mwaller

I can find no reference to any sort of oil additives in Nissan's technical literature. The service manual just calls for a GL-5 80W90 gear oil; no distinction is made bewteen models that are equipped with standard and viscous limited slip differentials. If I replace the gear oil in my Performance model Z, should I use a friction modifier additive?
If this additive is needed, why doesn't nissan call for it? Is there more than one type of viscous limited slip differential?
Thanks,
Mika

Phi-1002-

That additive is for friction clutch type limited slip diff's. You shouldn't need any additives. Your VLSD doesn't use clutches.

mwaller

Interesting... that would explain it. Thanks very much for the info.
So, do I understand correctly that there are different types of "viscous limited slip differentials?" If the VLSD in the Z doesn't use clutch plates, how does it differ from the standard differential on the base model?
Mika

TJZ

There are different types of LSDs in general, with viscous and clutch type being two of them (different types of lsds). the base model does not have a LSD at all.

Them Bones

Has anyone ever wondered how the viscous LSD and electronic LSD work together? In case you haven't noticed it in the manual, see page 5-22. The VDC system has a LSD function. If VDC is looking our for wheel spin, when does viscous LSD come into play???

Please see that section in the manual before replying (if you haven't already).

Thanks,
TB

Desmo

Some LSD's that use gears instead of clutch packs don't work when one tire is in the air or on a very slippery surface (ice). The worm gears inside the diff (or whatever mechanism they are using) need at least a little bit of resistance to push against before they can transfer torque to the wheel with traction.

In those cases a traction control system can apply brake pressure to a rear wheel that is freewheeling, thereby allowing the LSD to apply the engine torque to the other wheel.

But in the 350Z we have a viscious LSD, which shouldn't suffer from that limitation. It is simply going to try and limit the difference in speed between the left and right tire. So I would guess the VDC system is still able to help out by applying braking force to the single spinning tire. Maybe they do this to reduce heat buildup in the viscious LSD unit? Or maybe there are cases where engine torque can overcome the capabilities of the viscious LSD unit and that's when the VDC steps in with some braking help.

Phi-1002-

Desmo,

I think another reason for the brake LSD is that the VLSD has a little bit of lag time before the fluid heats up and it's viscosity increases. The increased viscosity due to heat creates the limited slip effect. So, until that happens, it's basically an open diff.

dr_gallup

One more thing for those who don't understand VLSD. The lubricant you are changing in the differential only lubricates the gears. The viscous limited slip function is accomplished using a non-Newtonian fluid (gets thicker as it gets hotter) which is completely sealed and not replaceable. Almost all your AWD cars use this technology except for some using torsen diffs and some really expensive electronically controlled diffs.

mwaller

Ah hah! That might explain the 1-piece "viscous coupling" depecited in the service manual. Is there any way to actually confirm that the coupling is sealed? Nissan service managers really have no idea what is in there... they keep telling me that there is "usually" an additive necessary for VLSDs..
Thanks,
Mika

Phi-1002-

Pop open the housing, drain it, and change your gear oil. You'll get a great view of the VLSD and yes, it most certainly is sealed.

If you're really talking to a service manager, find a different dealership to have your service done at!

Desmo

I don't think heat has much of anything to do with the viscous coupling. It's a fluid (silicone based I think?) that resists shear. Inside the VC unit are a series of plates spaced next to each other. The plates are alternately connected to the right and left tires. The plates may have vanes on them, or holes drilled through them, I'm not sure on the Z.

Anytime there is a speed difference in the rear wheels, the alternating plates inside the VC are spinning at different speeds. This causes a shearing force on the silicone fluid since the plates are so close to each other and immersed in the fluid. The shearing raises the viscosity of the silicone fluid and effectively locks the plates together. Heat is generated during the process, but it is not what controls the process.

btw, the correct definition of a non-Newtonian fluid is a fluid whose viscosity is not constant at all shear rates.

So yeah I agree there is a lag time between a difference in speed and torque being sent to the tire with the most tractions, but I don't think it's very long and I don't think the brakes are being called upon to reduce the time. Otherwise you would have the outer rear brake pulsing everytime you went around a somewhat tight corner.

JL