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3.90's vs 4.11 rear change

This is a discussion on 3.90's vs 4.11 rear change within the Drivetrain forums, part of the LSx Technical Help Section category; I've decided to change the rear on my '99 M6 1LE. I need to get confirmation from this forum that ...

  1. #1
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    1999 WS6

    3.90's vs 4.11 rear change

    I've decided to change the rear on my '99 M6 1LE. I need to get confirmation from this forum that the stock rear is 3.42. With recommendations of how much higher to go and why. The original window paper says it came "Tenser" limited slip rear, but I'm not impressed that that did much!! Is a version of Posi-traction that is better available? Any recs on this?

    Thanks, bma2

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    O U 8 1 2 Spaz's Avatar
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    i believe these are the rpo codes on your door sticker... see if any of these match up...


    GU5 : AXLE REAR, 3.23 RATIO
    GU6 : AXLE REAR, 3.42 RATIO


    it's a copy and paste and i don't have time at the moment to verify...

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    Firebird Encyclopedia 9T8W66's Avatar
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    All factory M6 cars from 1994 - 2002 came with 3.42 gears.

    4.10's are by far the most popular swap for M6 cars.
    I currently run 3.90's and I'm very happy with the added performance.
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    Your car probably has a Torsen (TORque SENsing) limited slip differential. Yes it's a form of Posi. If stock then it has 3.42 gears.
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    1999 WS6

    3.90's vs 4.11 rear change

    Looking for more 'pop' at the low end on the street. So I'd like opinions on changing my 1LE M6 3.42's to either 3.90 or 4.11. And is the 'Torsen' posi system 'OK' or is there something significantly better? I've been UNimpressed with Torsen at the drag strip. I've run 12.08/125.8mph on street tires and I never flat shift my engine. Don't really intend to return to the 1/4 mile... it's just for fun and trips with the wife. Opinions please.

    Thanks, bma2

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    Member DaddySS's Avatar
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    3.73 will give you a little more pop and make 6th gear more usable
    3.90 tends to be noisy because of the number of teeth, harmonics - several comapnies don't sell them for that reason.
    4.10 will give you more pop, can be noisier than 3.73, you will need to shift to 5th at the track, and of course will cost a bit in gas mileage.

    If it's just a fun car then it sounds like 4.10s are for you.

    The Torsen has no clutches so no maintenance or slipping, the other unit in use is the auburn which is a standard clutch based set up.

    Our sposnor New Era has the gears and the posi unit - and I recommend the GM (AAM) gears - much more tolerant in set up and generally quieter.

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    Thanks DaddySS.
    So if I go with 4.10's, do most find interstate trips at 80-85mph are still OK in 6th gear?
    And is the Auburn rear a huge improvement over the 'Torsen'? or only relevant at the 1/4 mile?

    Thanks, bma2

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    Your dealership guy konigandy6's Avatar
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    Quote Originally Posted by bma2 View Post
    Thanks DaddySS.
    So if I go with 4.10's, do most find interstate trips at 80-85mph are still OK in 6th gear?
    And is the Auburn rear a huge improvement over the 'Torsen'? or only relevant at the 1/4 mile?

    Thanks, bma2
    I run 4.11's and at 70mph I'm spot on 2k rpm. It is very tolerable for highway speeds.

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    ?

    when i bought my 98 z28 m6 it came with 4.11s what drive and performance difference does it do

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    Quote Originally Posted by DaddySS View Post
    3.73 will give you a little more pop and make 6th gear more usable
    3.90 tends to be noisy because of the number of teeth, harmonics - several comapnies don't sell them for that reason.
    4.10 will give you more pop, can be noisier than 3.73, you will need to shift to 5th at the track, and of course will cost a bit in gas mileage.

    If it's just a fun car then it sounds like 4.10s are for you.

    The Torsen has no clutches so no maintenance or slipping, the other unit in use is the auburn which is a standard clutch based set up.

    Our sposnor New Era has the gears and the posi unit - and I recommend the GM (AAM) gears - much more tolerant in set up and generally quieter.
    Good info
    But 4.10's will top 4th gear between 112-115mph at 6000rpm. A stock LS1 isn't going to hit that speed thru the traps. so hitting 5th gear isn't an issue. And a modified car with a higher rev limit would still be good.

    BTW my 3.90.s only added 200 more rpm in every gear. 4.10's should only be another 50-100 rpm above that.

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    Your dealership guy konigandy6's Avatar
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    Quote Originally Posted by 9T8W66 View Post
    Good info
    But 4.10's will top 4th gear between 112-115mph at 6000rpm. A stock LS1 isn't going to hit that speed thru the traps. so hitting 5th gear isn't an issue. And a modified car with a higher rev limit would still be good.

    BTW my 3.90.s only added 200 more rpm in every gear. 4.10's should only be another 50-100 rpm above that.
    I have to just shift into 5th crossing the trap.

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    Quote Originally Posted by konigandy6 View Post
    I have to just shift into 5th crossing the trap.
    Shouldn't be shifting into OD gears at WOT

    Seen cars with 4.56's go 131MPH in 4th (granted, 28" tire, but still).
    All depends on set-up.


    On a 26" tire and 4.11's....75MPH should be exactly 2000RPM. That is where mine sits....and I still get decent highway mileage.
    Thinking I want 4.56's though.

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    Quote Originally Posted by Mr. Luos View Post
    Shouldn't be shifting into OD gears at WOT

    Seen cars with 4.56's go 131MPH in 4th (granted, 28" tire, but still).
    All depends on set-up.


    On a 26" tire and 4.11's....75MPH should be exactly 2000RPM. That is where mine sits....and I still get decent highway mileage.
    Thinking I want 4.56's though.
    Agreed

  14. #14
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    So it appears 4.10's in present stock rear are the ticket. Since I don't intend to flog the car at the strip, it should survive? I'm running the stock 17" WS6 wheels with 285's(Continental). Gonna leave the Torsen rear 'as is'. Local mechanic is offering $530 labor if I get all the parts---is that appropriate?

    Any other things to consider?

    Thanks, bma2

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    Member JRENIGAR's Avatar
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    oh hell no, thats way too much. I payed 150 for mine. Id suggest if your the type that does his own headers that type of thing then this is no big deal either. Get a pinion depth guage from jegs or summitt for 99 and do it yourself. Its about a 2-3 hour job and you could save about $430 on the labor rate you quoted. Just be sure and read the breakin directions for the gears and folow it to the T...

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    What have you done to your car to trap 125 mph??

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