8.5 rear end build for 550 hp

Discussion in 'Got gears?' started by lsSkylark, Sep 20, 2020.

  1. lsSkylark

    lsSkylark New Member

    Hello all. I need some advice. 1st let me start by saying I have been on the board for a while researching for my 71 skylark build but this is my 1st post. I need to upgrade the rear end in my car to support 550 est hp. Engine wise its a gen 4 6.0 ls swap with the basic upgrades, trickflow performance cam(elgin sloppy stage 2). Headers, etc. This same build was done for hotrod mag and produced 514 hp. This is a street car the might see the track once or twice just for bragging rights lol. Whats the best way to go to build the rear to support this or should I just get a 9" ford rear built? I do plan to go with 3.73 gears and a true trac posi. I'm sure these questions have been asked a thousand times but thanks for Any help that can be provided.
  2. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    I used Moser axles and upgraded to 30 spline. (28 is stock)

    New 30 spline Eaton Posi

    Yukon 3.73 gears

    TA rear girdle cover.

    I have about40 passes on it with no complaints.

    Contact Monzaz for the axles. He can probably get you a good price.

    The 8.5 is a great rear. I say stick with it.
    lsSkylark likes this.
  3. Bens99gtp

    Bens99gtp Well-Known Member

    My car has enough power so far to run s vest 1/8 mile in the low 6.50s.......thst getting to 10.40s........running 8.5 30 spline

    I ran stk axles into the high 10s though I wouldn't tell someone else too

    Te 8.5 us alot stronger than most give it credit for
    FireRedGS455 likes this.
  4. Brandon Cocola

    Brandon Cocola Well-Known Member

    I have a 8.5 in my Camaro with a 550hp 454 and I have a stock type posi with 2.73 gears that I need to change and it lives up fine. I have had 275 drag radials on the street it didn't hook up.

    What tires are you planning on running.
  5. lsSkylark

    lsSkylark New Member

    Thanks all for the advice. Will be running 295/35/20s in the back.
  6. BillA

    BillA Well-Known Member

    Since you’re buying a new carrier get the 30 spline version and order new axles. 30 spline Moser axles are about $200 a pair.
  7. BrianTrick

    BrianTrick Brian Trick

    Just build it once. I’m sure after while,you will add more power. We all do. I have built plenty of these rears for members on this site. I would just build one with a set of Motive Performance gears,black/orange box,not the cheaper black/blue box. Get a 30-spline Tru Trac posi unit and a set of 30-spline Moser shafts to match. That rear will take plenty of abuse.
    DasRottweiler likes this.
  8. BrianTrick

    BrianTrick Brian Trick

    98075FFE-92EA-408C-AB22-C076112254F5.jpeg Oh. One more thing:
    275/6o/15 drag radials WILL hook up. Just ask Nick.
  9. hugger

    hugger Well-Known Member

    The Pro tire does really well,...but the standard S/S is soso,...unless the track is on point
  10. DasRottweiler

    DasRottweiler -BuickAddict-

    Is this the Tru Trac posi unit you are referring to?
    Will this carrier fit my 71'Buick 8.5 using stock 28 spline axles? Currently running a stock 2 series posi (cone style) with a spacer and 3.42 gears...upgrading with more go power and don't want to grenade the current carrier...Any help is appreciated, JIM
    Check out this page from Summit Racing https://www.summitracing.com/parts/dtl-912a556
  11. DasRottweiler

    DasRottweiler -BuickAddict-

  12. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

  13. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    DasRottweiler likes this.
  14. DasRottweiler

    DasRottweiler -BuickAddict-

    Yes, but Brian is recommending an Eaton Detroit Tru Trac ...I have this limited slip carrier saved in my Summit Wish List and that was the direction I was going in.
    Check out this page from Summit Racing https://www.summitracing.com/parts/etn-19557-010
    The carrier you linked to is for trucks...
    Just curious which is a better unit for my purpose, the limited slip or the TruTrac...JIM
    Last edited: Oct 6, 2020
  15. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    The only reason it may say trucks is because it is 30 spline. That is the part number that is installed in my car right now. I'd be curious to know the differences in the True Trac and why it is a better choice.
    DasRottweiler likes this.
  16. DasRottweiler

    DasRottweiler -BuickAddict-

    Not sure if it is a better choice, it is cheaper. Jim
  17. Bens99gtp

    Bens99gtp Well-Known Member

    The true trac because od its helical gear have the forced better distributed, the is no clutches or cones to wear out either.

    The others units we are more familiar with use that spring pressure to preload the system but once enough force is there to overcome that preload you can still send power to 1 wheel......we have all seen this at the track where a car in the burn out is spinning both tires slides forward and then starts to only spin 1 tire.........the true trac actually sends power to the wheel with the most traction which allows the one with less to join in what it can......not sending power to the wheel with least traction and then trying to drag friction against the case or clutch to try to get some power to the wheel with traction.

    Now with street tires its hard to over come the preload bias but it can happen
    DasRottweiler likes this.
  18. BrianTrick

    BrianTrick Brian Trick

    I am doing the majority of my builds with the Tru Trac units now. None of the clutch style posi units come out of the box with good clutches in them. If I build a street/strip rear with one,I build and tune the unit with the good 18-plate knurled steel clutches. NOT the Raybestos particle clutches,NOT the poly-carbonate clutches,and NOT the slotted rose-petal clutches(not shown). I also pitch the preload springs in the can. This will lesson or eliminate clutch chatter while corner because the clutches aren’t under constant pressure by the springs. When under force,the side gears and spider gears push against each other,which loads the clutches,making friction. The Tru Trac unit can do all of this without doing any extra work,or spending more money to buy the good parts(clutches). Here are some pictures of an original Eaton posi unit for 12-bolt Chevy that I built for some serious street/strip abuse.

    Attached Files:

    DasRottweiler likes this.
  19. DasRottweiler

    DasRottweiler -BuickAddict-

    Thanks for the clarification, Brian.
  20. No Lift

    No Lift Platinum Level Contributor

    I started twisting the 28 spline factory axles in my '76@4250 lbs with Drag Radials when I started running 11's with 455 power. I got Moser 30 spline axles from Monzaz and a later model 30 spline Truetrac. I'm high 10's now with no problems. Works great and there is no reason to not upgrade to 30 spine axles if you get a new carrier. No posi additive needed for the Truetrac. I'm not 100% but the '71 should have bolt-in axles which is a plus. Getting traction on the street is a completely different thing than at the track.


    Don't worry about the posi, the spacer is the fuse in that setup. If you don't plan any BIG power additions I'd just keep an eye on E-Bay for a used posi from any GM 8.5 10 bolt 28 spline unit. Even better, swap it out with the posi(?) from your Formula :rolleyes:
    I was taking the car to the track alot back then and IMO that is a big factor in the twisting splines.

    Atco 2015-11-15 1976 200.jpg

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