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How much abuse can a GM 7.5" Diff take?

  
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How much abuse can a GM 7.5" Diff take?

 
TheSilverBuick TheSilverBuick
Guru | Posts: 1227 | Joined: 02/06
Posted: 11/05/07
12:42 PM

I'm going to have to change it out soon I know, but as the title says how much abuse can one take?

It's a '77 Skylark with 14" rims and 235/70R14 tires, a pegleg 3.23 gears what would be considered safe behavior versus unsafe behavior for the Differential?

I am assuming from a standing start the street tires would break loose before the rear end, is that a safe assumption?

I am also assuming a hard down shift (manual transmission and a Buick 455's torque) at speed will dynamite something back there, safe bet there too?

Any idea's what the exact weak link is? Will the pinion shear? The splines of the axles shafts? The ring or spider gears?  Anyone want to start a betting pool?

The car will never see Drag radials or slicks so will a 8.5" Ten Bolt be sufficient?

Thoughts and ideas are appreciated.  
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The Silver Buick- '77 Skylark coupe w/ a Fuel Injected Buick 455&TKO-600, '72 Centurion Conv't - 455w/TH400, '67 T-bird 4Dr (suicide) w/428&C6. '69 Firebird replaced!

http://www.bangshift.com/forum/index.php?topic=6189.0

CSIROC CSIROC
Guru | Posts: 793 | Joined: 11/05
Posted: 11/05/07
03:23 PM

I'd say if you stomped on it from a stand still...the tires would be first to go.  If you stomped on it while rolling at speed and downshifted...you may be right with the breakage.  The weak link, from what I've heard, are the axle splines...though the pinion is kinda puny.  I believe the highest you can get are 28 spline axles.

I'm planning to put an 8.5" behind my 425 once its in the Delta 88...so I certainly hope its sufficient.  
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mistermisfit-#002 mistermisfit-#002
User | Posts: 95 | Joined: 01/07
Posted: 11/05/07
04:43 PM

from a standing start yes tire would go first, but continued abuse like that, who know exactely, but not very long.

the hard down shift, possibly. why destroy it untill you have the replacement ready to go.

as far as weak links, there are the c clips for the axels, the axels, pinion and ring gears. basically the entire unit is crap for performance.

i got an 8.5 10 bolt out of a GNX for my el camino, full spool, 33 spline axels, c clip eliminators. a little pricey ( i paid 2700 for the entire package drum to drum) but it included a 3.5" aluminum driveshaft with 1830 u joints and spli yoke. can you say bullit proof?

i would recommend a 8.5 10 bolt, c clip eliminators and a posi of some sort say an auburn.

hope this helps  

TheSilverBuick TheSilverBuick
Guru | Posts: 1227 | Joined: 02/06
Posted: 11/05/07
06:16 PM

Sounds good.  And I definately don't plan on anything destructive until I have something lined up to replace it, but you never know Crazy I live in a small town and have two other cars so if it happens to go I shouldn't be stranded far or with out a ride. I'm told the Rear Axles are the same for X-bodies from 72-79 so I should have some good options for an 8.5" 10 bolt.

Besides breaking things when you anticipate it is always fun Laugh  
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The Silver Buick- '77 Skylark coupe w/ a Fuel Injected Buick 455&TKO-600, '72 Centurion Conv't - 455w/TH400, '67 T-bird 4Dr (suicide) w/428&C6. '69 Firebird replaced!

http://www.bangshift.com/forum/index.php?topic=6189.0

monte85 monte85
User | Posts: 97 | Joined: 10/06
Posted: 11/07/07
11:41 AM

My s10 has a 7.5 inch rear axle and a bearing has been wineing in it for the last 40,000 miles and it still hasn't locked up (when i bought it for the large sum of $100 the pinion seal was bad and it didn't have a drop of fluid in it).  All I did was put a new seal and fluid in it and its still going, even with its massive 2.5L engine that has tens of horsepower!!!  And maybe over 100 ft-lbs of torque!!!!  OH YEAH!!!   Smile  

smallengineguy smallengineguy
New User | Posts: 9 | Joined: 12/07
Posted: 12/08/07
03:35 PM

I just picked up an 8.5 out of a Buick GN to go under my 78 Regal w/a 406 Chevy in it. The 7.5 did what I wanted it to do for me, I am surprised how much it can take!  

chvy2mch chvy2mch
New User | Posts: 24 | Joined: 01/08
Posted: 01/24/08
09:14 PM

why try to kill yourself by breaking the rear u never know the drive shaft could come through the car the 8.5 10 bolt will work fine they make a lot of stuff for them if you decide to go drag racing take time to weld the axle tubes all the way around since g.m just pressed them in i have seen these rearends behind low nine second cars and hold up Smile  

55Guy 55Guy
Enthusiast | Posts: 444 | Joined: 08/07
Posted: 01/25/08
07:16 AM

That rear won't take anything for abuse.

