Performance 8.5 input needed....

Discussion in 'Got gears?' started by StagedCat, Mar 10, 2018.

  1. StagedCat

    StagedCat Platinum Level Contributor

    So here's the basics, 71 Sportwagon est. 4,300-4,500lbs, TH400, 700-750hp TA engine, street/strip....
    Want some recommendations on what I will need for it to survive or is it even worth it?
    Go 12 bolt or 9"? Thinking 3.90-4.11's with drag radials. Thanks for any input.....Tom
  2. BrianTrick

    BrianTrick Brian Trick

    Not a problem. Get a set of 30-spline Moser axles,and either a good,tuned Eaton Posi unit,or a Detroit Tru-Trac.
  3. monzaz

    monzaz Jim

    Be aware you have a HEAVY car...and posi unit will only take so much no matter WHAT SIZE diff you use. They all have some form of planetary gear to make it more street able...So determine the amount of street vs strip the vehicle will do and buy accordingly.
    If you vehicle only goes to the corner car hop a FULL spool would be the way to go if the car will live at the track mostly.
    And in the case of the full spool we can go up to 33 spline with the A10/ set 10 bearing used in wagons (I looked up in a book and cutlass vista crusier uses the set10/a10 but say Buick does not...I think that is wrong info . I was sure they had the same axle bearings in the wagons

    J D RAce
    300sbb_overkill likes this.
  4. StagedCat

    StagedCat Platinum Level Contributor

    Thanks guys, that's my concern, the weight...What are your thought on heavier axle tubes and welding to 3rd member housing?
    And what are/is A10/ set 10 bearing(bearing size), not familiar with this? Its hopefully gonna be a 50/50 - 60/40 street/strip ratio.....
  5. BrianTrick

    BrianTrick Brian Trick

    Check and see if you have the bigger axle bearings. The Cutlass flat-top wagons did not,but the Vista Cruisers did. They also had the shock mounts curved inboard,to help clear the deeper backing plates for the wider 2.5” brakes.
    Here is a custom 33-spline posi unit that I built for an 8.5”,using the 71-72 Vista housing.

    Attached Files:

    TexasT likes this.
  6. monzaz

    monzaz Jim

    50/50 GOOD LUCK... Just keep money in the back set aside for broken Just the way it goes. hope these dimensions are correct. I know it is way bigger than the 1.382 of a stock a-body axles size

    Bore Diameter (in): 1.562 in.

    Outer Diameter (in): 2.875 in.

    Width (in): 0.763 in.

    J D RAce
  7. monzaz

    monzaz Jim

    Did you build your own 33 spline 8.5 unit too. Jim
  8. BrianTrick

    BrianTrick Brian Trick

    Yes. It is pictured above. Strange/Us Gear 33-spline side gears,and the correct matching spider gears. Eaton knurled steel clutches and tuned properly. No preload springs.
    You can’t make a 33-spline with the SET9 bearing because the ID of that bearing is too small to make that thick of an axle.
    If you have the typical 71-72 Skylark/GS rear,with the SET9,just get 30-spline axles. The only reason I suggest 33-spline with the SET10 because the cost of the axles is the same. There is a jump when you go to 35 because they use a different stock to make that axle.
    Last edited: Mar 11, 2018
  9. monzaz

    monzaz Jim

    So your using 12 bolt Chevy 33 spline side gears in the eaton and having to open up the carrier axle shaft engagement diameter to fit the axles then. Yes?
    I did a couple of 33 spline 12 bolt chevy original 2 series eaton posi unit for 33 spline...guy was using 2.56 and turbo buick car...weird but that is what he wanted. (there are no 2 series aftermarket carriers)
    I did not see 33 spline 8.5 spider gears in the strange catalog...?? Jim

    Here is my thing - Just because you can, should you? thoughts below -

    Why would you go with 33 spline Eaton or any posi for the 10 bolt 8.5 when making the doughnut hole larger in the side gear is just going to weaken it? Yes, the axle will not break but this compromises the side gear threshold.
    I would say 30 is about it for a posi in the 8.5 Even the 33 in a 12 bolt I question thins out the carrier case bearing journal too and the same doughnut hole affect.
    I know you are selling them and may hold up for you and it is a again cool factor etc.

    Just my opinion on the deal. ...this to me is another case of bigger is too big, like carbs, intakes, cam shafts, etc. were going too far and weakening another part of the system of holding the rear together.

    Moser engineering has the 35 spline for 12 bolt Chevy also.... TERRIBLE design...12 bolt Chevy should not have any more than 33 spline with the carrier bearing being used . I have repaired lots of these rears here after customers drive them hard and the wavetrac unit carrier bearing system / races are paper thin and does not hold up well to heat. Races get blued from heat that can not bleed off fast enough etc. this is a perfect example of TOO FAR... just a step too far in the wrong direction deal.

    Spool is a totally different thing in the transfer of power as it eliminates the spider gear weakness all together. But again even for the 12 bolt Chevy 35 not go there. Another factor is spools usually are used in quick accel and decel racing so heat is not usually ever a factor because of round times being spread between races. If you think you need that much axle shaft FOR THE STREET you better upgrade to even a bigger rear Dana 60 ot aftermarket MW 9.5" or 10" ford ... lol. Then you are pretty much not even talking street car anymore.

