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Old 02-13-2005, 01:11 PM
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F-body differentials, 9 bolt Borg Warner vs. GM 10 bolt

I have an 85 Trans Am in which I'm stuck with a Borg Warner 9 bolt rear diff with 3.27 gears. I upgraded everything on the suspension, including the torque arm, with components designed to fit the stock F-body rear end. I want to keep these suspension parts the way they are, so I'm forced to choose between a 9-bolt and a 10-bolt which both came OEM for this car. I am now planning on building a monster motor for the car, and I will need to upgrade the rear diff for sure. I have some questions about doing this, I guess I'll just list them off and let you answer them as you see fit.

Are all 9-bolts posi's from the factory, or are some of them one-tire-fire setups?

Is there any parts for upgrading the 9 bolt's strength, or would I be better off swapping to a 10 bolt, where parts are plentiful?

Would any of the suspension mounts be different if I swapped out to a 10 bolt?

Will the 9-bolt or the 10-bolt provide enough strength to handle over 500 HP and 500 ft-lbs if the unit is properly built up?

What is the cost difference between keeping the 9 bolt and building it and swapping out to a 10 bolt and building that?

If I am forced to consider a beefy diff like a Ford 9" or a 12 bolt, how much extra weight would this add, how much extra HP loss would I have, and how much extra work is it to change all the suspension components to fit the larger housing?

Finally, where is the best place to buy parts for a rear diff?

I realize this is a lot of questions, your input would be greatly appreciated.

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Old 02-13-2005, 01:35 PM
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Re: F-body differentials, 9 bolt Borg Warner vs. GM 10 bolt

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Originally posted by Mad Maggot


Will the 9-bolt or the 10-bolt provide enough strength to handle over 500 HP and 500 ft-lbs if the unit is properly built up?

Maybe for awhile on street tires! Good Luck on slicks! I would look for an early 90's rear end out of the salvage yard. They have the 28 spline axles and a slightly stronger diff. unit. I have had good luck swapping a zexel-torsen posi from a 2001 SS camaro into one of these rearends, but my sons Formula is honestly closer to 300hp than 500hp!
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Old 02-14-2005, 07:23 AM
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You can have a 9"/12 bolt/Dana 44 fitted with the torque arm mount etc. for a direct bolt-in replacement. It's about $100 extra for the mount. With 500 ft. lb. I don't think I would gamble on a 9 or 10 bolt (>7.625 gear dia.). The 9 bolt in my old camaro broke three times running 14.50's with drag radials.
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Old 02-15-2005, 03:40 AM
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For the 500 horsepower level you would be better off losing all thought of BOTH the 9-bolt AND the 7.625.

Neither of these axle will last very long at that power level.

Like said before in this thread, start with an 8.5 at least, and loook into the 9-inch or 12-bolt from someone like Moser/Currie/ Strange and the like.
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Old 02-15-2005, 07:30 PM
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thanks for the info so far, too bad none of it is really good news...

I would be looking at a prohibitively expensive bill for changing to a Ford 9" or a 12 bolt, it costs around $2,000 US for a decent one, plus the expense of making everything fit. I live in Canada, where the government chokes the life out of all of us with enormous taxes. With the exchange rate conversion, the G.S.T. (tax), and huge border crossing and shipping charges, I wouldn't be surprised if I have to shell out 4 grand CDN for this kind of a setup, and that's AFTER taxes! Not possible, folks... I already have a 9 bolt in my car and I can find a 10 bolt in a junkyard cheap, so I'm pretty much stuck with these options. I'm guessing it would cost about $1,000 or so for a new posi unit and some gears, and maybe down the line I will replace the axles. If I were to find a used 9" or 12 bolt, I would start paying upwards of $600 for the unit from the junkyard, plus I would have to rebuild it with all new (and expensive) components as well. Then I will have my hands full changing all the suspension components, which will also cost more for mounts, etc.

In retrospect, I already have a 400 horse, 400 torque motor in the car (that I am selling to fund the bigger motor which I already have parts for), and the 9 bolt I currently have is bone stock and it has lived for over a year with this motor with me stepping on it. Keep in mind I will only use street tires (BFGoodrich Radial T/A 265/50R16), and I will not neutral bomb or tranny brake this at all. It is purely for street use, and if I take it to the track I will be racing it the way it is driven to the track.

People have said the Australian 9 bolt is actually a pretty strong rear end for it's size, there were rumors that it was better than a Dana 44. Check out this site: www.9bolt.com and give me your feedback on the info that's there. Also check out the Hot Rod Magazine article there as well. The only drawback from building up the 9 bolt further is the lack of aftermarket parts for it, the guy at this site seems to be the only supplier of parts for it I have found so far.

Auburn Gear sells a posi unit for the 7.625" 10-bolt in it's Pro-Series that is designed for high performance and built with ultra strong parts. Flatlander Racing sells this unit for $440, which leaves me enough money for gears and axles as well. Their High Performance Series are for "mild" street engines... but they call 400HP a mild engine, so I'm led to believe that their Pro Series will be strong enough to handle my output. There are a lot of other aftermarket parts for these rear ends as well that claim to be much stronger than stock. I just want to know if it is a bunch of bull, and if these parts really are as strong as they say they are.

If I am ABSOLUTELY guaranteed that I will eventually blow up the 9 bolt or 10 bolt rear end, regardless of buildups, then I will have to scrap all plans for this motor upgrade and start selling stuff off, either that or I will have to wait until I can bust my can enough for the extra cash for a 9" or 12 bolt.
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Old 02-15-2005, 08:16 PM
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Again, the early 90's 7.65" is probably the best bet (w/28 spline axles) if you must use a factory rearend. If you buy the Auburn Pro diff. , and some quality gears that will help alot! I would also run a girdle style rear cover to help keep ring gear deflection to a minimum! Still, as a safety measure, look into some C-clip eliminators! At least then if you do break an axle, you won't be replacing a quarter panel! (or worse) I know many S10 owners with V8 that absolutely punish the little 7.65 rearend (on street tires) and they hang right in there! We're still not talking about 500hp. motors though!!
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Old 02-15-2005, 08:25 PM
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I live in Canada

I bought the Ford 9" bolt in housing from Moser. I opted for a few goodies seeing I was going to the trouble. Aluminum center section, Detroit locker, 35 spline axles, Daytona carrier, and SSM lift bars. $3850 Ca delivered to the door.
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Old 02-15-2005, 10:46 PM
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Guess I wasn't far off in my guess of $4,000...


DOH!!!!
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Old 02-17-2005, 08:02 PM
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A know a few guys in Orlando with 9 bolts running low 12`s. It has shocked me to this day!

My machinist told me as long as your tires are spinning off the line you should be Ok. NO Nittos.

Here is the cheapest way to do this. Mid 90`s camaro and TA`s with sticks have a 10 bolt posi 3:42 gear set up. Call your junk yards!!!!!!! AND the brakes are aluminum disc! BUT, you still need to be easy off the line. I bet it`ll run you about $300. I picked one up for $200 but I had to change the gears because my rear was from an automatic 4th gen with 2:73`s. I should of looked harder...live and learn! Also, the newer rear is 2 inches wider on each side so you can put fatter tires on without using a spacer.
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