Transmission A8 limit

Discussion in 'SRT Hellcat Performance' started by Hector02, Aug 28, 2015.

  1. Hector02

    Hector02 Gold Member

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    Hi
    what is the limit in whp for the A8 transmission on the Charger HC?
    What can be done to the transmission to support more whp?


    Thank you!
     
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  2. SRT_HC_707

    SRT_HC_707 SRT_HC_707 Staff Member

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  3. TallCool1

    TallCool1 Gold Member Hellcat Car Club Gold Supporting Member Nat'l Hellcat Tech Advisor HCC Charter Member

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    Someone posted quite recently a shop that does performance mods to ZF trannys (can't find it now.)
     
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  4. TallCool1

    TallCool1 Gold Member Hellcat Car Club Gold Supporting Member Nat'l Hellcat Tech Advisor HCC Charter Member

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  5. mowe

    mowe Gold Member

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    with a few 800-900 hp modified cars we have seen them have issues. I have seen them not want to shift into gear.... Guess we will see
     
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  6. Hector02

    Hector02 Gold Member

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    Thanks guys!
    Just want to see what upgrade can be done to be safe.
     
  7. TIGwelder

    TIGwelder SSDD

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    This Is something I posted elsewhere quite a while ago. Hopefully the links are still good



    I figured since this is new for the Challenger Hellcat, I would dig up some info and we can discuss this new transmission. Here are some informative

    links, a video showing the internals and some specs.

    An interesting thing is the converter can lock in first gear. Were the older units the same way?
    Also it can shift from 8th to 2nd.

    Along with a separate chain driven pump and other optimizations it looks like they worked hard to reduce parasitic loss.

    Main info:
    8HP Automatic Transmissions

    Video:
    ZF 8HP90 - Video

    NEW LINK Cars | 8-Speed Automatic Transmission - ZF Friedrichshafen AG

    Specs:
    8HP90_DataSheet.pdf

    ZF8 Info at Allpar
    845RE / 8R70: ZF 8-Speed Automatic Transmission for Chrysler cars

    ZF 8 Speed Automatic Transmission - Equipment and Tool Institute Tech Day - June13 2012
    Chrysler PDF Document


    ==============================================


    Let's look at the transmission ratios for the Hellcat

    1st -- 4.714
    2nd -- 3.143
    3rd -- 2.106
    4th -- 1.667
    5th -- 1.285
    6th -- 1.000
    7th -- 0.839
    8th -- 0.667

    That first gear is crazy. I decided to do this because I wanted to know what was up with all the second gear starts I'm seeing. Then I wanted to compare it to other stuff. And then I just went silly with it :)



    Let's compare it to the NAG1 WA850

    1st -- 3.588
    2nd -- 2.186
    3rd -- 1.405
    4th -- 1.000
    5th -- 0.831

    So second is bit taller than first in the NAG1. ( Hellcat 2nd 3.142 vs NAG1 3.588 = 0.445 difference)


    Let's check out old school with the 904/727 for reference

    1st -- 2.45
    2nd -- 1.45
    3rd -- 1.00
    ==============
    4th -- 0.69 ( later 4th used with above ratios, even the SRT Ram10 )



    =========================================================



    Final Drive Ratio
    -----------------
    For the Hellcat with the 8 speed it's 2.62
    Tire: The 275/40R20 Pirelli P Zero is 28.7 inches in overall diameter.


    Let's look at gear multiplication, first gear comparisons for several ratios/setups.


    Hellcat
    ( 1st x Final )
    4.714 x 2.62 = 12.35068

    (2nd x Final )
    3.143 x 2.62 = 8.23466 (How I've seen people documenting 1/4 mile passes)


    Old NAG1
    ( 1st x Final )
    3.588 x 3.06 = 10.97928


    And to show the gearing now to my old school brain, let's compare it to some older stuff.

    OLD school 727/904 with various gear ratios

    ( 1st x final )
    2.45 x 4.10 = 10.045
    2.45 x 4.56 = 11.172 ( Close to NAG1 )
    2.45 x 4.88 = 11.956

    4.88s and still not there yet!!

    2.45 x 5.13 = 12.5685


    Hellcat vs Old School
    To compare a 1st gear start - Your final drive ratio would be around 5.04 with a 28" tire ( wow )
    To compare a 2nd gear start - Your final drive ratio would be around 3.36 with a 28" tire


    ================================

    So back to the Challenger Hellcat. Let's look at max speed in each gear.

