OzzyTom

Picked up my rebuilt diff yesterday.... with a new set of 3.73 gears.

Pricey rebuild, but I don't have too many options in my neck of the woods.
Local diff guru doesn't like aftermarket gear sets much, and prefers the GM originals... stronger, quieter and generally more reliable.
But as he could not source a new or decent 3.73 GM gear set.... aftermarket it was.

He ended up sourcing a gear set from Motive Gear, and installed a complete overhaul kit... new bearings, clutches, seals and an 800lb spring kit.

He has set the LSD quite tight to provide better lsd action.
He said there might be some shudder on tight turns the way it's setup, but will work better for launching with 450~500 horses.

I'm a bit concerned it might be too tight.
Is 800lb too much spring pressure on the clutches?
any comments from those that know?

I've got a few more items to order for the front and rear suspension also, then I'll install everything over our winter season in the next couple of months.
Hoping to get my vette really flying in time for the spring cruising season.

demonx

I can't answer your question, but i just got all excited when I saw your ride was a right hand drive!

Good to see another Aussie on here! Nice car!

wolfsdad

I cant answer your question either but being in the UK, like the RHD thing going on!

i would be mighty pi**ed if my newly rebuilt diff shuddered when turning! especially if it cost me a lot of heard earned green stuff.

There are many guys on here running hi performance setups and I will bet that none of them have a shuddering diff.

Shuddering means something isnt right.....most people get their diff's rebuilt when they start to judder

Im sure Gary (gtr1999) will chime in and put your mind at rest.

OzzyTom

G'day Allan (demonx)

I see you also have impeccable taste.....
a red chrome bumper convertible, and it's a small block manual too.

Yes, there's a few of us "right hooks" here on this site.
I'd have to say.... if you can't find an answer for a technical corvette query here, you won't find it anywhere else.

OzzyTom

G'day Jack from UK.
Yes... I'm hoping someone who has set up diffs in vettes with around 500 horses will give some feedback.

demonx

Ozzytom.

Mine is a lefty, it just took me by surprise as I was admiring your interior photo in your sig thinking what doesnt look right, then I noticed it was a right hook, then I noticed you were from oz!

http://www.nswcorvettes.com.au/forum/index.php

Have you ever tried these forums? I come to the forum we're on for general corvette stuff, but the nsw forum is great for anything Australia specific. Lots of great guys there too.

OzzyTom

I've scanned the ozzy vette forums, and spend a bit of time on one in particular, more so for social intercourse.
But there is NO comparison when it comes to tech info.

I only look at the C3 Tech/Performance threads on this site... and I can't read them all... there's just so many posts every day.
And the moderators do a top job at keeping everyone in line here.
This is a quality site.

GTR1999

Better use 2 bottles of additive and if he used a new eaton with fiber clutches,good luck. I suspect you may get some hammering if the lash is tight with those springs.

I never used Motive gears, if you have a picture of the RG look to see if there is a "G" within a shield logo stamped in them- (US - Tom's) He's correct about the GM gears they were the best.

OzzyTom

G'day Gary,
Diff builder also mentioned the possible need for extra additive IF there was an unacceptable issue with shudder after run-in period ... otherwise, a 600lb spring kit would need to be put in if it was bad.

Fibre clutches? The old ones that came out look like steel plates.
Looking at invoice, part number for installed clutch kit is PD-GM12PC18
Any idea what they are?

Gear set was P/N V885373 . I never saw it.
But I was told it was Made in USA.

regarding original diff gears.... I was told that diff gears used to be cut in 5 stages in the past. 5 incremental cuts to reduce wear on cutters, which also reduced the stress on the grain boundaries of the steel cell structure ??? The surface finish on the teeth of GM gears is much smoother than aftermarket gears available today.
Most diff gears made today are apparently cut in 2 stages... quite a coarse cutting process which inevitably results in more wear on cutters, and hence not as good a surface finish on the gear teeth which contributes to detrimental noise issues.

So Gary, what spring pressure kit do you normally install in the diffs you build up?

tracdogg2

Yes, 800 lbs is way too much for the clutches. I'm curious how he got them in there. With the backlash correctly set on the spider gears
(.001-.005) the stock 200 lb barely fit. 400 lb springs bend the retaining plates and won't fit without opening up the backlash. 800 lb require extra thick retaining plates and a lot of backlash.
Your LSD is no longer limited slip. It's locked. With the excessive backlass required to install the 800's you may start snapping off spider gear teeth. Clutch disc wear will be massive. On a properly set up posi 200 lbs is plenty for the street and none is required for performance driving or racing.
The only time 800's will fit is with carbon fiber discs (which don't work) and powder steel gears. Tom's can''t even give these gears away.
Mike

OzzyTom

Well Mike, I can't answer how he got them in.
Looking at the invoice, the 800 lb spring kit also included plates.

