3:73 Ratio for 8 1/4

Transmissions and Rear Ends

  1. MBDale

    MBDale Well-Known Member

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    Sure would like to see a 3:73 ratio for the 8 1/4. I’d buy it!
     
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  2. Duke5A

    Duke5A Well-Known Member

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    Maybe in the aftermarket? Factory only used a handful of ratios with this axle. 2.24, 2.94 and 3.23 come to mind. Find an aftermarket one used would probably be a pain too since anyone doing any serious performance builds go for stronger axles.
     
  3. Raff

    Raff Well-Known Member

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  4. 80mirada

    80mirada Well-Known Member

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    They offered 3.73 geared 8.25 under the last year Cherokee, and later the Liberty

    5018719AA 3.73 ring and pinion set. MOPAR discontinued

    DANA 10009173 is currently $110.99 at rockauto.com

    There are other options for a 3.73 gear set for an 8.25. For used you can search 2004 Jeep Liberty 3.7L V6.
     
    Last edited: Mar 4, 2019
  5. Duke5A

    Duke5A Well-Known Member

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    The late model 8.25" axles have a different spline count. Don't think the ring and pinion will swap into an earlier carrier.
     
  6. 80mirada

    80mirada Well-Known Member

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    They swap to the earlier carriers, the only difference is the carrier's axel splines
     
  7. BudW

    BudW Moderator Staff Member

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    80Mirada is correct. The ring and pinion gear set will fit FMJ’s fine. The carrier (either open or limited slip) will not, due to the axle spine difference.

    Dakota’s and many Jeep’s use 3.5 and 3.7’s gear sets (the two most popular gear sets for both vehicles).

    There has bee a couple of forum members who has taken a Jeep Liberty 8¼” differential, with disk brakes, and fit it into an FMJ. Liberty’s use coil springs – so a person would need to cut off a lot of brackets and obtain, then weld-on a set of spring perches onto it.
    Getting the parking brakes to work correctly afterwards takes a bit of creative work.

    I plan on doing some work on this for a future article (both the disk brake portion as well as fitting the Liberty differential into an FMJ).


    Different topic, the propeller shaft flange/yoke changed company-wide in mid ‘90’s, to a flat flange/yoke.
    3732251.jpg
    FMJ style differential yoke
    EB75019.jpg
    New style differential yoke
    5183074ab Flange.jpg
    The propeller shaft end (flange) that attaches to the new style yoke.

    The flange/yoke will easily interchange with any of the older yokes, as long as it is ’70 or newer (29 spline) for any 7¼” thro 9¼” and including Dana 60/70. The ’68 and older yokes are 10 spline and will not fit.

    All FMJ’s came with 7260 U-joints.

    If a person is needing to get a propeller shaft made, or modified, and also needs a yoke – I would encourage you to upgrade the yoke and U-joints to 7290 or to 1330 style joints – if your car makes a lot of power.
    BudW
     
  8. AJ/FormS

    AJ/FormS Well-Known Member

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    I tell you what;if you don't already have 'em, just install some 3.23s,and up your stall rpm to in the range of 2500 to 2800; and you'll forget all about 3.73s. I love the 2800; it has a nice zing to it.
    Let me paint a picture for you. These figures are just an exercise.
    Say your engine makes 140 ftlbs at 1800, a typical stall number and you have an A998 with a 2.74 low gear, and 2.76s. When you floor it, the TM (Torque Multiplication) to the rear axles will be;
    140x2.74x2.76=1057ftlbs, kindof ho-hum. But let's factor in the convertor, which might have an instantaneous TM of 2/1 and a sustained TM of plus 5%. So then your numbers might be ;
    140x2.74x2.76x1.05=1112, and the instantaneous might be lbs 2117 , still not impressive.
    Ok so now let's swap to 3.73s.
    140x2.74x3.73x1.05=1502 with an instantaneous possible 2861, so that looks pretty good; if it squawks a tire it will motor pretty good, but if you have a limited slip,it will be a while before it gets up on the power
    Ok so let's back up to 3.23s, and increase the stall to 2800 where the smogger might put out 240ftlbs.
    240x2.74x3.23x1.05=2230 with an instantaneous 4248@ zero mph, see now that's gonna spin both rear tires and you are gone!
    Ok but if the 2800 is so good, what does it look like with the 2.76s? Glad you asked
    240x2.74x2.76x1.05=1926 with an instantaneous of 3669, pretty sweet looking, and spins both tires as well.
    So why did I recommend 3.23s? cuz 3.23s will get you about 60mph at 3900rpm, about where the 318 cam power-peaks. So that would probably get you the lowest ET possible in the zero to 60 time-trial. Whereas the 2.76s will hit 60 at about 3300, not very impressive.
    With 3.23s ;65~2600rpm
    Of course if you have more than a smogger 318, everything changes.
    Just something to think about.

    EDIT;
    just to re-iterate, the 140/240 numbers are fictional numbers I pulled out of a hat, just for this exercise and demonstration purposes only..... altho they might be close.
    I guessed 140@1800 cuz the secondaries are not likely to open at that low an rpm.
    I guessed 240@2800, cuz you can get your secondaries to at least be partly open by then.
    If you have a DP carb, you can force the secondaries to be open at 1800, but a Low-C engine with 2.76s will probably just bog. Even with 3.23s, I doubt it wouldn't at least lay over for a bit. With the 2800, a DP is or can be awesome, if it's properly matched to the engine size.
     
