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  1. #1
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    Changing your 15+ zf rear....

    Here's a quick diy for anyone wanting to swap out their rear.
    First let me start by saying in my opinion after running the 3.70 at the track and after having the 3.09. I personally went back to the 3.09. The 3.70 is great for around town fun , a quick boost in acceleration for a stock set up car. As far as the track , imo, it's just too much gear. It shifts too fast and caused me excessive wheel hop in second gear. Yes second. I launched pretty good in first , consistently at a 1.7 60ft , but hitting second just hopped and skip shift right to third. Any way for me , I think 3.09 is better. I'll get back about that after ATCO.
    Now for the guys coming from a 2.62 open or lsd, the 3.09 is an ideal gear for the heavily modified 5.7 or the hellcat. A 3.70 or 3.90 may be beneficial to the mildly modified 5.7's looking for a good kick in the a$$. Again this is just my opinion.
    This was done by , Bad B-rad and myself. So shout out to Brad!!
    First things first, we ran the car in reverse up onto ramps. Disconnected the exhaust to allow for a bit more room. This is on a 6.4 , I believe the 5.7 can disconnect where I circled. There are the two clamps holding it to the mids ,and two bolts at the active valves ,and then two more at the rear. Sorry I forgot to get pics of these , but you'll find them. We found it easier to pull the bolts , rather than slip off the rubber. We also pull the further back body cross member.
    Next up is dropping the driveshaft,(I believe it was a t40, but not sure) a mallot will do the trick here also. Then support the rear with a hydraulic jack that can roll when you need to.
    Sent from my SM-G955U using Tapatalk

    Then you have two vertical bolts holding the housing in , remove those.
    Now you can begin with an open end 21mm wrench to loosen the two rear horizontal bolts and slowly walk the jack forward to release the bolts( they do not come out as you will see , they hit the tire bin).
    Now you can slightly lower the housing and cock it to get the driver's side axle out
    Repeat the same procedure for the passenger side
    That is it , follow the same in reverse for the install and your off !!!! Good luck!!!!
    Last edited by ronan978; 10-25-2017 at 10:05 PM.



  2. #2
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    Excellent write up sir thank you very much! Now I just need to save up and find a 3.09 rear! LOL!
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  3. #3
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    The tool for the driveshaft bolts is a 45ip ,TORX PLUS.
    Not regular torx, not tamper proof, it is a torx plus, it is a little different then the regular torx.
    Also remember to draw up the(driveshaft) bolts evenly, I have only had the driveshaft give me an issue one time.
    One more thing, we couldn't put the car in neutral, so we had to roll the car forward a touch, or jack the rear and let the tires hang to line up driveshaft to rear bolt holes, just like when getting the 2nd axle splines to line up going into the rear.
    (When installing the rear,the first axle will go in easy, just rotate the front of the pinion, if it is not lined up, for the 2nd axle you may have to jack up the one wheel on the side of the axle you are installing 2nd, so you can turn it to line up the axle shaft splines to the rear.)

    I think it holds a quart of fluid, so now is a good time to change, or check it, what every fluid you use, make sure you have or add the limited slip additive
    Last edited by Bad B-rad; 10-25-2017 at 10:28 PM.
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  4. #4
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    Great write up Ronan978 .. I've read your posts on results of your 3.70 swap and am not surprised at the lack of improvement.. you stated you believe the 3.70 is too much gear for the A8 and I submit it's not too much but the wrong gear for a stock or in your case a mildly modified 6.4 and here's why .. I used an online gear calculator over a year ago when deciding on a gear change for my A8 Scat. assuming an average trap speed of a stock 6.4 is around 113 mph . below are the rpm's crossing the stripe at the end of the 1/4 using different gear ratios.

