lower rear gears for M6 Hellcat

Discussion in 'SRT Hellcat Performance' started by carcass, Jan 24, 2016.

  1. carcass

    carcass SRT Hellcat Supporter Gold Supporting Member

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    I've read several threads about the 3.09 gearset swap for the A8 and folks seem to be happy with the results. But I cant seem to find any confirmed lower gear direct bolt in swap for the 3.70 in the M6.
    Has anyone done a lower gear? I think first gear is way to tall in my car, and I think its at fault for the clutch problems in part.
    where to get lower gears and what is available if anything?
     
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  2. carcass

    carcass SRT Hellcat Supporter Gold Supporting Member

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    will the 3.90 differential from the SRT392 car fit in the Hellcat? is it the same positraction carrier and ring gear size?
     
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  3. TrackDay

    TrackDay Silver Member

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    Yes. The difference in leaving a traffic light is very noticeable. 1st gear is not enough to me with the 6 speed.
     
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  4. carcass

    carcass SRT Hellcat Supporter Gold Supporting Member

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    What gear ratio do you have? And where did you get it, what car was it intended for from Dodge, is it a direct bolt in?
     
  5. TrackDay

    TrackDay Silver Member

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    My personal car is an A8 but I've driven a M6 with a 3.9. The rear diff from the 392 will bolt in as the 3.09 does for the A8.
     
    Last edited: Jan 26, 2016
  6. PurpleCat16

    PurpleCat16 Active Hellcat Member

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    The M6 in the Hellcat also has different Gear Ratios in the Transmission than the M6 in the SRT 392. The 3:92 Rear Axle Ratio will no doubt improve the ET in the 1/4 mile, while sacrificing top speed and and the engine RPM will be higher on the Freeway....
     
  7. Wack_Cat

    Wack_Cat Senior Hellcat Member

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    So has anyone done a swap in a cat yet. If so what is the part number and how do you like it?
     
  8. carcass

    carcass SRT Hellcat Supporter Gold Supporting Member

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    Bumped to the top for this thread hopefully real world experience and some photographs of an install will prove this swap legitimate I would like to lower the gear ratio of my manual hellcat but the difference between 3.7 and 3.9 may not be worth the cost I have used a gear ratio speedometer calculator every which way from Sunday and the difference is small but no doubt helpful $1500 helpful ??
     
  9. carcass

    carcass SRT Hellcat Supporter Gold Supporting Member

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    I haven't found any actual dragracing statistics regarding this swap.
    Please forgive me if that information is on this thread but linking it here would be great if anyone can help
     
  10. NorCaletch19He77Kat

    NorCaletch19He77Kat Silver Member

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    I will do the swap in the next month--a swap to the 390 gear wont cost any top speed--the car will pull to the same top end.. the lower gear will dramtically improve 1st gear drivability and stop 90% of clutch issues as well--the gear from the factory is for fuel mileage period believe or or not... yes--they are still concerned with C.A.F.E. even with the HellCat since it affects overall corporate ratings.
    I do believe the ratio spread in the tranny is the real culprit, but I have seen no sensible transmission solution so far.. similar issue to the diff in top loaders--wide ratio vs close.. wide ratios always drove better actually.
    will update when complete, but I have done a similar swap in many performance cars with no real sacrifice on top end.
    with the 390 gear installed, my next step will be to increase rpm limit to 6500--the engine can easily handle the rpm increase..
     
  11. carcass

    carcass SRT Hellcat Supporter Gold Supporting Member

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    Love to see some back to back 0 to 60 times and 60 foot marker times at the track !!
     
  12. carcass

    carcass SRT Hellcat Supporter Gold Supporting Member

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    First is so tall of a gear I often times find myself shifting straight from 1st to 3rd, the factories skip shift that went from 1st to 4th was too much but 1st to 3rd is more comfortable while driving lazily
     
  13. carcass

    carcass SRT Hellcat Supporter Gold Supporting Member

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    Jam transmissions has a number of cool calculators for gears, this starting line ratio for first gear is interesting for the difference between 3.7 and 3.9 rear gears with a 2.26 first.
    StartLineRatio
    8.362 for the 3.7's
    8.813 for the 3.9's
    with a 28.6 inch tire and 4500 lbs a start line ratio in the eights?
    I'd like to try some 4.10's or so.
    calculator says the start line ratio would go to 9.26.
     
  14. carcass

    carcass SRT Hellcat Supporter Gold Supporting Member

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    Link to DSS website for their ford nine inch conversion differential setup.
    Stronger and more gears, but certainly wont pass for stock.
    I like the options of different posi units too, the true track seems a lot better than an old fashioned clutch posi, I wonder how traction control computer would be affected.
    The Driveshaft Shop | 2009-2014 LC Challenger Automatic 9-Inch Direct Bolt-in Rear Conversion Kit -includes Axles / Hubs and 4'' Aluminum CV shaft ////// DIRECT BOLT-IN KIT\\\\\\ - Differential Conversions
     
  15. PurpleCat16

    PurpleCat16 Active Hellcat Member

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    You will lose some top end, but how often do you drive over 150... You would probably see an even more dramatic increase in 1/4 mile performance with the 6 Speed from the 392 and the 3:90 Gears.... which also has a very tall 5th and 6th so you would also get better top end and highway mileage would improve.... the drawback being that the Transmission and Differential from the 392 are not built with the same higher quality parts as those for the Hellcat
     
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