My brain's a little fuzzy this morning, but I think your '77 is still an A-body car, '78 was when they put those cars on the G-body platform. If this is the case, then finding a stronger replacement won't be too hard. You could get a 8.5" 10-bolt or even a 12-bolt from an early 70s A-body that'd bolt right up under your Skylark if it's an A-body.  

trpositd trpositd
User | Posts: 70 | Joined: 11/05
Posted: 01/25/08
01:51 PM

I have a 77 Nova with a 7.5 rear.  I added a PowerTrax locker, 3.73 gears, and one of those reinforced support covers about 5 years ago.  My car runs low 13s (hopefully 12s soon) and I've never had any trouble.  I use a line lock for burnouts and launch at 2000 rpm on the foot brake.  I run slapper bars and BFG Drag Radials.  The spider gears are the weakest part - and the PowerTrax replaces them.  I've read (either in this mag or a sister mag; I still have the article somewhere) that this rear can take up to 400 hp at the wheels with these types of mods.  I still run the c-clips, which is not ideal, but I'm not in any danger of blowing the rear at this power level.

The Skylark, Nova, and 70-81 Camaro are basically interchangeable underneath, so upgrading to the 8.5 should be easy.  I've seen a lot of properly prepared 8.5 rears lift the front wheels and run 10s with ease.  Plus, the 10 bolt supposedly takes less power to turn than a 12 bolt or 9 inch Ford.  Whether you would actually notice any difference, I don't know.  

TheSilverBuick TheSilverBuick
Guru | Posts: 1227 | Joined: 02/06
Posted: 01/26/08
08:36 PM

Icon Quote55_Hardtop_Guy:
That rear won't take anything for abuse.

My brain's a little fuzzy this morning, but I think your '77 is still an A-body car, '78 was when they put those cars on the G-body platform. If this is the case, then finding a stronger replacement won't be too hard. You could get a 8.5" 10-bolt or even a 12-bolt from an early 70s A-body that'd bolt right up under your Skylark if it's an A-body.


It's definately an X-body.  Skylarks were never a G-body.  My best bet is finding an 8.5" from a similar year Nova, or bigger engined '76-'79 X-body.  
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The Silver Buick- '77 Skylark coupe w/ a Fuel Injected Buick 455&TKO-600, '72 Centurion Conv't - 455w/TH400, '67 T-bird 4Dr (suicide) w/428&C6. '69 Firebird replaced!

http://www.bangshift.com/forum/index.php?topic=6189.0

4spds4evr 4spds4evr
New User | Posts: 35 | Joined: 01/07
Posted: 03/03/08
08:37 PM

I've been running a 3.08 limited slip 7.5 behind a 4-speed G-body '85 Cutlass with a 370-ish hp 355, and it's taken a pretty severe amount of abuse in the past two years including clutch sidesteps...I've even grenaded a Saginaw before the rear went. I'd say with 3.23s, it'd probably be okay for a starting rear-end, but I wouldn't ask it to run slicks or really anything severely sticky for tires. I did, however, buy an 8.5" before I pressed my luck too far.

An 8.5" for your Nova should be relatively easy to find, and failing that, there are only about a million companies making X-body 9" rearends, and for about the same money for a good name-brand built 8.5", and they have warantees! Depending on what you plan to do with the car, that might be a better option.  

mattbln mattbln
New User | Posts: 1 | Joined: 03/08
Posted: 03/31/08
06:43 PM

1980 monte carlo  450 -500 hp 383ci 3600 stall,runs 11.30's .   factory 7.5 rearend richmond gears,mini spool   ,moeser axles. I have broken it 2-3 times , but made it through 9-10 races in 2007. it still in one piece.  

BPearce BPearce
New User | Posts: 1 | Joined: 03/08
Posted: 03/31/08
08:55 PM

1984 firebird 355 running 12.0 at 116mph the diff only has richmond 4.56 gears i leave on the foot brake at 3800 and i have only twisted one axle so far it has about30 passes on it  

TheSilverBuick TheSilverBuick
Guru | Posts: 1227 | Joined: 02/06
Posted: 04/01/08
01:40 PM

Well I've now made 3 quarter throttle 1/8th mile passes on it, chirping the tires in the first four gears with no noises or clunks coming from it.  I think it was Jeff Smith who told me that I should have no problem with an 8.5" rear with a few upgrades.  That way I can find one in a Nova of a similar body and the spring/shock mounts all line up.  Later this week I plan on installing a new throttle cable that will give me the other 3/4 of the throttle (basically the secondaries on the Q-jet barely move with the pedal to the floor) and see how it goes Grin  
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The Silver Buick- '77 Skylark coupe w/ a Fuel Injected Buick 455&TKO-600, '72 Centurion Conv't - 455w/TH400, '67 T-bird 4Dr (suicide) w/428&C6. '69 Firebird replaced!

http://www.bangshift.com/forum/index.php?topic=6189.0

jdh355 jdh355
New User | Posts: 1 | Joined: 04/08
Posted: 04/01/08
04:25 PM

yes the tires will break loose first but if it
dose hook up the axles may break in the splines.
i had a 79 malibu with the 7.5" in it and i broke
three sets of axles on the street with street tires
had a 355 with about 425 horse power.  

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