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Discussion Starter · #1 ·
So I received my 02 Firebird (3.8) back from the shop today with a whole new rear end. Mine had been making so much noise you could hear the echo under a bridge like a metal grinder.. awful!
I had the whole rear end replaced and upgraded with the 3:42 gears instead of 3:08...


So I am creating this to help people make a decision on this upgrade.

So far I have noticed that the torque has increased quite a bit, especially in the lower RPMs. I haven't had it dyno tested or anything yet but I can tell a defrence even without it.. so I would say this is a must for anybody still running 3:08 its worth the time and money.

If I could have done anything different I would of gone with a 12 bolt rear end I've heard good things about the strength.
Also, if you are planning on doing a v8 swap in the future you really want to do a 12 bolt or 9" rear end with 3:73 gears so it can handle the power of an LS1.

 

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1994 Firebird Formula 381ci LT1 / TH400+GV O/D
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The change in the "feel" is easy to calculate. 3.42 / 3.08= 1.110. It feels like you added 11% more torque at the rear wheels, when you launch the car. The tradeoff is that you have to shift the car sooner bcause you reach redline RPM at 11% lower MPH. There is no real change in the engine's HP or torque. You just changed the way the drivetrain applies the torque to the road. There is nothing to be shown by a dyno pull. Have you had the PCM programmed to correct the speedometer/odometer?

I am not sure why you wish you bought a 12-bolt. While the 10-bolt is weak, it's not generally a problem with the 6-cylinder cars. All you would be doing is adding unsprung weight to the rear, hurting ride and handling, and adding mass to the drivetrain that absorbs HP, leaving less HP to the asphalt.

Even in the V8's if you don't abuse it (high RPM clutch dumps on sticky tires) a stock HP engine isn't likely to break it. There are 4th Gens that have run 10.xx-second 1/4-miles on the stock 10-bolt. I ran a 125-shot of nitrous on top of a "bolt-on" LT1 that was probably in the range of 300-325HP without the nitrous, (over 425HP with the nitrous) and never broke the 10-bolt. And that was through a T56 manual trans. I have a Strange 12-bolt now with 4.11 gears, but the engine is also at 800HP/800lb-ft.IF you feel you need to beef up the 10-bolt, here is a 2-part article describing what you can do:

http://www.highperformancepontiac.com/tech/hppp_0610_ls1_pontiac_firebird_performance_rearend_build/

http://www.highperformancepontiac.com/tech/hppp_0611_firebird_rearend_rebuild/
 

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Discussion Starter · #3 ·
Thanks for the insight! I'm not to familiar with rear ends and gears this is my first experience with them. The info I was going off of was info I got from fellow f-body owners and the internet. I would hate to lead somebody in the wrong direction so I'm glad you cleared some things up for me!

I have not had the PCM tune yet, I am going to add some more mods to the car before I decide to tune it up.
 

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*DARK KNIGHT*
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yeah, when mine was a v6, i did the 373 upgrade with posi...but i also had to get a tune so that my gauges would read correctly...and do u now have posi?
 

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*DARK KNIGHT*
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oh...you would feel an awesome difference with the posi...after a rear gear upgrade a tune is needed to let the car know its got different gearing, the speedo and odo dont read exactly right without it
 

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If you did not upgrade to an Auburn or Torsion posi differential, which differential did you use, and what gearing brand? A little more info would be insightful.
 
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