I've tried to search the forum as much as I can so my apologies if this is a duplication.

Nearly done with my LS swap and discovered the other day that what I thought was a posi differential in my '69 is in fact an open differential. Ugh.

Interestingly it still has the red "limited slip additive" tag, but the code for it is an AK (3.36 open diff) and jacking it up and turning the tires confirms that it is indeed an open diff.

So this leads me down the fun path of what to do. The LS3 will be ~550hp flywheel with a 6 speed manual, so probably just shy of 500rwhp. It will be a track day/road course car but with the occasional autocross and drag strip run. I do not intend on ever running it with slicks at the drag strip, but it does have modern 285 summer tires.

So, is it just as simple as swapping in a new Eaton 19670 limited slip carrier? Will that be strong enough?

If it is not strong enough, what do I need to do? I'm very hesitant to let this become a "while you're in there" project where cost spirals out of control.

Thanks for your advice!

 

It's the shock loading of drag racing that blows up rear ends so if you're just going to road race it I would think it'll do the job as is.

 

Quote: turning the tires confirms its not a Posi. I find that hard to believe, but maybe. So, you have both tires off the ground. Driveshaft locked in Park. You try to rotate one tire and the other side does what?

A. Spins the opposite direction?
B. Spins same direction?
C. Or does not turn at all?

 

I found it hard believe too! But the answer is A. It's definitely an open diff. The spinning the opposite direction and the stamped code on the bottom confirm this.

 

Cheapest way is to find a good used diff for a few hundred bucks. They're out there if you look.

No stronger of course....but might give you something to start with.


JIM

 

^^^ and you will have a spare as I did.

 

I have most of this (original GM/made in the USA) stuff still available, if you or anyone else is interested send a PM

https://www.corvetteforum.com/forums...iff-parts.html

 

After doing a Google search on "How To Tell If You Have a Posi" it seems you can believe about 49% of the methods described (outside of removing the cover). And as we all know the internet never lies or misleads people and is always 100% gospel truth, right?

Most of "the net" questions involved how to find a posi at a junkyard by just jacking up the the rear axle. Keep in mind the axle tags or code numbers could be meaningless. As Forrest Gump said about a box of differentials, you never know what . . . . .

So, it seems that "the net' agrees by 51% that:
with both tires off the grd, spin one side, the other should rotate the opposite if a Posi.
(or very difficult to rotate- clutch packs)

 

All depends on how heavy your right foot will be. It is possible to use what you have WITHOUT abusive driving but as I always ask why build a 500hp car?

The posi you mention is the aftermarket Eaton available from any Eaton dealer so you can shop around if that is what you want. I would guess they are in the $550 price range. It is my understanding they are cast in china and assembled in mx, no surprise there with todays market. The cases are ok, they are not as good as the cases Eaton made for GM but not too bad. I don't use the spiders or fiber coated clutches but many will. Will it bolt in, yes. I recommend you fully polish any posi you choose. You should clean the flange and stone it, same for the RG, countersink the flange holes and use ARP RG bolts with #271 Loctite. Make sure the bolts fully seat before you final install them. Will a 10 bolt posi hold up to a lot abuse with 500+ hp? Again that depends on use, traction, transmission type- I would never rate a stock posi for that power level. I do know plenty of guys that do push them though - with some mods to them in the 500-600hp range.

What I did with my son's 500hp 75 was build a 1480 12 bolt using Tom's parts. This kits comes with Tom's 3.5" - 134" wall 1/2 shafts, and 31 spline outer axles. I used Tom's offset arms with upper link kit. In addition you can weld in frame gussets, reinforce the front diff tongue bracket, move to solid mounts, get set of adjustable shocks, and stiff rear spring. Our car runs in the upper 10's but the days of going to the track are very limited with life's other issues going on now. I did some other things inside that diff too that typically aren't done and so far it has held together with a lot of abuse. I hate the term bulletproof since anything will have a breaking point.

If you can put together a car and build an engine you can build a diff like that yourself. You might have to sub out some machine work but it's something you can do with coaching and patience. I have taught plenty of guys at my Carlisle seminars over the years. One thing, if you think you want a 12 bolt then you better call Tom soon before he closes, I don't know what his vette inventory is but most likely not like it once was. There are ways to cut corners on this type of work by not using the best parts or procedures, I would NOT recommend you do that if you choose this path.

Do your homework and see what your budget allows. You will save a lot by doing it yourself. Keep in mind the weakest link will always break. Stock 17 spline outer axles break at the base of the threads or twist splines on the later 70's axles, we broke solid spicers on one pass - both inner and outer, bent 1350 axles, cracked trans cases, etc. That's why I like my old 300hp 72, I start it up and go out cruising all day, I don't have to race or prove anything to other drivers and my rebuilt drivetrain is fine.

Good luck with your build, YOU can do it! post some pictures of what you end up doing I would like to see them.

 

Awesome feedback, thank you Gary.

I certainly don't intend on hammering on it at the strip and really shocking the drive line, but sounds like a straight up stock eaton unit would be pushing my luck.

Guess I need to give Tom's a call.