I was assured it would NOT be totally locked
That would introduce appalling understeer through the mountain twisties

I'll have another chat tomorrow with diff installer regarding this.

Concerns me if you consider 200 lb springs more than adequate?

tracdogg2

These are only pre-load springs. Once there is a load put on the gears the springs have no use. The clutches work by the effects of side loading. The more load applied (stepping on the gas), the greater the force is applied to the clutches. The springs are for light load applications. Best example is one tire on pavement, the other tire on ice. Under light throttle the tire on ice will spin since there is insufficent loading of the clutches. This is why they have pre-load springs. If you hit the gas there will be sufficent loading and the springs aren't needed. Launching a car at the strip the springs are useless. Carbon fiber clutches don't create any friction. They must have at least 400 lbs preload to attempt to work. They will still slip with 800 lb. springs. This is why they don't chatter.
Chatter is caused by too much pre-load pressure and/or insufficent friction modifier. The early slotted steel clutches were proned to chatter but could be controlled by adding more modifier. They were more prone to breakage which is the best reason to replace them with solids.
Excessive backlash in the spider gears will create a hammering effect. Every cracked posi case that Gary and I have posted shows the crack at the cross shaft hole. The shaft takes all the abuse from hammering. The crack always runs from the hole to the imperfection in the case where the sand castings were assembled. This is why all cases need to be polished and radiiused. Something that is rarely, if ever, done by the mass rebuilders. I can't stress enough how important setting the spider gear backlash is.
I know of only a hand full of people that can properly set up a posi. Get the backlash set correctly and toss the springs. Your rear end will thank you for it.
Mike

GTR1999

I never use springs in my posi work. Those fiber clutches are total crap but are very good if you're into trap shooting!

Mike is correct, IF you truely have 800 lb springs corners will be interesting. There's only 2 guys I know that polish and tune posi these days.

I think I should fly over there and hold a posi/Diff class!!!LOL

tracdogg2

LOL!
Alternate use #137 and counting.

Solid LT1

I only know of 200LB and 400LB pre-load springs for Eaton posi units. I used to rebuild without preload springs but now find that after new clutch pack plates start to break-in that posi pre-load goes off greatly and since I don't want to tear down my differential every 6 months to re-shim it, I go with 200lb pre-load springs for my Autocross Vette and 400lb springs for guy who like to Drag race.

OzzyTom

Hi Gary,
well I spoke to my diff builder....
It definitely has the 800 lb spring kit from GM fitted....
and the retaining plates are about 3 times the thickness of the stock plates to prevent deformation.

The clutch plates are definitely steel, and also sourced from GM.

As to how they fitted them: with difficulty I guess. Maybe they are shorter in overall length compared to the 200lb springs? I'm not sure.

You mention that you don't use springs in your buildups?
How does that work?
I thought the springs were there to initiate the LSD action, and more spring tension is required in applications where more torque is applied.
Why are 4 spring tensions available (200, 400, 600 and 800) if it's not necessary?
What am I missing?

Maybe you should consider a career as a tutor

OzzyTom

Well you guys have succeeded in getting me concerned.
I found another article too, which also makes me nervous.

http://www.duntovmotors.com/Corvette...ndHandling.pdf

In the article above they only use the higher spring pressures with the carbon fibre clutches.....

OK.... How much work is involved in replacing the spring kit?
Does it entail a complete strip down of the diff and a neccessity to relash and setup pattern spread? or is it reasonably straight forward.

I figure 400lb spring pre-load kit should provide less stress (on car and my mind)

gkull

I did in the stock unit and moved up to the Eaton. Then I had problems with the cross pin elongating the hole through the posi case. It was every year or two that the case would then crack and break. I finally said screw it and went with the $700ish (USD)Toms big pin polished posi unit.

now 4 years later and my 434 ci it's still going

It's the whole richmond gear bigger main caps ................... custom hardened yokes are the key to making any stock case last

GTR1999

Forget the fibers.
To replace the clutches you have to remove the case from the diff and basically start over with the setup and lash setting. Look up my thread on rebuilding these and you'll see what's involved.