how much power can a 8 1/4 take?

Transmissions and Rear Ends

  1. Ed Dorey

    Ed Dorey Well-Known Member

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    I'm just curious as to how much hp the 8 1/4 can take. I'm going to be building a blown 408 and have a manual trans. Should I change it out during the build to a 8 3/4 or ford 9" before?
     
  2. Grandmas84

    Grandmas84 Well-Known Member

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    An 8.25 is plenty strong enough. Id say good to around 400 horse. Run what you got while on the lookout for an 8.75.
     
  3. kitcar Chris

    kitcar Chris Well-Known Member

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    Ask slant six Billy....lol
     
  4. kkritsilas

    kkritsilas Well-Known Member

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    It may last for a while (pretty short while I would say), but the blown stroker 408 (easily over 500 HP, possibly into the 600 HP range) and manual transmission will put shock loads into it that it will not stand up to (assumption is use of slicks) for very long. Run it until it breaks if you want, or get something stronger.
     
  5. slant6billy

    slant6billy Well-Known Member

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    HAHAHA
     
  6. slant6billy

    slant6billy Well-Known Member

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    OK. 8 1/4 sure grip... meaning your divide the power between 2 back wheels and not 1. You can't put a one legger to that blown 408. I wouldn't think you would. My 8 1/4 has a sure grip and 4:30 gears and 325/ 50 drag radials. My failure point: the leaf spring on the passenger side got too soft and lifted the passenger side rear tire and planted the driver side, and the tires are way too sticky. mace battle hammer.jpg

    So get some good springs back there. Gator just put super stock springs on his 79 and that is super stiff. Calitrax or some other suspension enhancement- a must. So my engine is not well built. 85 AHB 318 with a .30 punch, 284 duration/ 484 lift, 71 340 heads, 340 iron intake, mp distributor, blaster coil. Trans is a 904 non lock up with a slightly higher stall than stock with a home made halfassed shiftkit. Never dynoed, but my SS/T puts just over 300 hp to the rear wheels and the car will over take the SS/T with some ease. So 400hp- perhaps? Your 8 1/4 needs everything else working or it will fail like above. I'm just taking it easy the rest of the drive season. NO power launches! My 8 1/4 is good now, but it kills me that I could easily kill it with my right foot. I had an 8 3/4 from a 67 GTX and I put a 742 center chunk in it with 4:11 sure grip. Gator is the Man! Not only did he hook me up with the 8 1/4 axle to get me going, he is prepping my 8 3/4.

    So, 8 1/4 sure grip with strong suspension, 8 3/4 sure grip- ideal, or ford 8.8 from an exploder(with disk brakes) -you just need to re-perch the rear.
     
  7. BudW

    BudW Moderator Staff Member

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    I say run what you have until it goes. Meanwhile, gather parts for the changeover until that time.

    Slant6billy had an unusual failure with the axle – the C-clip ring on axle broke – which I don’t think I’ve seen on a Mopar before.

    Might look at cost for a new Dana 60 – to use as comparison for 8¾” (or other) differential.
    If costs for changeover comes close to a new Dana 60 then I might rethink which option to go.
     
  8. Ed Dorey

    Ed Dorey Well-Known Member

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    thanks. I've got awhile before it's on the road again but really appreciate the tips.
     
  9. 79410aspenrt

    79410aspenrt Well-Known Member

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    stroker, blower, and a 4 speed............only 1 diff i would buy, Moser 60.
     
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  10. MiradaMegacab

    MiradaMegacab Well-Known Member

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    Between the above choices, I'd use the ford 9" ........

    What kind of blower are you going to use?
    Details please:)
     
  11. kkritsilas

    kkritsilas Well-Known Member

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    Why the Ford 9" over the 8 3/4"? 8 3/4" is just as strong as the Ford 9" (if not stronger), and while it may not have quite as many gear sets, and other parts, available, the selection of parts is not at all limited. There may be some minor differences between the 742, 741, and 489 versions, but all are plenty strong. T

    he ultimate is a Dana 60 (or the modern 3rd party equivalents), but that is a very heavy unit, and unless that amount of strength is needed (which I don't think is for your blown 408" small block), it is probably overkill.
     
  12. BudW

    BudW Moderator Staff Member

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    The weakest of the Mopar 8¾” series, the 741, has a thicker pinion shaft (which is the weakest part of the 8¾”) than the Ford 9”.
    Also, tests show the 8¾” is a stronger unit than the Ford 9”. Now with that said, the aftermarket is MUCH larger for the Ford 9”, which is fine with me. It makes finding 8¾” parts easier for me.

    I don’t think the strength is that much different between those two, IMO.


    Now if someone is saying the Ford 9” is stronger than the Dana 60 (9¾”) then someone has been sniffing too much paint thinner or something.

    I would agree that a Dana 60 is overkill for most of us – but if you need something stronger than an 8¼” differential and money to buy an 8¾” or Dana 60 is about the same, then I’m going with the Dana 60.


    Note: I have a Dana 80 in my diesel pickup (11¾”) and the differential alone weighs in as much as most cars today weigh in at.
     