    It's basically
    MPH = ( RPM x tire dia ) / ( gear ratio x 336 )
    or
    MPH = ( 6000 x 28.7 ) / ( gear ratio x 336 )
    where 'gear ratio' will be ( trans ratio x Final )

    or ( 172,200 / (( trans ratio x Final ) x 336 )) @ 6000 RPM
    AND ( 177,940 / (( trans ratio x Final ) x 336 )) @ 6200 RPM


    1st -- 4.714 x 2.62 = 12.351 ( x 336 = 4,149 ) = 41.5 MPH @ 6000 -- 42.8 MPH @ 6200
    2nd -- 3.143 x 2.62 = 8.235 ( x 336 = 2,766 ) = 62.3 MPH @ 6000 -- 64.3 MPH @ 6200
    3rd -- 2.106 x 2.62 = 5.518 ( x 336 = 1,854 ) = 92.9 MPH @ 6000 -- 95.9 MPH @ 6200
    4th -- 1.667 x 2.62 = 4.368 ( x 336 = 1,467 ) = 117.3 MPH @ 6000 -- 121.29 MPH @ 6200
    5th -- 1.285 x 2.62 = 3.367 ( x 336 = 1,131 ) = 152.2 MPH @ 6000 -- 157.32 MPH @ 6200
    6th -- 1.000 x 2.62 = 2.620 ( x 336 = 880 ) = 195.6 MPH @ 6000 -- 202.20 MPH @ 6200
    7th -- 0.839 x 2.62 = 2.198 ( x 336 = 738 ) = 233.3 MPH @ 6000 -- 241.11 MPH @ 6200 ( we're past the aero limit here )
    Then we enter driving in a vacuum land :)
    8th -- 0.667 x 2.62 = 1.748 ( x 336 = 587 ) = 293.3 MPH @ 6000 -- 303.13MPH @ 6200

    That's right..let it rip to redline in 8th with zero wind resistance and look at the speed!! haha!


    Note: I watched the K&N dyno run, the shift into 6th was around 148MPH, 7th at 191
    So my above results are close. There is some rounding in the above numbers.
    Also the Tire dia used plays a part ( no resting weight, air pressure, using tire specs, etc ).

    ==========================

    So what did I learn.

    - Well, I have to much time for one :)
    - Starting in 2nd has a gearing disadvantage to older NAG1 and gears used before. They have 2nd gear start in valet mode and it would be nice to be able to toggle that feature separately in transmission settings. If 2nd gear start is used then gearing could be changed.
    - It's insane how final gearing in first gear is compared to old school cars. 5.04s on a 28" tire, really? Need to stop thinking old school gearing.

    Questions:
    When does the converter lock? I read that it can lock in first gear ( "The tall first gear allows for early lockup of the torque converter clutch. -- Allpar ). Under what conditions though? When does it lock on launch or hard acceleration?


    Maybe somebody will throw in a spreadsheet to play with ratios/tire sizes
    Anybody want to tackle the 6M??
    Mistakes? Let me know!

    Discuss :)


    ===============================================
    Edited:
    v1.02 - Changed/recalculated old NAG1 gearing to 3.06, not 3.09
    - Changed "steeper" to "taller"
    - Added MPH calcs for 6200 RPM
    - Added link for Allpar ZF8 info
    - Added converter lockup quote from Allpar ZF8
    - Added Chrysler document link
     
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  8. TIGwelder

    TIGwelder SSDD

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  9. Carnage

    Carnage SRT Hellcat Supercharged Moderator Staff Member Hellcat Car Club Gold Supporting Member HCC Charter Member

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    man talk about need for speed haha
     
  10. Lucky Campbell

    Lucky Campbell Rookie

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    Max RPM 1 - 7 is 7000, max RPM in 8th is 5600
     
  11. Hector02

    Hector02 Gold Member

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    I was looking at various site but neither tells what exactly can be done so the transmission support more than the 700 lb of torque
     
  12. sinister707

    sinister707 Silver Member

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    i'm a rookie at this so please be gentle :)

    so i read the post and most of the documentation... is it really a flange that separates our transmission from supporting AWD?

    Reason i ask is because as you know at SEMA they unveiled an AWD Challenger... I'm looking at making my somewhat of a handling vehicle as opposed to a dragster. AWD would help :)
     
  13. DGatzby

    DGatzby Gold Member Hellcat Car Club HCC Charter Member Midwest Regional Pres.

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    We have a member doing 9.7 second quarters, in a A8 Charger so go for it.

    Way to go Linda and TC!
     
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  14. ArizonaGasMonkey

    ArizonaGasMonkey Hellcat Member

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    TigWelder that's great info to share.
     
  15. Racist Cat

    Racist Cat Silver Member

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    I always drive my car in manual mode, and the TCC locks around 1,300rpm in 1st give or take a little.

    I also have an eco diesel. I think the 8spd auto in that sucker is very similar to the HC Trans.

    There are twin turbo kits out for the Eco diesels that put them around 450rwhp. (If the oilfield was doing better I'd have a kit on mine)
    The hp isn't the deal, but the torque getting close to 1,000lbs is interesting. Suncoast already has parts available. That is, if it's the same Trans.
     

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