    Last edited: Mar 15, 2019
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  9. MBDale

    MBDale Well-Known Member

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    Wow guys thanks for the wealth of knowledge here! I’m going to stick to my 3:55 ratio and hope for high 12’s on a drag radial.
     
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  10. Oldiron440

    Oldiron440 Well-Known Member

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    Aĺl that sounds good but I have seen stock 318s on the engine dyno and even with no smog control the torque peak was a flat lazy 200 ftlbs, I highly doubt you would see a 300 rpm change from a gear change.
    I have a friend with a 318 m body stock class drag car that was able to move his stall around a 100 rpm with rear end gears, but we are talking twice as much + hp and 4.56 4.88 gears. One thing it has no effect on flash stall, the only effect is after wheels are turning.

    Sometimes AJ I think I would be nice if you used facts instead of of your brain dribble, rear axle gears and lower first gear are going to make a larger impact on 60 foot times in a stock powered vehicle every time, I know I've done it. When you have less than 200ftlbs of torque your not going to get big swings in rpm in the converter from a gear change. And saying 300 rpm is just BS.
    I know your data is your own , your own figures but you shouldn't represent it as fact because it isn't.
     
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  11. AJ/FormS

    AJ/FormS Well-Known Member

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    Hang on. Hi 12s? with a smogger-teen and 3.55s? And I'm guessing a street-chassis.
    To net a 12.9 yur gonna need a P/W of about .096 That is to say;
    at 3600pounds car and driver, 3600x .096=346hp.
    At 3400 pounds; 3400x.096=326 hp
    The mph is gonna be ~106, and 3.55s will get you ~4900 in Direct with 5% TC slip.
    To do all that I don't think you'll be using a smogger-teen. I think yur holding back on us.lol.
    You're gonna need a Hi-output 360. And you're gonna have to hit 106 closer to the power peak,4900 ain't gonna cut it.
    So, spill the beans Mr. Winnipeg ,lol.
     
  12. MBDale

    MBDale Well-Known Member

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    394.8 hp, 438 tq. 9.5-1, 2.02 j heads. Lunati 474in .490exh I believe on the cam, 3000 stall. 360, .30 over. Speed pro pistons H116CP30 is the pn.
     
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  13. Oldiron440

    Oldiron440 Well-Known Member

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    Before spending money on a 8.25 axle parts put that money in to putting a 8 3/4 in your car. The Bbody axle assembly is nearly a perfect fit and twice as strong as the 8.25. You can pick up a spool for little over a hundred dollars. That and whatever gear set you choose. Depending on how much you race 4.10 gears would be a big impact on ET and are livable on the street.
    The thing is once you do the axle upgrade you can start building your HP up and not worry about it for a long time.
    I believe that there is a member here that has a 8 3/4 axle assembly with 410 gears in it ready to go.
    Good luck, looks like a fun car.
     
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  14. AJ/FormS

    AJ/FormS Well-Known Member

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    AHA!!
    Since you are only an hour away, come on over some time; I might have something of interest to you. But right now it is stuck in a snow-bank.
    .8 eh?!
     
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  15. Oldiron440

    Oldiron440 Well-Known Member

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    If you had the correct converter and gear in the car it should be in the 11s with a mph of 115 or so. With a 3.73 gear you would be at 5500 rpm and 410s would be 5900 rpm in the traps.
    These are stick rpm your converter will slip10 present or so, you can add 10 percent to the rpms for a converter. I have no idea of the rpm range of your cam but 300 over in the traps is golden.
    This was figured out using a 27" diameter tire
     
    Last edited: Mar 18, 2019 at 8:13 PM
  16. MBDale

    MBDale Well-Known Member

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    This is great advice Old Iron! Thanks! I’m running cop rims with 255/60/15 Radial TA’s and have 255/60/15 Drag Radials in the basement right now. Run them at the track, here’s cam specs.

    84211D17-5E0F-4886-ADD6-A05FCE93B868.png

    1C7C8549-F113-4AF3-BD75-55C57479C98F.jpeg
     
  17. MBDale

    MBDale Well-Known Member

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    Yes! I think that’s a good idea!
     
  18. Oldiron440

    Oldiron440 Well-Known Member

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    I used the Moroso power speed calculator for the information and I used 3500 lbs as a vehicle weight as an estimate. I think the 4.10 gears would be perfect for the your car. I also think he has $600 on the axle assembly without brakes and that's a good deal.
     
  19. ZieglerSpeed

    ZieglerSpeed Well-Known Member

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  20. Oldiron440

    Oldiron440 Well-Known Member

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    I was going to say that's a nice street cam, the spec's are similar to the old hemi grind cam that mopar sold. I've use that in several 440 projects and I have a buddy that ran it in his 340 Dart for years.

    It would be nice if you would post information on your motor build, you must have had it run on a dyno to come up with the numbers you did, post them. A good converter and gear your car will be a blast.
     
    Last edited: Mar 19, 2019 at 10:05 AM