    3.09 @ 113 mph in 5th gear = 5273 rpm
    3.70 @ 113 mph in 5th gear = 6315 rpm ( too high )
    3.70 @ 113 mph in 6th gear = 4895 rpm ( too low )
    3.90 @ 113 mph in 5th gear = 6656 rpm ( way too high )
    3.90 @ 113 mph in 6th gear = 5159 rpm

    too high or too low rpm and you lose acceleration.
    the 3.70 with close to stock hp is the wrong choice for the drag strip in 5th or 6th gear .(it could be a good choice for a high hp N/A or low hp F/I )
    also, look how close the finish line rpm is between the stock 3.09 in 5th gear is to the 3.90 is in 6th ...almost the same except the 3.90 accelerates faster . I gained .2 with my 3.90 swap.
    an N/A 6.4 with 3.90 and a cam that would go to 7k would be an interesting experiment.
    I'm trying to shine some light on why things turned out the way they did . not Sh*T on what you've done. hopefully I accomplished that...

    also have you looked into the Per4mance Development rear differential brace to fix your wheelhop ? I have one on my 700 whp Scat and it really tightens up the rear.. no hop period..
    Thanks Bad B-rad, ronan978 thanked for this post
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  5. #5
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    Quote Originally Posted by MXS67454 View Post
    Great write up Ronan978 .. I've read your posts on results of your 3.70 swap and am not surprised at the lack of improvement.. you stated you believe the 3.70 is too much gear for the A8 and I submit it's not too much but the wrong gear for a stock or in your case a mildly modified 6.4 and here's why .. I used an online gear calculator over a year ago when deciding on a gear change for my A8 Scat. assuming an average trap speed of a stock 6.4 is around 113 mph . below are the rpm's crossing the stripe at the end of the 1/4 using different gear ratios.

    3.09 @ 113 mph in 5th gear = 5273 rpm
    3.70 @ 113 mph in 5th gear = 6315 rpm ( too high )
    3.70 @ 113 mph in 6th gear = 4895 rpm ( too low )
    3.90 @ 113 mph in 5th gear = 6656 rpm ( way too high )
    3.90 @ 113 mph in 6th gear = 5159 rpm

    too high or too low rpm and you lose acceleration.
    the 3.70 with close to stock hp is the wrong choice for the drag strip in 5th or 6th gear .(it could be a good choice for a high hp N/A or low hp F/I )
    also, look how close the finish line rpm is between the stock 3.09 in 5th gear is to the 3.90 is in 6th ...almost the same except the 3.90 accelerates faster . I gained .2 with my 3.90 swap.
    an N/A 6.4 with 3.90 and a cam that would go to 7k would be an interesting experiment.
    I'm trying to shine some light on why things turned out the way they did . not Sh*T on what you've done. hopefully I accomplished that...

    also have you looked into the Per4mance Development rear differential brace to fix your wheelhop ? I have one on my 700 whp Scat and it really tightens up the rear.. no hop period..
    Much appreciated, definitely makes sense. I'll see how it goes now at the track and take it from there. I did notice I had to be in autostick to avoid shifting to sixth right before the line.
    Any light shined down is still light!! Hopefully this thread can help others make an informative decision on gears and options , along with what to expect as far as rpm crossing the line and cruising. For instance, with the 3.70 I was at or around 2350 @ 80mph, I believe the 3.90 is more like 2500, and the 3.09 is around 2000. This should be/turn into a thread worth reading.


    Sent from my SM-G955U using Tapatalk

  6. #6
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    Some GOOD info here!!!!!!!
    I want to say I have a 2016 R/T 5.7, and I was told I cant just swap the rear,(going from2.62 open to a LSD)
    Well I swapped from my 2.62 open to the 3.09, worked fine, just changed the Diff.
    Then I swapped the 3.09 to the 3.90. So it will work for R/T cars also not just Scats.

    Ronan to the rescue!!!!!! People had questions, Roan supplied the answers!!!!
    Also Shout out to MXS67454!!!!!! Very informative write up on the RPM @ finish line!
    That explains the hop.
    Are the rpms with the gears the same for 5.7 Hemi, just at a lower MPH(rpm), like I cross at 105MPH, so is that 4th and 5th, vs the Scat's 112MPH @ 5th or 6th?
    I have 3.90's, would I be better off with the 3.70's?
    Last edited by Bad B-rad; 10-26-2017 at 07:48 AM.

  7. #7
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    Quote Originally Posted by Bad B-rad View Post
    Some GOOD info here!!!!!!!
    I want to say I have a 2016 R/T 5.7, and I was told I cant just swap the rear,(going from2.62 open to a LSD)
    Well I swapped from my 2.62 open to the 3.09, worked fine, just changed the Diff.
    Then I swapped the 3.09 to the 3.90. So it will work for R/T cars also not just Scats.

