Rear Differential Fluid

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Nov 24, 2005 | 05:09 PM

Maine Vette

Thread Starter

Drifting

Joined: Aug 2005

Posts: 1,289

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From: East Waterboro Maine

Rear Differential Fluid

First off, happy turkey day everyone!

I went out to refill my rear differential today and noticed that the red tag that goes on the fill plug says "Use limited slip differential fluid only". What's up with that, I thought the early 70's cars came with posi rear ends. I know some cars came with tags that said "use posi fluid only"
Do I still (or should I) add the posi addative I bought? My car is a 71 coupe with the stock 3.08's in the rear.

Thanks

Nov 24, 2005 | 05:25 PM

turtlevette

Melting Slicks

Joined: Feb 2003

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St. Jude Donor '03,'11

Quote:

Originally Posted by Maine Vette

Do I still (or should I) add the posi addative I bought? My car is a 71 coupe with the stock 3.08's in the rear.

Thanks

limited slip and posi are 2 terms for the same thing. Its got clutches in it. The more additive you put in the more you make them slip and then it becomes less "limited slip" and more slip.

Nov 24, 2005 | 06:44 PM

Maine Vette

Thread Starter

Drifting

Joined: Aug 2005

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From: East Waterboro Maine

But doesn't the posi addative make the differential fluid more slippery to prevent the plates from binding and chattering? If so, wouldn't the addative make the posi rear end act more like a limitied slip rear end?

Nov 24, 2005 | 07:03 PM

marshrat99

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From: Dayton Oh

I use 80W gear oil in my 79 and add the specified amount of GM additive. If you don't use the additive then you'll hear a grinding or whirling sound when you make turns.

Nov 24, 2005 | 07:08 PM

turtlevette

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St. Jude Donor '03,'11

Quote:

Originally Posted by marshrat99

I use 80W gear oil in my 79 and add the specified amount of GM additive. If you don't use the additive then you'll hear a grinding or whirling sound when you make turns.

it ain't grinding its chattering or popping because the clutch plates are actually doing their job. Adding a bunch of slippery additive causes the clutches to slip easier and thus to be less effective, but the lack of noise makes the show and shine guys happy.

Nov 25, 2005 | 10:38 AM

GTR1999

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From: Connecticut, USA

Use the additive it will get in between the clutches and cause the slip needed for cornering.
The stock setup uses4 400 lb springs to keep tension on the clutches all the time. Great for straight line but not too good if you just want to drive your car on the street without the hammering too much tension will cause. If you have a 70-79 they use the snowflake desing whihc was used to help get additive in between the clutches. Problem is they are weaker and will break under load,now the amount of load required to break them I don't know. I've found them broken on stock mid 70's vettes with little hp ot torque.
If you find the additive doesn't help and you want to correctly fix the hammering then you'll have to remove the diff and rebuild the posi to eliminate the problem. A lot of work and money so you have to decide just how much it's worth to you. The way I set them up has eliminated the hammering in the all the jobs I built
good luck,
Gary

Nov 25, 2005 | 01:17 PM

GTR1999

Tech Contributor

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From: Connecticut, USA

This picture may help you understand the clutch types better. This is a pic of a posi I just took apart. It has what is left of the original snowflake clutches in it. The clutch on the right is one of the few that survived, the rest came out in pieces like the one on the left. Also look at the pinion washers to the right,one is missing a good piece of it.
Gary