the diff because turning makes one wheel spin faster than the other. This is exactly what happens when a wheel starts to slip.