True Power

Ok, I'm just starting to dip into the wonderful world of rear ends. I have a couple of questions...
What is the difference (in performance) between a 7.5" and 8.5" 10bolt rear end. What makes one better than the other?
Where would you find a 12bolt rear? Is this better than a 10bolt?
That's basically it. I have a 1985 Camaro IROC-Z with a 3.23 posi rear, so I am "fairly" sure I have the 7.5" 10bolt rear. I was planning on installing some 4.10s, but I am wondering if it would be smarter to install a 8.5" or 12bolt first. Thanks in advance

ede

gm sort of screwed all the hot rodders when they put the weak 7 1/2 10 bolt in our cars. it's weak and it has a torque arm. only thing you can use from gm is the 10 bolt and the 9 bolt. the 9 bolt is lot stronger, but you can't buy good gears fro it. there was a few f bodys with dana 44's but you might as well give up on finding one of them. strange makes a raftermarket 12 bolt and moser makes a 9"ford rear that will fit our cars with the torque arm, but you'll need a lot of money to get one.

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-=ICON MOTORSPORTS=-
1st & 3rd

five7kid

iTrader: (14)

Although not as desireable when compared to 60's and 70's muscle, you still have to be making some serious power before the 7.5 10-bolts' absolute capabilities are exceeded. See the ultimate 10-bolt tech article.

If you've got a manual tranny maxed-out big block that you race regularly or a laughing gas big bore stroker small block, you might consider a specialty rear like ede says (see latest Chevy High Performance mag for their 3rd gen article - includes more sources for such rears). Otherwise, optimize your 7.5" 10-bolt when you put your gears in.

Don't overlook suspension mods that eliminate wheel hop.

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82 Berlinetta, orig V-6 car, now w/86 LG4/TH700R. 2.73 unlimited slip. Cat-back from '91 GTA, Accel HEI SuperCoil. AMSOIL syn lubes bumper-to-bumper. Daily driver, work-in-progress (LB9 w/ZZ3 cam, TBD heads, exhaust, paint, etc.).
57 Bel Air, my 1st car. 0.030 over 396, Weiand Action+, Edelbrock 1901 Q-Jet, Jacobs Omnipack, 1-3/4" headers, TH400 w/TCI Sat Night Special conv & shift kit, 3.08 10-bolt, AMSOIL syn lubes bumper-to-bumper. Best 15.1 @ 5800' Bandimere. Daily driver while Camaro was being put together.

AlkyIROC

The size of the ring gear is what determines the strength of each diff. Our little 7.5/7.625 is very small. A 8.2 or 8.5 10 bolt is better. A 12 bolt is 8.75 and the Ford 9" is 9". Getting even bigger is a Dana 60 with a 9.75. Now using all those sizes as a comparison we'll look at ratios. A standard ratio available to just about everyone is 4.10:1. This means there are usually 41 teeth on the ring gear and 10 teeth on the pinion. To squeeze 41 teeth on a 7.5, the teeth need to be very small when compared to 41 teeth on a 9". These smaller teeth can't hold as much power as the larger ones. It's usually the small pinion gear teeth that break first.

There are other differences in all the diffs also. The 9" uses the same carrier for all it's gear ratio so the pinion location remains the same. GM diffs use a 2, 3 or 4 series carrier when going to lower and lower gear ratios. This allows the pinion to contact the gears the same without changing the thickness of the ring gear like ford does.

Third gen cars, using stock style suspension (no ladder bars or 4 link systems), are stuck with a factory 7.5/7.625 10 bolt or 7.75 9 bolt. If you can find one, a 8.5 Dana44. Aftermarket allows a 12 bolt or ford 9". You won't be able to adapt any other diff to fit because of how the torque arm is attached to the front of the diff.

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Stephen's racing page

87 IROC-Z Pro ET Bracket Race Car
383 stroker (carbed) with double hump cast iron heads and pump gas
454 Big Block buildup now in progress for the 2001 racing season

Best results before the 383 blew up
Best ET on a time slip: 11.857 altitude corrected to 11.163
Best MPH on a time slip: 117.87 altitude corrected to 126.10
Best 60 foot: 1.662

Racing at 3500 feet elevation but most race days it's over 5000 feet density altitude!
Member of the Calgary Drag Racing Association

87 IROC bracket car
91 454SS daily driver
95 Homebuilt Harley