    NO STREET car in my opinion should even need 35 spline axles. The street can not even produce the stick to break a properly set up 30 spline axle rear on the street.
    (disclaimer - you can not be hitting chuck holes and bouncing the rear tires because of bad suspension components on hard clutch dumps and such that is not a far comparison etc. )

    Giving info to chew on, everyone has to make there own decision.

    Remember I am not out attacking write from wrong...We are in a discussion forum and asking and answer questions.
    J D Race
  10. StagedCat

    StagedCat Platinum Level Contributor

    Brian and Jim, thanks for the info.......
  11. monzaz

    monzaz Jim

    Yep, Still your car, you have to decide what is good for you. :)

  12. BrianTrick

    BrianTrick Brian Trick

    Yes,I honed the carrier bore,and yes,it is no thinner than a 33-spline 12-boot Chevy carrier. I have seen some Yukon 33-spline carrriers break the hub off,but Inhave not seen an Eaton do it,so there might be better material in the Eatons. I have an 33-spline Eaton in mine,and I haven’t broke it yet.
    I have a 35-spline Detroit Locker for 12-bolt Chevy,but it also takes a different bearing set,to keep the hub thick.
    I spool is much better material than any posi unit,so there is a lot less chance of failure.
    To come back to the original question: Will the 8.5” 10-bolt hold up? Yes it can.
    Last edited: Mar 12, 2018
  13. monzaz

    monzaz Jim

    QUOTED ' I have a 35-spline Detroit Locker for 12-bolt Chevy,but it also takes a different bearing seat,to keep the hub thick.' but how thick is the race? That is what was failing in the 35 spline Moser 12 bolt Chevy WAVE TRAC carrier bearing and race system. It is very thin and does not handle heat well. For drag racing it might work well but any kind of longer race versions it is usually doomed. The wave trac uses the same bearing a races as your Detroit 35 spline.

    Just remember circumstances will always play a big part of what will and will not survive. So many different scenarios can and will apply pressures on parts no one can ever tell what will hold under what applications thrown at parts.

    So we can only do what we think is right under scrutiny of parts and what they will be used to accomplish. picking the right parts and size parts and quality parts for the right jobs is critical, true.
    As you stated, who's parts are built correctly, castings and metallurgy design from defects etc. ... again will always through a wrench into the works.
    Sometimes all the skill and all the best parts are superseded by a big bit of luck!

    J D
  14. BrianTrick

    BrianTrick Brian Trick

    Yes,those Wavetracs have that larger hub. Moser tried to keep the bearings a top secret,like you have to get them directly from Auto-Tech,but I already have some from doing the Detroit Locker. Yes,those bearings are much thinner,and I question what abuse they will take. It would be ideal to have a larger bore in the saddles. Then you can run a better bearing. Moser should offer their 12-bolt housings with larger bearing bores. That Wavetrac IS a very nice unit though. I have built some 9” rears with that,including one for myself.
  15. monzaz

    monzaz Jim

    ditto Told them to do it BUT tey are not going to as there are more people going to by a crap 9" so that is that. :(

    I totally agree too, ring and pinions are not the issue with the 12 bolt just the limit on the axle shaft size with full spool.
    Last edited: Mar 14, 2018
  16. Bens99gtp

    Bens99gtp Well-Known Member

    Not to hijack this thread, but my question does go along with the OP some what.

    What does it take to be able to put 33 spline in an 8.5 for 's spool setup. My rear now is set 9 bearings. Our new motor is looking to be north of 700,725. Only sees race time. We have been super lucky with the stk axles to run low 11s high 10s. But we rotated axles evey 25ish runs.

    I know 30 fit right in.....but if 33 cost the same for axles and basically same for spool.......what does it take/cost to get them in the housing?
    From what I took from above 33s fit through a set 10??? Is this just a housing end swap??

    At our level is it even worth doing?
  17. BrianTrick

    BrianTrick Brian Trick

    You can’t run a 33-spline in a SET9 housing,but I wouldn’t be too worried about it. A set of 30-splines will handle that.
    If you were to cut the ends off,you would get a Moser #7900,which takes the SET20. Even bigger bearing. It still has the same bolt pattern for the factory backing plate.
  18. Bens99gtp

    Bens99gtp Well-Known Member

    I see they have this 7900 and 7900fm for a flush mount. Not sure which would work better, assuming they take the same bearing. Wonder then if this would allow some of the bolt on rear disc brake kits???

    Any idea what cost to get the ends cut and welded back on would run.......the ends are only 100 bucks, if the work to install would be reasonable , and it not change axle price might be a cheap insurance to get 33s overy 30s.

    I do not feel secure in my welding ablities at this time to be able to add these welder is being very temperamental right now and I haven't got it fixed yet
  19. BrianTrick

    BrianTrick Brian Trick

    Everything needs to be cut square and put in a jig. If you have a chassis shop nearby,they should be able to handle it.
  20. Bens99gtp

    Bens99gtp Well-Known Member

    Any know a good chassis shop in middle to norther Ohio that might be able to handle this? I would like to balance out all my options

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