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  1. #1
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    Question Final drive ratio change on N54 6MT, from 3.08 to 2.56, thoughts?

    What do you guys think? Stock 6MT final drive is 3.08, Stock DCT 135/335 final drive is 2.56...i can get my hands on a built Quaife unit that uses the 2.56 final drive...

    I'd like to make the 1st gear a bit longer so it is more useful, and there is no way at the moment to change just the 1st gear's gear ratio as MT trannies aren't yet rebuildable...it'd also raise the car's overall top speed, it'd hit 400rpms less with the 2.56 final drive than the same speed than with the 3.08 in every gear...it'd also improve fuel economy if I ever let my wife drive it LOL

    I realize this may impact acceleration negatively but given the torque on my N54 is already over 500ft-lbs at the wheels with the current 3.08 final drive and 4.06 for the 1st gear the only thing that really does/will happen is either drivetrain damage or just really $#@!ty launch...

    Here are some specs on the 6MT gear ratios (final drive: 3.08):

    1st--4.06
    2nd--2.4
    3rd--1.58
    4th--1.19
    5th--1
    6th--.87

    The car is NOT a dedicated drag car, it is a street car...Thoughts?
    Click here to enlarge

  2. #2
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    What MPH does it hit in each gear with the 3.08?

    You are talking about lengthening the gearing by 17%, that's a pretty big change.

  3. #3
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    Here's what someone else said in some random thread:

    =====

    yeah it would be insanely tall but I just figured that if you trap around 120-125 in the 1/4 mile the 2.56 might be better because i did some calculations. 3.08 diff would be 108 mph in 3rd and 142 mph in 4th while the 2.56 is 85 mph in 2nd and 129 mph in 3rd.


    So with that in mind you are doin 1 less shift but of course you are getting to 129 a little slower in the 2.56 diff. but I rarely track and the 2.56 might be a little slower around the track but much better for cruising around town and on highways.

    ==
    Click here to enlarge

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    It might be quicker as well. Considering the time savings in shifting and the wide power band. There is no downside to it other than the work and cost.

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    Here it is:

    6MT 3.08 Final Drive:
    Click here to enlarge

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

    135/335 7DCT 2.56 Final Drive:
    Click here to enlarge

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

    6AT 3.46 Final Drive (not interested, just for sake of comparison):
    Click here to enlarge

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

    These are at 7k rpm, with Cobb, redline is raised to 7200rpm
    Click here to enlarge

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    Here's the 2.56 Final Drive with the 6MT 335i gearbox speeds at raised 7200rpm redline..i like where 3rd gear redlines a lot with this setup:

    Click here to enlarge
    Click here to enlarge

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    Got the results based on this calculator:

    http://www.apexgarage.com/tech/gear_ratios.shtml
    Click here to enlarge

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    1 out of 1 members liked this post. Yes Reputation No
    If you think you can get out quicker with the new gearing I'd say improving ET is a given even w the longer gear.

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    2 out of 2 members liked this post. Yes Reputation No
    It'll probably help a bit with the 1/4 times but good lord that is really tall gearing. Can you even shift into 6th gear at 60mph and cruise on the highway without lugging the engine?

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    2 out of 2 members liked this post. Yes Reputation No
    I'd give it a shot, actually for the type of pulls I do that gearing would be great.

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    Click here to enlarge Originally Posted by dzenno Click here to enlarge
    Here it is:

    6MT 3.08 Final Drive:
    http://www.bimmerboost.com/images/im...517tires-1.png

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

    135/335 7DCT 2.56 Final Drive:
    http://www.bimmerboost.com/images/im...517tires-1.png

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

    6AT 3.46 Final Drive (not interested, just for sake of comparison):
    http://www.bimmerboost.com/images/im...517tires-1.png

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

    These are at 7k rpm, with Cobb, redline is raised to 7200rpm
    Where did you find this calculator?

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    Click here to enlarge Originally Posted by Rob@RBTurbo Click here to enlarge
    If you think you can get out quicker with the new gearing I'd say improving ET is a given even w the longer gear.
    I'd wager more on improving trap than ET as he is bound to lose something in the 1/8 but gain on the back end.

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    1 out of 1 members liked this post. Yes Reputation No
    Unless you are making a dedicated drag machine, I think you are going to zap the fun right out of your car. That is a really huge difference in ratio, and probably not optimal for a street car. Honestly, I feel like 3.08's are already at the limit. Just my .02.

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    1 out of 1 members liked this post. Yes Reputation No
    I'm trying to go shorter gearing, and you're trying to go longer.. lol.. seriously though, like said above, I think this may effectively make your car a bore to drive around, may be good at the strip or what not, but if it's a daily driver... eh.. not so sure
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    Wish there was someone local willing to let me try their 2.56 diff on my car, I'd give them my Wavetrac 3.08 to drive on in the meantime Click here to enlarge
    Click here to enlarge

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    1 out of 1 members liked this post. Yes Reputation No
    send me your email and i'll send you a nice shift point calculator to see what's theoretically faster.

