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  • Sluggo
    started a topic 8 3/4 swap details

    8 3/4 swap details

    I finally got a 71 or 72, 8 3/4 3.55 posi installed in our 78 Magnum GT!

    Wanted to share my experience here for anyone considering this swap. I had posted questions a couple times, and now I have answers to give.....

    First the good/easy: As others have said, this is pretty much a bolt in (when the donor rear is from a 71-74 B body). The perches are in the correct spot, the axles length is correct, and if your car has the 727 trans, the driveshaft does NOT need to be cut (I don't have any experience with the other transmissions, may be the same?). You do need the "adapter" or "combo" Spicer u-joint as the 8 3/4 yolk is bigger than the one on the 8 1/4. I was also able to re-use the metal brake lines, the brake hose/vent, and the parking brake cables. The parking brakes cables snap right in, but do need to be re-adjusted for the larger 11" brakes.

    I did have to drill out the center hole on the perches. When I bought it the guy told me it was from a '73, but when I broke it down to change the axle bearings/seals/fluid, it said 71 on the ring gear. I'm assuming its a 71 or 72. So the answer is, yes, pre- "iso clamp" perches (pre-73 I believe) do need to be drilled out. (You need a metal hole saw and have to make a jig to cut the hole straight. There are instructions at Moparts.com if you search)

    Also, for whatever reason, the pinion angle was way off. Like 8 degrees We had to cut off and re-weld the perches at 2 degrees.. So answer number two, check the pinion angle and adjust to 2 degrees. I don't know if all earlier B rears are like this, or mine may have been modified already.

    And lastly, an FYI: both types of Mopar sure-grip rear ends require at least four ounces of additive. I didn't add any at first and it made horrible noises, binding, etc. 4 ounces of Mopar additive from the dealer and the rear is now perfect and quiet.

    The speedometer is almost 20 mph off (going from the 2.45 to 3.55 ratio). I still need to get the "yellow" speedo gear and that will be fixed. The car launches so much better, and is so much quicker. In my opinion, unless you travel on the highway most of the time, this is the single best change anyone can do to these cars. 3.23 would offer most of the benefits and rev slightly less.

    The lock-up torque converter is not real happy. It does lock, but it stays locked too long now and you can feel some bucking when you're decelerating. I may disconnect it (saw the mod somewhere). Not a huge problem, I'll probably get used to it.

  • rl78
    replied
    sluggo, thats for the info and pictures. your car looks great.

    Leave a comment:


  • Cranky
    replied
    Bump.....Sluggo, where ya been?? Your car looks really nice in those pics and have you ever posted any pics of it on the site?

    Leave a comment:


  • Sluggo
    replied
    Found a couple of pics. My Dad took them and they really don't show any of the mods. It does show the nice thick axles with new sealed bearings.

    One of the tailpipes is contacting the shock. Once I get that fixed, and get air shocks bag on it, I'll take some better ones.

    http://a.imageshack.us/img193/562/rear12.jpg

    http://a.imageshack.us/img193/9040/rear22.jpg

    Leave a comment:


  • Sluggo
    replied
    Our Magnum had the rear stabilizer bar, so that's why I wanted to keep the original iso-plates.

    Thanks for the tip on the trans kit. I've wanted to change the fluid and filter for awhile now and that seems like a good excuse to do so. Got a buddy that's a trans guru, I'll let him do it.

    Leave a comment:


  • Magnumguy
    replied
    I'm taking my white Magnum in to get that kit installed after I return from Carlisle I've gotten the shudder pretty bad in it, so that's why I'm taking my red XE

    Leave a comment:


  • R-body_mopar
    replied
    Originally posted by Magnumguy View Post
    Sluggo, you can get a kit from Superior Trans. Parts. This will ~ readjust or eliminate the lockup (it's hydraulic, not electric BTW), pumps in park, along with improvements in shifts and shiftpoints.

    Kit # is KTF-TC

    Has anybody used this??? How does it work?

    http://www.superior-transmission.com...Page.cfm?ID=44

    Leave a comment:


  • Magnumguy
    replied
    Sluggo, you can get a kit from Superior Trans. Parts. This will ~ readjust or eliminate the lockup (it's hydraulic, not electric BTW), pumps in park, along with improvements in shifts and shiftpoints.

    Kit # is KTF-TC

    Leave a comment:


  • Cranky
    replied
    I've never dealt with the iso setup before.....is that the reason for the large holes?

    Leave a comment:


  • CudaZappa
    replied
    too bad you didn't grab the 71-72 shock plates when you did this swap. You could've eliminated that iso-crap in back and avoided drilling the spring perches.

    Leave a comment:


  • Cranky
    replied
    Pics Pics!!

    Leave a comment:


  • DynoDave
    replied
    Thanks for this info. I have a '72 Road Runner 3.23 Sure Grip 8.75" rear with springs laying in the garage, waiting to go into the Cordoba. Sounds like it won't be too bad. Had this same powertrain in a Royal Monaco, and the difference in performance between 2.76 and 3.23 is a very enjoyable one!

    Leave a comment:


  • Sluggo
    replied
    You are right on both Cranky...you do need a hole saw, I think it was 1 9/16 so you have to center it somehow.

    I bought and tried shims from a local rear end place first, but it didn't change it enough. If its only a degree or two they work fine. Anymore than that and you have to cut and re-weld. Like I said, I think someone may have modded this rear before.

    Leave a comment:


  • Cranky
    replied
    Don't understand why you need a jig to drill the hole larger.....just drilling the hole larger, the drill bit will follow the same hole but a hole saw might need something to keep it inline. Pinion angle can be changed with wedges which are available. As for the additive....I've been using STP for over 10 years and it seems to work better. Just my observations but congrats on a successful swap.

    Leave a comment:

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