Diff decisions...

[mungus]

Tossing up what to do about my 9" diff.

2 centres came with the car, one LSD with the bigger 3.086" side bearings that's maybe worth doing up, and 3.00 open small bearing one that I'll just flick on eBay. My issue is whether the better centre is capable of coping and then what's cost effective.

Has the correct bolt on shroud / rubber damper at the front end, and an Auburn LSD running the standard 3.25:1 gearset. However the ring gear has some minor surface rust from PO storage and I'd rather run 3.5 with my .64 OD TKO anyway. So I figure the gear set is going...

Not a Nodular carrier, but that's Restomod anyway. At least it has the bigger side bearings. I could upgrade the pinion support to a Daytona but I gather the carrier and axles are the weak point anyway.

So what's the consensus. Is a 28 spline non N diff OK for say 380-400 hp & 480-500 ft/lbs with a manual gearbox. Street cruiser with 235 rear tyres? Can't promise I'll never light up the rear but mostly I'd be pretty easy on it. My feeling is that is probably is up to it, right or not?

And if as a platform, it is up to it, then is it cost effective to rebuild it with new gears etc or better off choosing a quality reconditioned LSD centre anyway? Going to 31 spline costs a lot but if need be Ill consider it. (assuming the carrier itself is good enough). No idea if the LSD itself is good as I never drove it nor swung a TQ wrench on it. However it wasn't rusty and the bearings and backlash were fine, so possibly it is... Obviously if it's bad then 31 spline really does becomes a consideration. as the only extra is for the axles.

I've never rebuilt a 9" although if I take my time, my workshop and I are probably up to it.

Do most here consider that job worth the trouble or just pay for the work?

So my options are;

Sell both and put the $$ into a recond period correct carrier with a 31 LSD.
Rebuild & re-gear mine with the 28 LSD. (Cheapest?)
Rebuild mine with 31 LSD & axles.

Views or experiences appreciated.

[hybrid]

235s aren't going to trouble an 8" diff let alone a 9" of any spec.
400HP also won't trouble a 28spline 9".
But it's more about how often you step off the clutch at 4000RPM than the power. That's what most often destroys a diff.
To rebuild a diff, you need to know how to set the gear lash properly. That is the only tricky part.

[Shaunp]

Many of the Australian Trans-am cars still run 28 spline 8" in the 5L class, these cars would have 400 hp no trouble as they turn them to 7500rpm they also run lockers, they very seldom fail. John English's blue 67 ran a 351 and it did 2 seasons on an 8" till it ran out of oil. I wouldnt worry about hurting one in a road car. What is the ratio set in you TKO, Id say a 3.5 is the tallest diff you want, you may even want a low one like 3.8

[boofhead]

.64 OD - you could go to 4.11:1 and still highway cruise at a reasonable RPM.

If your drag racing then you pick the gear based on power peaks and RPM level crossing the line.

[Shaunp]

.64 OD - you could go to 4.11:1 and still highway cruise at a reasonable RPM.

If your drag racing then you pick the gear based on power peaks and RPM level crossing the line.

Yep it needs to be 3.8-4.11 with a .64 OD

[mungus]

Thanks for the replies guys.

I found an axle makers torque calculation today, and that confirms my engines predicted TQ won't bust a 28 spline axle. Nice to have that theoretical confirmation too. (attached) Apparently the factory 28 spline half shafts are good for 4800 ft/lbs and my car works out to 4,520...

Not too worried about a home rebuild, and I'm using the spare open 3.00 for practice, setting lash etc. Have made a frame etc. I'm lucky in that I have the less common normal casing with the bigger bearings. So it will look factory but be a little beefier than usual. So I guess it's just a rebuild kit and gear set and I'm away. Bearings seem OK but no way I'm going through all this and refitting them.

Ratio wise I'm thinking about cruise RPM being just on the XR270HR cam, so about 1,800-2,000. My 390 with a mild roller will torque through the first 4 gears quick as, so I don't need too low a ratio to "haul arse".

When I ordered the TKO600 I had been told by the PO that one of my Diffs had a 3.9 in it. That's why I went for a .64 instead of a .82. Upon examination I found a I had a 3.00 and a 3.25 POSI. Bugger! I'm thinking somewhere around 3.5 to 3.9 at most. I want a nice relaxed cruise and she'll still launch well with all that TQ. I keep playing with the Tremec APP on my iPad, mulling over what gear set to get. Time will tell, I guess swapping gearsets later is not crazy expensive, just time.

[Shaunp]

So buy a true trak centre and some 3.9 gears not expensive, forget the clutch type LSD

[Shaunp]

https://www.summitracing.com/int/parts/ ... /overview/
https://www.summitracing.com/int/parts/ ... /overview/

https://www.summitracing.com/int/search ... ion%20Kits

[Shaunp]

Also handy to have a dial indicator torque wrench for setting pinion pre load, they are about $300 US that's what I use I have Snapon one, or you can use a fish spring scale and a foot long breaker bar. I break the crush sleeve with a rattle gun first, and once your into the sleeve you just pulse the rattle gun till you yo get the right pre load on the bearing, or you can use a solid sleeve and shims. Think this is the one I have

https://store.snapon.com/US-Reading-Sta ... 34618.aspx

Conversely Aussie diffs at the Gold coast will do the job including parts for about $1500.

[mungus]

I've got an inch lb wrench for firearms work. I'll have to check on the torque range though. Thanks.
My Auburn LSD is a cone type and is OK, so happy not to spend on a Tru Trac, even though I do likem!