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    here it is:

    dzenno at gmail dot com
    Click here to enlarge

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    sent...also,this is a pretty good gear calculator:

    http://locost7.info/gearcalc.php

    I think you'll find you're theoretically faster with shorter gears.

  19. #19
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    Click here to enlarge Originally Posted by black bnr32 Click here to enlarge
    sent...also,this is a pretty good gear calculator:

    http://locost7.info/gearcalc.php

    I think you'll find you're theoretically faster with shorter gears.
    This one is awesome actually...just did entered numbers for 6mt (3.08) vs. 7dct (2.56) final drive with the 6mt gear ratios and it shows that on 275/40/17 tires I wouldn't need to get out of 3rd gear until 130mph which is really cool...3rd gear all the way from 85mph->130mph, no 3-4 shift into 4th and torque to the wheels due to overal gear ratio is higher than if 4th gear is involved...i think that this'd actually see a noticeable improvement in the 1/4...probably not so great in auto-x but maybe also great in road coarse, depending on the coarse setup obviously..

    Here's the results from the above calculator if I go with a 2.56 final drive (3rd gear would allow for 130mph which is probably within max limits on RBs which would eliminate 4th gear altogether):

    Click here to enlarge

    Click here to enlarge

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

    For reference here's the current 3.08 final drive situation (requires a good bit of driving in 4th):

    Click here to enlarge

    Click here to enlarge

    I'm starting to like this 2.56 final drive on a 6mt given the power/torque band with RBs...its not bad at all actually for daily driving either, might actually like that better too...
    Click here to enlarge

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    i think your gonna screw yourself on dragway performance

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    1 out of 1 members liked this post. Yes Reputation No
    You said this is a street car, so that being said...How bad do you spin launching hard and the street from a dig or slow roll in first and second? (on your street tires)

    If you spin a lot, which you likely do at your power level i would think that it would probably help you times as you will hook up much better plus save time in the shift. I don't think you need to worry about taking the fun out of the car as you can always downshift.

    Are you currently using the JB4 for traction control in 1st and 2nd?
    Never thought I would see the day...
    Click here to enlarge Originally Posted by Terry@BMS Click here to enlarge
    Life is so much more fun with a nemesis. I miss Shiv. Click here to enlarge
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  22. #22
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    Click here to enlarge Originally Posted by LostMarine Click here to enlarge
    i think your gonna screw yourself on dragway performance
    you may be right...it'd be reducing torque to the wheels in 1st by about 20% which is great for 1st gear as there's way too much anyway...then in 3rd it'd be down 17% in torque to the wheels due to overall ratio and that can be bad, as it'd be 85mph all the way to 130mph in 3rd gear...on top of that there's a high cost involved...i'm thinking boost redux in 1st to reduce the torque at the engine before it gets multiplied by the gears to the ground is probably a MUCH better way to go here to take care of traction in 1st, 2nd and then get the full torque to the wheels by means of the current oem final drive (3.08) at full boost in 3rd and 4th...


    Click here to enlarge Originally Posted by Forced Air Click here to enlarge
    You said this is a street car, so that being said...How bad do you spin launching hard and the street from a dig or slow roll in first and second? (on your street tires)

    If you spin a lot, which you likely do at your power level i would think that it would probably help you times as you will hook up much better plus save time in the shift. I don't think you need to worry about taking the fun out of the car as you can always downshift.

    Are you currently using the JB4 for traction control in 1st and 2nd?
    Street tires, its b-a-d Click here to enlarge a bit better in 2nd with 295s out back but regardless 1st is useless...

    dialing in boost by gear would probably be the best thing to dial in here...1st gear limited to like 2psi Click here to enlarge

    3.46 from the 6AT might actually be a better choice combined with a dialed in boost by gear, or it might not...final drives still aren't cheap enough to play with on this platform
    Last edited by dzenno@PTF; 01-26-2012 at 12:58 AM.
    Click here to enlarge

  23. #23
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    Do it. Take any factory 125 plus manual car and look ar the gearing. Zo6 come to mind and their gearing is even longer then what you want. Hell if i ever need to drop the pumpkin im doing it.

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    Click here to enlarge Originally Posted by Laloosh Click here to enlarge
    Do it. Take any factory 125 plus manual car and look ar the gearing. Zo6 come to mind and their gearing is even longer then what you want. Hell if i ever need to drop the pumpkin im doing it.
    What would be a factory 125+ manual car to look at that is geared for the strip?

    Here's numbers for a C6 Z06 (combined 3rd gear is shorter (4.4) than what it'd be my car with the 2.56 final drive (4.0):

    Click here to enlarge

    Click here to enlarge
    Click here to enlarge

  25. #25
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    Click here to enlarge Originally Posted by yandyr Click here to enlarge
    I'm trying to go shorter gearing, and you're trying to go longer.. lol.. seriously though, like said above, I think this may effectively make your car a bore to drive around, may be good at the strip or what not, but if it's a daily driver... eh.. not so sure
    After trying shorter gears, I'm going to go longer as well especially with way more power and torque than gearing setup for stock.

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