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  • Gear Change Question

    Hello to all. I have a 79 Cordoba with that has a 360 4bbl (340 resto cam, Performer Intake & Non-Lean Burn) 2.5" dual exhausts , A999, and a 8 1/4 with 2.45 gears. One of my thoughts for down the road is to have the gears changed. I was thinking of having 3.23s installed to keep it somewhat highway friendly and have kind of a hotrod. I'm just wondering where is a good place to order the parts to have this work done and I'm also curious how much should this swap cost me if I have a third party do it? Thank you in advance.

  • #2
    I'm gonna wait for Cranky to weigh in on this one. Anybody with that many jackstands and pinion knowledge is somebody ya listen to on stuff like this. Best I can do for now is add in some nifty online tool links for this.

    https://www.ringpinion.com/calculators/Calc_RPM.aspx

    http://www.summitracing.com/expertad...nd-tire-height

    http://www.csgnetwork.com/multirpmcalc.html

    http://www.differentials.com/technic...tio-calculator

    http://www.businesspath.com/business.../rpm-calc.html

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    • #3
      magnumminded, I have not done this swap, but I can offer a couple of thoughts!

      First of all, the 8.25" rear is not very HD, so now would be a good time to find a 9.25" rear end from a police/trailer tow package (3.23 with sure grip!), and swap the whole rear, not just gears! At this time, it would be good to upgrade the rear springs to HD 5 leaf, and add a rear stabilizer bar. Vast improvement in handling!

      I think changing the gears in the 8.25" rear to a lower ratio will require a different carrier, not just the gears, as the 2.45 uses a different center section. Not sure, though!

      And, if you change gears, it will have to be set up properly for preload, backlash, pattern, etc. Again, ask Cranky about this!!!! He knows his stuff in this area!

      Another thing to consider at this time is the speedometer gear, as this will drastically change the speed reading; pull the appropriate gear from the transmission in the donor car, (if you can find one!)

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      • #4
        If you do go with a different ration and keep the 8.25, actually, if it's a 2.45, something about the case or carrier is different than any of the other ratios.
        .


        bringin' em back ~ to the Dodge Mahal !!....

        Where old Magnums can find a home.. :angel:

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        • #5
          Thank you all for the replies. The reason why I was considering this change down the road is because I was going to give this car a more 70s look with wider tires in the rear and smaller tires in the front. The car already has new leaf springs so I really don't feel like having them changed again. As far as the 8.25 is concerned I'm not looking to drag race the car but, a little grunt off the line would be nice if that's achievable without changing the whole rear end plus the 8.25 was used in Dakotas, Jeep Cherokees (XJ), and now the Jeep Liberty. Maybe a little peel out to release stress is what I'm thinking but, again its all just brainstorming and trying to come up with a plan. Plus, I was going to try to find a shop to do it and maybe one that will warranty their work.

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          • #6
            Anything above 2.71 gears will require a different diff or carrier (the carrier is what the ring gear bolts to). Yes, the 8 1/4 is probably tough enough for what you plan on doing but a heavy car with even a mild engine can be a bit hard on them. My 95 Dakota has one and it's held over 1600 lbs in the bed many times but there's only a V6 in front of it with 3.55 gears. If you're going to spend some money on the one you have, you might think about grabbing one from another vehicle but parts will run just as expensive as the others so why not go with something bigger and better and forget about it? Ever think about throwing in an OD trans into the car? The A500 (904 based) OD in our Durango does rather nicely with the 3.90 gears. Launches well and will still knock down decent mileage with the basically old school 318 with FI and hualing 4600 lbs around.....and it'll get 13mpg doing 80 lol. That ain't too bad for a brick

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            • #7
              Cranky, hello and thank you for the post. If I take what you're saying is correct then, the whole axle needs to be changed to upgrade the gears? If that's the case I don't know then. It kind of sucks since I got the air conditioning just recharged in this car. My thought was to take it to a drivetrain shop, have them change whatever is needed to be changed in the current axle and be on my way. As I said, the prior owner just changed leaf springs and I don't want to have the leafs changed. As for an overdrive I really don't want to go that route hence why the lowest gear I could tolerate would be either 3.21 or 3.23s.

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              • #8
                Just to add a little visual...



                ...what is labeled here as a "case" is the part you would have to change along with the ring and pinion gear. Some series (ratios) of ring gears are thinner, or thicker. To make up that difference, the flange on the case has to be thicker or thinner, to keep the overall thickness the same.

                In your situation, going from a 2.xx series gear to a 3.xx series gear, cranky is saying you will also need a different case. A good time / excuse to make the jump to a sure grip differential / case.

