rosros

I took a couple days off this week to concentrate on the vette. Got a couple of projects going on simultaneously.

I had the differential fluid change because of grinding/chatter noise at start up and goes away after driving for awhile. Shop used Kendall Syn with one bottle of posi additive. The grinding noise was still there so I had the shop add another bottle. The grinding noise is barely noticable for the first mile and then goes away. Should I have them add another bottle?

I understand most people add one bottle extra to most syn differential lubes to quiet down the chatter.

Should I look into amsoil/addiitive or go to GM oe gear oil. I spoke to the service mgr at Chevy and he said their latest gear oil recommendation solves the chatter even on older vetts.

I will be going back to the shop tomorrow for them to redo their Map sensor and would like direction on what to do with the differential fluid while the car is there.

aklim

Some synthetic fluids already have the additive so it isn't needed. 1 bottle usually is enough. 3 would be a problem.

Rob31

Is that Noise on acceleration or decel . Does it go away when you coast in Nuetral ?

87 vette 81 big girl

1 bottle of GM Limited Slip Additive is more than enough in 99 % of street driven vehicles.

If chatter is still present while turning corners at speeds from 1 mph to 20 mph,
The differential fluid is likely contaminated with foreign matter &/ OR very dirty gear oil yet.
Re changing the gear oil would be the next move I make.
Use a quality modern synthetic gear oil like royal Purple 75-90w, Redline Shockproof gear oil, Casterole Type "C", or Mobil 1 synthetic gear oil.
If the chatter still persists afterwards,
New posi traction clutches are required in your C4 Dana 44 or Dana 36 Rear differential.
Entire Rear diff assembly & batwing must be removed from Your C4 Corvette to make necessary repairs.
Special Kent Moore service tools or similar are required when replacing C4 Dana posi- Trac lock clutch packs and rebuilding the rest of the rear differential.
If you have ZERO experience rebuilding any hypoid ring & pinion differential,
best to leave it to an experienced veteran professional mechanic.

Brian R.

rosros

It happens when you first start the car or if the car sat for a short time. the chatter is present during accel,decel, coasting at low speeds. It goes away in a short time. The chattering just recently started.

JrRifleCoach

Around here the rule of thumb is two bottles LSD lube to the diff service.

87 vette 81 big girl

2 bottles of GM Limited Slip additive may cure all.
Have to try it if you want to.

I myself never used anymore than 1- bottle of GM LSD additive in my own C4 or other cars.

The brand or type of gear oil may be the determining factor in results also.
I use only Royal Purple gear oil.
Its very slippery to begin with & has Limited slip additive pre mixed in.

BR

PAINTER MAN

Flame Red

I used two of them little bottles.

rosros

Got two bottle in now and sounds a lot better and quiets down quicker. Sometimes there is no chatter. Chevy says it may take a few days for the noise to go away. I do not drive much, but if still continues, will do another drain and refill, this time with the GM stuff.

aklim

That is crazy. It shouldn't take that long for the fluids to mix up in the differential.

grandspt

I had a noise going around corners, I changed the fluid with GM stuff and one bottle of the additive, my car still made the noise.
I tried the old ATF fluid in the rear end trick and drove in a figure eight in one direction then swapped to the opposite direction about 3 sets of opposite directions. I then drove straight for a mile. Drained the fluid and then refilled it again but this time with Mobil 1 synthetic and one bottle of additive.
It has been four years and no noises from the rear differential.
My car sits a lot in the winter time and the clutch packs stick.
The ATF fluid basically frees them up. It was absolutely worth the effort because it saved me from pulling the rear end out of the car!!!

383vett

Unless the owner does a few figure eights after adding the additive, it might take a few days of normal driving for the new stuff to get between the clutch packs.

aklim

I forgot to put the stuff on and noticed it. Dumped the additive in and the strange sound went away. Thought it mixed up pretty quickly but I could be wrong.

RedLS1GTO

While true that I wouldn't recommend the job to someone who doesn't have some experience, you don't need to go spend $$$ on specialty tools for a Dana rebuild.

You can make the tool needed for next to nothing. Some threaded rod, washers, and nuts and it works like a champ.

C66 Racing

There is a downside to adding too much friction modifier. The diff clutch plates are what give the diff its limited slip ability. They are designed to grip to each other and pass torque to the tire with grip when one tire looses grip. When your fluid properties are not correct (too slippery), these clutch plates lose their friction with each other, thus the diff becomes more like an open diff. Don't know the spec for the C4 diff, but on my 02 Z06 the limited slip diff is supposed to pass 75 lb-ft of torque to the loaded tire. I check this regularly with a torque wrenc on my track day car - if I'm not getting max torque to what for me would be the outside loaded tire coming out of a tight turn then I'm losing performance compared to my competitors.

While that might not be as much as factor on the street, you should at least recognize that adding too much modifier, while it won't hurt the diff, does cost you performance. The goal is to just have enough additive to prevent chatter and no more.

I use AMSOIL Severe Gear Extreme Pressure Synthetic 75w90 (Product Code SVGQT) with no extra modifier and my diff doesn't chatter.

Crepitus

Phoenix Performance/Carbotech Engineering #66 SCCA T1
2004 SCCA MidWest Division T1 Champion

OT but that is so cool!

C66 Racing

Thanks - that was my last full season of competing. I'd like to be able to say 2011 SCCA Northeast Division T1 Champion - but really don't have the time to compete full time right now, and my oldest (of two) daughters just started college this year taking a significant chunk of my personal racing budget with her. I do race enough to keep my license active and do plan to add to that 04 title at some point!.