1978 Trans Am 10 bolt posi questions

Discussion in 'Transmission & Driveline Topics' started by Gasoholic, Feb 8, 2020.

  1. Gasoholic

    Gasoholic Veteran Member

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    I found this on Craigs list.
    https://detroit.craigslist.org/okl/pts/d/farmington-gm-rear-end-differential-10/7056085648.html

    I spoke to owner. He still has it. Says its a 10 bolt posi from a 1978 Trans Am but he wasn't sure of the gear ratio. He said he thought it was a 2:73. He said it was the OE rear end. He says it is in excellent condition and it does look pretty clean in the pics.

    I did some searching and I dont see where the Trans Am came with a 2:73 posi. From the info I found it looks like it "could" be a 3:23 or 3:42 with a Pontiac "Safe T Trac" type limited slip?

    Any one here know anything about what rear ends came stock in the 78 Trans Am? There are casting numbers included in the pics. But its all greek to me.

    I'm considering this to put in my 78 Type cause it would save me some money. Just not sure if its a good investment. I was really looking for a true trac type posi and going with lower ratio like 3:91.
    I'm going to be running this behind a 4l80E trans and stock LS3.
     
    Last edited: Feb 8, 2020
  2. 1981zapper

    1981zapper Veteran Member

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    Trans am did come with 2.73:1 and so did some camaro’s the only way to know for sure is to remove the cover and count the teeth or read the number off the ring gear that’s if it’s a GM gear

    also note if it is a 2.73:1 Gear and you wanted to upgrade to a lower gear ratio you will need to swap out the carrier
     
  3. SPG

    SPG Bumblebee Builder

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    He probably meant higher as changing the carriers defeats the entire point of buying a different rear end unless this guy doesn't have one already...plus there isn't anything lower than a 2.73 for a 10bolt anyways unless it's made custom.
     
  4. big gear head

    big gear head Veteran Member

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    Standard gear ratios for the 8.5 are 2.41 and 2.56 for the 2 series differential and 2.73 to 3.73 for the 3 series. You can install after market 4.11 to 5.13 gears on the 3 series. Pontiac called the limited slip differential a Safety Track, but it's the very same Traction Lock that is used in other car 8.5 corporate rear ends with 28 spline axles. The casting number on the 2 series is 410409 and the 3 series is 410408.
     
  5. Gasoholic

    Gasoholic Veteran Member

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    Thanks!
    If I can get some free time I might see if he'll let me inspect it in person and pop the cover off.
    I appreciate the help!
     
  6. Skip Fix

    Skip Fix Veteran Member

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    What BGH said. My 78 TA and the 180 HP 400 came with 2.56s. 4000 rpm was 130 mph but all the stock motor could pull!
     
  7. badazz81z28

    badazz81z28 Veteran Member

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    WAYYYYYY too much money for that rearend!!
     
  8. big gear head

    big gear head Veteran Member

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    Ooof, yeah, you're right. That's more like a $500 to $600 rear end at best. No brakes included, axle condition unknown, gear ratio unknown, clutch condition unknown.
     
  9. badazz81z28

    badazz81z28 Veteran Member

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    Hard to believe I was buying 3.42-3.73 posi rears on 2001 for $100-$250. For this rearend....I wouldnt pay more than a few hundred bucks unless it had a performance ratio in it,.
     
  10. Gasoholic

    Gasoholic Veteran Member

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    Yes, thanks for the advice!
    I was thinking I would make him an offer. But more I think about it, its not really the way I want to go.
    I was considering it because I've been looking for ways to cut my expenses. If I can save some money on my trans and rear end I might be able to get my heads ported and a new cam for the LS3.

    The rear end I've been looking at from quick time is around 3K and the 4L80e trans from gear star is another 3K. When all is said and done I will probably have the better part of 7K between the 2. And that leaves me with 3K to spend on the Cal Tracs, split mono leaf springs, shocks, Holley TermX Max controller, all my mounting brackets, new LCA and UCA bushings, coil springs, ball joints, body mounts, sub frame connectors, brakes, tires, uggggh. Yeah I don't have enough money. But hey thats never stopped me in the past :)
     

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