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1968 Firebird rear end options? Rate Topic: -----

#1 User is offline   tumbler1 

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  • Year: 1968
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  • Model: Firebird
  • Engine: Pontiac 400
  • Transmission: Muncie M-21

Posted 05 June 2010 - 10:28 PM

I need to replace the rear end of my '68 before I can drive it on the street. The rear end that came in it was 3" too wide, so it has to go to save the brand new sheet metal hanging above the rear tires. I have a 1975 Pontiac 400 bored 30 over with a few power adders (little under 450hp I am guessing), backed by a Muncie M-21. I have been told the GM 8.5 10 bolt is the lowest I could run, but it would be better to run a GM 12 bolt. I recently have been hearing about eh Ford 9" rear ends, and they look pretty solid. I am not trying to keep it all in the GM family, just getting the parts to make it a great runner. Any help is greatly appreciated.

Also, How wide is the rear end supposed to be on the '68?
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#2 User is offline   Pete W 

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Posted 05 June 2010 - 10:42 PM

Hey Chris, in my 69 I run a 12 bolt with 3.55 gears and it is excellent, lows of low end pull and still can do 60mph on the highway.
A 10 bolt is ok if you are not going to stomping on it a lot but if you plan to smoke em I would look for a 12 bolt.
Not sure on the measurements but Fred will know. http://www.firebirdnation.com/forums/public/style_emoticons/default/12EMOTICHEERS.gif
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#3 User is offline   tumbler1 

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  • Transmission: Muncie M-21

Posted 05 June 2010 - 10:55 PM

 Pete W, on 05 June 2010 - 10:42 PM, said:

Hey Chris, in my 69 I run a 12 bolt with 3.55 gears and it is excellent, lows of low end pull and still can do 60mph on the highway.
A 10 bolt is ok if you are not going to stomping on it a lot but if you plan to smoke em I would look for a 12 bolt.
Not sure on the measurements but Fred will know. http://www.firebirdnation.com/forums/public/style_emoticons/default/12EMOTICHEERS.gif


Thanks Pete. As I am sure you can guess, I will be roasting the rears, and will put every drop of power possible out to the rear end. So Id say the 10 bolt is out. Period.
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#4 User is offline   Injuneer 

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Posted 06 June 2010 - 07:14 AM

I have a Strange 12-bolt behind my 800HP/800t-lb nitrous motor, and when the car still had the manual trans and a twin-disc McLeod Street Twin, the 12-bolt handled 5,000RPM clutch dumps on slicks with no problem. I wouldn't hesitate to use one behind your engine.

The Ford 9-inch is also a great rear, particularly the ability to quickly swap gears, simply by pulling the carrier/front cover assembly, and changing to a new assembly with a different gear ratio. Good for adapting to changing track conditons, but not a real advantage for a street car. The 9-inch also has minor issues with:
-pinion is not on the centerline. It is offset to the drivers side of the car, putting the driveshaft at an angle.
-pinion is 3/4" lower than the typical 10-bolt or 12-bolt, adding yet another complication to the driveshaft angle and getting the pinion angle set correctly.
-I think that you can only get a limited slip type differential up to the 31-spline axle configuration. For stronger axles, you need a locker or a spool - not street friendly
-higher mechanical HP losses (10%) compared to the 12-bolt (7%).
-weighs more than the 12-bolt

In a "basic" configuration, the 12-bolt and 9-inch have similar strength. Once you approach 1,000HP, the 12-bolt is overstressed, and you have to start moving up to more expensive options on the 9-inch. A buddy of mine was running 1,350HP through a Moser 9-inch, but it was a $4,000 setup.

Another rear axle to look at is the "Strange S60". That is a rework of the beefy Dana 60 rear. Even lower mechanical losses (5%) and beefier than the 12-bolt or 9-inch in "basic" setup. Does weigh about 25# more than the 12-bolt though.

My choice would be a Strange 12-bolt.
1994 FIREBIRD FORMULA

FRED

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#5 User is offline   tumbler1 

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Posted 06 June 2010 - 09:54 PM

I am only a college student, and can't fork over $2,000+ for a rear end. How would a normal GM 12 bolt with posi hold up to my drive train? Would it be enough? Thanks for the help!
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#6 User is offline   Pete W 

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Posted 06 June 2010 - 11:40 PM

It will be fine Chris, but keep in mind every time you hammer the car you are stressing metal so at some point something is going to break. http://www.firebirdnation.com/forums/public/style_emoticons/default/12EMOTICHEERS.gif
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