                It would not necessitate a complete axle change, but it may be the more economical answer. It all depends on how inexpensively you can get a new or used gear set and case, plus labor, Vs. swapping a complete axle assembly.
                WPC# 12304 / 1970 Plymouth Duster / 1972 Dodge Charger Rallye / 1977 Chrysler Cordoba

                www.cordobaclubusa.com

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                • #9
                  Kinda weird that places that sell parts call them by different names....like on the 8 3/4, the 'carrier' is actually called the case (when it's empty) and the other axle one is called the integral axle design. I dunno...maybe I'm the one that's been wrong all these years lol. Many call the part that the ring gear bolts to the differential. Anyway, DynoDave is right about not having to swap the whole rear end for going to a 3.23 gear change....just the case/differential/carrier lol. And why would you have to change the leaf springs if you swapped in a completely different axle housing drum to drum?

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                  • #10
                    Originally posted by Cranky View Post
                    Kinda weird that places that sell parts call them by different names....like on the 8 3/4, the 'carrier' is actually called the case (when it's empty) and the other axle one is called the integral axle design. I dunno...maybe I'm the one that's been wrong all these years lol. Many call the part that the ring gear bolts to the differential. Anyway, DynoDave is right about not having to swap the whole rear end for going to a 3.23 gear change....just the case/differential/carrier lol. And why would you have to change the leaf springs if you swapped in a completely different axle housing drum to drum?

                    My thoughts on this matter were along the same lines, but geared (no pun intended!) towards an upgrade in the rear suspension/driveline/handling aspect of the swap. Of course, you would have to have the proper leaf spring plates to attach the factory rear stabilizer bar, plus the links, etc!

                    My next question is what new rear springs were installed?, and are they HD 5 leaf, or stock 4 leaf? The rear suspension on those cars was a little lazy as stock, and why I suggested an upgrade to HD 5 leaf springs and the 9.25" 3.23 rear WITH stabilizer. It would solve the problem of taking the rear apart to change carriers, and having to set up a new set of gears, something that few people know how to do PROPERLY, or have the tools to do it! Also, many of the 9.25" 3.23 HD rears already had Sure-Grip installed, another plus!

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                    • #11
                      The 9.25 in our Durango has a Sure Grip unit but it's the dreaded Track Lock unit. It's a one piece design 'case' lol with the clutches outboard' of the side gears and is a piece of crap. If you don't have the proper tools to replace the clutches in that thing, I would suggest you not bother and take it to a good rear end shop. Anyways, it started acting up at 25k miles so I changed the fluid and installed new with additive and within 5k miles it started acting up again. The first time the bottom of the housing had a lot of sludge in it. Anyways, it got another 2 quarts of premium gear oil only this time it got STP for the additive. Been fine ever since. Not sure what other limited slip designs the 9 1/4 used (if others were offered) but I wouldn't recommend the Track Lock to anyone.

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                      • #12
                        LOL...yes, carrier, case, or differential. I always thought that "differential" was the correct technical term, but locally we always called it a carrier. I didn't hear it called a case until I moved to Detroit.
                        WPC# 12304 / 1970 Plymouth Duster / 1972 Dodge Charger Rallye / 1977 Chrysler Cordoba

                        www.cordobaclubusa.com

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                        • #13
                          Where did you move from and why did you go to Detroit?

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                          • #14
                            Central Illinois, and I moved for work back in 1989. And I say "Detroit" generically. I've never actually worked or lived in Detroit, but in "S.E. Michigan", the greater Detroit area.
                            WPC# 12304 / 1970 Plymouth Duster / 1972 Dodge Charger Rallye / 1977 Chrysler Cordoba

                            www.cordobaclubusa.com

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                            • #15
                              Magnuminded, we did the swap on our 1977 Doba with a 400 and took the carrier out of a Dakota (up to 1996 will do the job). As you know they come with 3.55 and that gets the car revving a bit too high without the overdrive. But if you get a set of 3.21 gears they will work with that carrier. Otherwise, as mentioned before, put the 500 OD in with the 3.55's and we have that setup in a 76 Doba and a 79 Magnum and it works real slick. Either way the 2.4 gears have to go. The swap will make a whole different car for you. Don't forget 97 and up Dakota will NOT work. Good luck whatever you decide to do.
                              1975 Charger SE E68 323
                              1976 Cordoba 360-4 500 OD 355 Sure Grip
                              1977 Cordoba 400-4 727
                              1979 Magnum XE 360-4

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