Okay so every day I learn more about my Z. I want Amsoil Severe Gear so dug up some info on my car (kudos to the previous owner to have kept the records so clean and orderly!). I have a '99 Z28 with:

-GU5 Axle Ratio 3.23
-G80 Axle Posi
-Stock Torsen rear diff (no clutches---at least I hope I'm right about that.)

Question 1: Amsoil provides an additive called SlipLock. Should I purchase it for my type of differential or is that pointless? I read Amsoil Severe Gear already has the additive mixed in.

Question 2: 75W90, 110, or 140? Reason is there's a whine at 50mph+ and want to get rid of it, firstly by doing the oil quick-fix---hopefully through the toughest oil I can find. I read that by getting 140, I could lose gas mileage. Is this all that can happen or will it really ruin or even lock the diff? I don't race the car it's basically a daily driver, with occasional bursts of course Also the climate here in LA has been pretty cold, usually at 35 deg in the morning to 65 deg at night---does this factor in my choice of oil? Anyone care to share?

 

The Torsen has no clutches unless you have an upgraded T2R differential carrier, which I doubt you have. The Torsen does not need to have friction modifier added to the oil. A synthetic 75W90 is all I ever used when I was using a Torsen. If you have a whine at 50+ MPH, you most likely have a bit of wear on the ring face, and since the Torsen is a pretty tough unit, I think you could safely experiment with a thicker oil. However I doubt you'll be able to mask the sound of ring gear wear with the oil, unless it's a mild whine.

 

Okay, great, so no additive needed. I've never actually drained the old fluid on a diff, is there a plug or will I have to remove the cover? And yes there's a mild whine, especially when I'm at 50. If I go 60+ it's still there but starts disappearing. When I let go of the gas it stops but I can still faintly hear it. It's pretty annoying

 

What you are describing is definitely the sound of ring gear whine. If it was axle bearings, it would be a growling that gets linearly louder with vehicle speed. If it was pinion bearings, it would be a rumble & growl in much the same way. If it was pinion gear spacing, you'd hear it go away only when you are coasting with no load on the gear faces and it would come back when you decelerate and engine-brake. If you hear it only when you are cruising, and it's very faint on deceleration, it's probably some mild gear face wear on the ring & pinion. I lived with that sound for about 40,000 miles on my Trans Am; and with factory 3.73's, it was pretty pronounced, especially at highway speeds.

 

Hmmm...so at a stop, no whine. As I accelerate it starting humming that grows louder up to 45-50mph then starts quieting down past that---I can still hear it but probably because it is rotating quickly by then. It sounds like a spaceship, or an Idol contestant humming to Whitney Houston's "will always love you" song. The sound gradually climbs based on my speed. When I let go of the accelerator it stops but I can barely hear it. There's very little sound when I start braking to a stop. Thanks for your help, is this repairable by a novice like me or a pro needs to see it?

 

It's repairable with basic tools such as the appropriate sockets, a torque wrench, pry bars, depth gages for checking back lash, and some knowledge on how to set up a differential. However, it generally takes some guidance and the assistance of someone who has is good at setting up a differential; Otherwise you risk getting the shim stack incorrect, and if the gear faces do not engage properly the first time, and if you do not follow break-in procedures properly, you soon end up with gear whine, or worse.

...so it is probably worth paying someone replace the gears for you, if you really want to go that route.

 

I found a really good site on explaining the differential setup. I'll read into it to see if maybe I can try it myself----I won't jump into anything until I'm sure I know exactly what I'm getting myself into though. I know there's always the offchance that I fall into a bind when working on cars, so hopefully I can prepare myself enough to avoid that. It'll take me some time to muster up enough confidence to actually start but I'll keep you posted..and thanks again man!

Oh here's the link...if you have another site you think is better let me know.

http://www.pirate4x4.com/tech/billavista/Gear_Setup/

 

Wow,
Best gear site I've seen yet!