    Ronan to the rescue!!!!!! People had questions, Roan supplied the answers!!!!
    Also Shout out to MXS67454!!!!!! Very informative write up on the RPM @ finish line!
    That explains the hop.
    Are the rpms with the gears the same for 5.7 Hemi, just at a lower MPH(rpm), like I cross at 105MPH, so is that 4th and 5th, vs the Scat's 112MPH @ 5th or 6th?
    I have 3.90's, would I be better off with the 3.70's?
    yes . the math applies to all A8 cars . a change in horsepower changes the rpm at the stripe. . speed,time,distance .

    3.70 @ 105 mph =5867 rpm 5th gear
    3.90 @ 105 mph =6184 rpm 5th gear

    3.70 is a better choice right now . you could increase hp about 30-40 from where you are now before forcing a shift to 6th.
    Last edited by MXS67454; 10-26-2017 at 09:17 AM.

  8. #8
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    Sounds like Ronan978 and I made a good trade, 3.09 to him, and 3.70 to me!!!!!
    I guess I will be pulling those 3.90's.(After I test that other tune)

    HUGE THANK YOU to MXS67454!!!!!!

  9. #9
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    Wow, that is much easier then the 1st gen LX cars.
    Would never have thought it could be done that quick and with so few tools, thank you for the excellent write up!
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  10. #10
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    Quote Originally Posted by MXS67454 View Post
    Great write up Ronan978 .. I've read your posts on results of your 3.70 swap and am not surprised at the lack of improvement.. you stated you believe the 3.70 is too much gear for the A8 and I submit it's not too much but the wrong gear for a stock or in your case a mildly modified 6.4 and here's why .. I used an online gear calculator over a year ago when deciding on a gear change for my A8 Scat. assuming an average trap speed of a stock 6.4 is around 113 mph . below are the rpm's crossing the stripe at the end of the 1/4 using different gear ratios.

    3.09 @ 113 mph in 5th gear = 5273 rpm
    3.70 @ 113 mph in 5th gear = 6315 rpm ( too high )
    3.70 @ 113 mph in 6th gear = 4895 rpm ( too low )
    3.90 @ 113 mph in 5th gear = 6656 rpm ( way too high )
    3.90 @ 113 mph in 6th gear = 5159 rpm

    too high or too low rpm and you lose acceleration.
    the 3.70 with close to stock hp is the wrong choice for the drag strip in 5th or 6th gear .(it could be a good choice for a high hp N/A or low hp F/I )
    also, look how close the finish line rpm is between the stock 3.09 in 5th gear is to the 3.90 is in 6th ...almost the same except the 3.90 accelerates faster . I gained .2 with my 3.90 swap.
    an N/A 6.4 with 3.90 and a cam that would go to 7k would be an interesting experiment.
    I'm trying to shine some light on why things turned out the way they did . not Sh*T on what you've done. hopefully I accomplished that...

    also have you looked into the Per4mance Development rear differential brace to fix your wheelhop ? I have one on my 700 whp Scat and it really tightens up the rear.. no hop period..
    Seems 3.45 rear would be ideal from your data points.

  11. #11
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    BTW, checking the pictures I'm surprised there is no C-clip on the axles to hold them in the diff....or did you remove them?

  12. #12
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    Quote Originally Posted by Chase300 View Post
    Seems 3.45 rear would be ideal from your data points.
    while there is no one size fits all ... 3.45-3.55 would fill a gap in the available ratios for FI cars specifically . with cam/head technology available today I still think the 3.70/3.90 is the way to go. would love to a serious N/A street car with one of the higher ratios but since FI is more cost effective I won't hold my breath.

  13. #13
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    Quote Originally Posted by Chase300 View Post
    BTW, checking the pictures I'm surprised there is no C-clip on the axles to hold them in the diff....or did you remove them?
    Best I could do with the zoom , but there are clips , just not removable prior to extraction. Kinda just pop clips. Being there's no diff cover , I'm guessing this is what zf came up with.

    Sent from my SM-G955U using Tapatalk

  14. #14
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    Def. one hell of a write up with a few guys adding on to make it complete. Makes me want to work on my car....keep 'em coming. Thanks
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  15. #15
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    I have a question. My 2015 Charger R/T has 220mm rear axles. I notice most differentials say they fit 230. Will I need to replace the axles to make the differential fit?

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