Aluminum 8 3/4 case

[BugEyedValiant]

What does anyone know about this piece. I already have a good 8 3/4 in my car, but I know it will not last forever. Actually I am at the limit for it(low 10s in a 3400# small block street car with a tranny brake) and want to know what its limit is. I e mailed Mancini about it and they said that it was available and costs about $750. I saw one at the PRI show and diddnt get a really good chance to look at it.

[djswwg]

I'd be very interested in an aluminum 8 3/4 diff case. Who makes this? I've never heard of one. Thanks, DJS.

[BugEyedValiant]

Mopar Performance #P4876445 uses 742 gears

[dschumers]

I think I read in one of the Mope rags that this thing is 14# lighter than a cast iron case. Is that right? That's some serious weight loss.

[Dave Hench]

Richard Ehrenberg (Mopar Action) memtioned in an article a alum. differential that he put under the "Green Brick". You might write him, and ask about yours. OBTW the one Rick talked about was a Mickey Thompson brand.
Hang on to yours, it is a piece of history.
Dave

[BugEyedValiant]

Oh Yeah.........It is a BRAND NEW item in the new 2001 Mopar Performance Catalog.

[Marc]

I'd be of the opinion that the aluminum carrier would be LESS stable than iron, if it's an issue.... I had sooooooo much trouble with ford 9" rears with every pumpkin imaginable that I never want to hear about them again.....
For strength what about billet caps and load bolts?

[djswwg]

Thanks for the reply Bugeyevaliant; why in hells bells would they use a 742 style housing? 489 is stronger and easier to get gears for. What gives?

[BugEyedValiant]

Ok here is what the book says:
This rugged aluminum center section is designed to hold up under high torque and save weight at the same time. Uses 1 3/4 large stem pinion and standard components.
*uses 742 style large stem ring and pinions *fits 8 3/4 banjo housings *weight reduction of 44%-approxomatly 11 lbs less than production cast iron unit *features 6061 billet aluminum caps * stiffer than cast iron housing, decreases pinion deflection by 10% * uses 742 style bearings and shims, sure grips and service parts.

this is all the info that I have on this part, as copied from the new MP 2001 catalog....and oh by the way, it takes a totally different pinion snubber too. Hope this helps

[72Challenger]

I think the answer to the "why a 742?" is because of the sure-grip. After spending all that money on a rear, wouldn't you want a rebuildable unit? Yeah, the cone styles can be rebuilt, MAYBE, but the strength difference between the 742 and the 489 is minimal.

[ChristianCuda]

72 Challenger the suregrips from the 742 to 489 are interchangable. You can swap all the differentials from each year across the board. now some of the 742's used different bearings but they can still be swapped.

Christian

Also The 742 is better (not necessarily stonger though) than the 489 with a crush sleeve. If the 489 would not have had the tapered pinion it would have been far stronger. Also I have seen 741's holding up under alot of drag cars. It wouldn't matter what Case it is It will hold up most likely unless you have 1000 HP but then you had better have a Dana. And that throws out the whole equation out the window.

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Http://ThunderRacing.Knudstrup.net/

68 'Cuda 383 Best of 8.363@83.78 in the 1/8mile which is a mid 12 with a 1.89 60ft
Formula S recreation not original but making like they should have at the factory) Working on adding EFI
69 'Cuda Race Parts Car
76 Duster 273 recent transplant
95 Neon 2.0 SOHC best of 16.96
84 Dodge Ram D50 transplanting 360

[Beeracuda]

The 742 is also the only one you can buy Pro Gears for. And you can only buy those in 4.86.

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Monty Brown
74 Barracuda
446 / PG
10.14 /134

[Jack Z]

A year or so ago, the people I checked with about racing gearsets for the Mopar 8 3/4" axle (Tri-City Competition, Tom's Differentials), recommended using the 742 case due to the use of shims on the 742 as opposed to a crush sleeve on the 489. And while it is true that the pinion shaft on a 489 type pinion is beefier than the 742 type, that's not what usually breaks so it's of no benefit. I can say based on first hand experience as well as comments from a number of other racers, that gear tooth breakage is almost always the problem. This gear tooth breakage is caused by the carrier flexing which changes the tooth alignment which overstresses the teeth. If this new aluminum housing is stiffer, that should be a big benefit in maintaining alignment.

By the way, if anyone is looking for racing type gear sets for the 8 3/4", Tri-City and Tom's both have proprietary thermal treating and coating processes that they perform on any ratio standard gear set that toughens them up a bit and makes them almost lke "Pro" gears. Although they still won't be as strong as a Dana 60 gear set.

[cdignition]

Just to keep everyone informed correctly..."pro" gears are slightly softer than regular street gears. and OEM gears are the hardest of all, usually...OEM rockwell C hardness is around 59-60....Street gears are around 55-58...."pro" gears are around 52-54...This is why you dont want to run pro gears on the street..wont hold up too long...

[LAR]

Right on! I had a friend that ran a 9 second 440 duster with a Dana. He had it set up with regular gears so that he could run it to cruises and on the street. First time out, third run, it wiped out the ring gear and locked the rear up. He learned right there about the hardness of the gears. PRO gears are softer (less brittle)so they can take the shock. Street gears are much harder (more brittle). Pro gears just wont last on the street too long. (How much street driving can you do in high 9 second Duster anyways!)