  13. MiradaMegacab

    MiradaMegacab Well-Known Member

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    Plenty of affordable aftermarket high performance parts are available for the ford 9". Break an 8 3/4 at the racetrack and chances of finding a replacement component aren't as great as the more common 9".
    Based on the OP's choices ( 8 3/4 vs 9") I suggested a 9" based on the above.
    The four speed is going to shock the driveline. The 9" does have better pinion support with an additional bearing for support.
    If the OP is buying a new fabricated rear or simply adding a new spool, there are affordable options for the 9", axles, brakes, gun drilled axles, more splines, spools, light weight spools, several posi units, new cases, gear ratios, ect..... I wouldn't trust old stock components (gears and axles) in a 8 3/4 or a 9" in a blown track car with slicks.
    I would suggest the Dana but, the OP might have access to an existing 8 3/4 or 9" and the OP might want the convenience of a drop out style rear. Especially with a new engine combo, he can fine tune gear ratio selections much easier by swapping chunks.
     
  14. kkritsilas

    kkritsilas Well-Known Member

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    For what its worth, break a diff of any type at the track, and you are going home. Parts availability at the track is of no consequence, as a broken differential will result in a variety of other things breaking, from universal joints, axles, yokes, and drive shafts to trashing most of the parts inside the differential. In almost all cases, you go home, as most of those parts won't be at the track, unless you bring a spare differential. So you break am 8 3/4", a Ford 9", or even a Dana 60, you go home and fix what is broken. Slant 6 Billy broke an axle (not due to any thing that eh differential did, if you read his description), and broke an axle. In this case it was on an 8 1/4". but even if he had broken a Ford 9", the odds of finding an Ford 9" rear axle of exactly right length at the track is exactly zero.

    Your argument is the same as the Chevy guys have regarding the small block Chevy. Huge marketplace, huge quantity of people with knowledge on how to make power with that engine. Still doesn't make the Chevy the best small block. SImilarly,, Ford 9" rears have a huge marketplace (for differentials), and have a lot of people who know how to set them up. Still doesn't make the Ford 9" better/stronger than the 8 3/4".
     
    Last edited: Sep 20, 2016
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  15. BudW

    BudW Moderator Staff Member

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    Well put, kkritsiulas.

    If you broke - it is a very good idea to go through everything before going back out, anyway, and that, time wise, won't happen at the track.
     
  16. MiradaMegacab

    MiradaMegacab Well-Known Member

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    I've seen plenty of racers with spare axles, pumpkins, u joints, transmissions, ect...
    Sheer some teeth off a ring gear and you simply swap chunks.. I've seen it done numerous times.... At the track.....
    The OP isn't running a 727 and won't roll a sprag......While other people said, "Run it till it breaks"... I've never suggested to do that... I've stated to replace stock axles, in a blown application with slicks, better safe than sorry.... Slant6 Billy was at a Mopar specific show and couldn't locate an axle... If he could've, he would have continued to race.... I've swapped 904 transmissions at the track, I holesawed the firewall where the bell housing bolts are for quick and easy access...... I don't have an argument, I have an opinion, as do you. As you correctly stated, the chevy engines have a huge market place and quantity. As do the ford 9" rears, that's why those parts are plentiful and cheap....
    The aftermarket parts for the 9" are plentiful and cheaper than a Mopar application......
     
  17. MiradaMegacab

    MiradaMegacab Well-Known Member

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    Slant6 Billy's axle failed cause he twisted the splines, not a C Clip failure.....

    100_4135.JPG
     
  18. Ed Dorey

    Ed Dorey Well-Known Member

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  19. Ed Dorey

    Ed Dorey Well-Known Member

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    With a 9" I can swap out the diff for different gear ratios more easily. I figure I won't spend quite as much money on the parts
     
  20. kkritsilas

    kkritsilas Well-Known Member

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    Those are essentially my points, just taken from a different perspective. If you had differential chunk go, you can replace it with another chunk. ASSUMING that you either brought one with you, or that you can find one at the track that has the same gear ratio that you had previously. If the gear ratio is wrong, then you can continue to race, you just won't do very well. Same goes for other things, getting the right length axles, an exact length drive shaft, the right size universal joints or a yoke with the rignt splines or size to work. Most of the time, you break a rear end, you go home. The fact that there are more parts for a Ford 9" doesn't mean those parts are available at the track for when you break something. Same is true for the 8 3/4". In this case, the size of the Ford 9" aftermarket, while much bigger, doesn't help when you need the exact part at the track.

    As for the "The aftermarket parts for the 9" are plentiful and cheaper than a Mopar application......" is true. It is also true of the small block Chevy. So to use your line of reasoning, we should take all of our LA V8s, or /6s, and replace them with small block Chevy. After all, small block Chevy parts are cheaper, and McDonald's gives small block Chevy parts away with every happy meal (or so it seems), right? NOT. If I want a small block Chevy engine, I will get a Chevy car. If I wanted a Ford 9" rear end, I would buy a Ford. Hard ass purist? Maybe. But there are viable Mopar designed parts that do just as good a job as a Ford 9", so why not go with them? They are more expensive, but if you want cheap and plentiful, you picked the wrong platform(s). Go buy a Camaro or a Mustang if you want cheap and plentiful.