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Thread: Limited slip for a 915...price?

  1. #1

    Limited slip for a 915...price?

    I have a 3.2 and 915 in my autocross/track car, that desperately needs a limited-slip.

    What do you think I should expect to pay to put in a limited-slip?

    thanks
    "Speed never killed anyone. Suddenly becoming stationary...that's what get's you."

    1973 Austin Mini
    1975 911S 3.2 "the blue goose"
    1973 911E sunroof 3.2

  2. #2
    i paid $1k for a factory ZF and then had it rebuilt.
    __________
    Sean
    1971T restoration in progress http://911restorationmadness.blogspot.com/
    1982 PCA club racing #372 D-Stock
    Registry #1595

  3. #3
    A quaife used to run around 1500, IIRC. I see ads for 1100 now.
    http://www.nbdgraphics.com
    73/83/93 RS clone, 3.6

  4. #4
    Quaife is a Torsen Type Diff doesn't have any lock up on coast - it behaves entirely the same way as an open diff once you lift off the accelerator.

    In a race car one of the benefits of an LSD would be to stop it locking an inside rear on turn-in so you could move the brake bias more to the rear and brake a bit later.

    A Quaife dosn't help in this area it only affects beaviour on the power side and this can't really be adjusted in the way a plate type diff can as the lock characteristics are a feature of the helix angles on the gears.

  5. #5
    Goldmember ttweed's Avatar
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    Dave,
    This is a complex decision to make, and depends on your budget and the use of the car. It is true that a torque-biasing (torsen-type) diff does not provide locking on decel, or when one drive wheel is off the ground, but if you primarily autocross, and just want to eliminate wheelspin on corner exit, it might serve your purposes well at a reduced initial cost over the plate-type LSD, and does not require any regular maintenance and rebuilding, so a used one can be very cost-effective. If you do a lot of big track events and want the added stability under braking that an LSD offers, definitely spend the extra $$$ and just deal with the understeer in slow, tight corners it will induce by setup changes and driving style when you autox the car. Personally, I would not mess with a used LSD. By the time you get done rebuilding it, you will be in it for nearly the cost of a new one. You can't go wrong with these guys:
    http://www.guardtransmission.com/
    Better than OEM bits, IMHO. From their FAQ:

    How do I choose a differential for my vehicle?

    The most typical installation choices for our differentials are as follows:
    (a) Street / autocross - Torque-biasing suitable for most applications, especially autocross. 40/60 (or 40%) LSD for all Turbo and high-performance street cars.
    (b) Street / track - Torque-biasing for light track use, with stock suspension. 40/60 (or 40%) LSD if car has stiffer suspension or power upgrades.
    40/60 (or 40%) LSD for any car used primarily for track.
    (c) Track only - 50/80 LSD Special order “Zero-Preload” LSD, with 4, 6, or 8 friction discs

    Limited-slip differentials provide lock-up on both acceleration and deceleration. The amount of lock-up on accel and decel can be adjusted by selection of the internal plate sequence and/or ramp selection. Lock-up on deceleration allows late braking and aggressive entry into high speed turns, reasons why all professional Porsche road- racing teams utilize LSDs, rather than TBDs.

    Torque-biasing differentials provide lock-up on acceleration only. The amount of lock- up increases as the amount of torque increases. On deceleration, lock-up is negligible, making the TBD ideal for the slower-speed turns of Autocross. (In the same slow speed turn, an 80% LSD would most certainly cause unwanted understeer.)

    A torque-biasing differential performs like an open diff whenever one of the drive wheels approaches lifting off the ground. Lock-up and traction are lost until both drive wheels are again planted firmly on the pavement. Distinct limitations exist when a TBD is incorrectly used in a rear-engined or high speed track race car.
    A great deal of confusion is caused by optimistic sales information distributed by companies that only offer torque-biasing differentials, or when a TBD is described by a vendor as a “limited-slip differential”. As Guard offers both differential types, we try to present as much information as possible, so that an informed choice can be made based on suspension set-up and intended use of the vehicle. Too often, we have seen the full potential of a car severely limited by use of the incorrect differential type, or by a low quality Chinese knock-off “LSD” that fails to provide long-term performance.


    Plan on spending around 2 "race-bucks" on this mod.

    TT
    Last edited by ttweed; 01-06-2012 at 10:39 AM.
    Tom Tweed
    Early S Registry #257
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  6. #6
    Tom
    Thanks alot. This is good info. My car is 90% autocross and 10% non-competitive track days, so I think the torsion type might be what I need. Additionally, it's hookup on acceleration coming out of corners that I'm looking for.
    I'll investigate further, and consult further with my mechanic as well.
    thanks again

    Dave
    Attached Images Attached Images  
    "Speed never killed anyone. Suddenly becoming stationary...that's what get's you."

    1973 Austin Mini
    1975 911S 3.2 "the blue goose"
    1973 911E sunroof 3.2

  7. #7
    Quote Originally Posted by davewadd View Post
    Tom
    Thanks alot. This is good info. My car is 90% autocross and 10% non-competitive track days, so I think the torsion type might be what I need. Additionally, it's hookup on acceleration coming out of corners that I'm looking for.
    I'll investigate further, and consult further with my mechanic as well.
    thanks again

    Dave
    Great photo ... looks fast.
    -Marco
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    Searching for engine #907495 and gearbox 902/1 #229687

  8. #8
    I am a little confused how a torque biasing diff is described as either 40/60 or 40%. 40/60 usually relates the ramp angles which determine the rate of lock up depending on the number and way the plates configuration.

    40/60 is usually the definition of the ramp angle of the diff. The shallower the angle the greater the load applied to the ramp driver and the greater the rate at which the diff locks up. The number of plates, thier configuration and the type of friction surface all effect the detail of how the diff operates. Preload is also another very important variable and needs to be carefully considered depending on the application.

    It is convention to define the Ramp angles in the format of Power/Coast. 40 means as a 40 degree angle for the power ramp and clearly 60 degrees would be angle of the coast ramp.

    As Torque Biasing diffs don't have any locking capability on coast I can't figure out how this description applies.

    40% is a description of 'locking factor' and a typical Quaife diff has a 4:1 Bias Split which is one of their stated advantages in poor traction conditions. I think the Wavetrac for example has a greater bias capability than a Quaife but I am not 100% sure

    A 40% locking factor works out to a bias ratio of 2.33:1 and this seems too low. A 4:1 Bias would be an 80% locking factor.

    A 4:1 bias means that 20% will go to the low traction side and 80% will go to the high traction side.

    If your chassis set up is relatively soft the Quaife could be an issue in a hard driven car as has been stated does it not bias torque on coast it also doesn't bias torque when one wheel is unloaded - this is defined in most technical articles as when a wheel is lifted but looking in more detail the situation is not quite so straightforward.

    In practice the inside rear wheel must have at least 20% of the rear weight applied in order to have maximum torque biasing and maximum thrust of both tyres. Less than 20% weight on the inside tyre means less thrust capability.

    For example if the weight transfer nets only 10% of the weight on the inside wheel, it reduces the maximum thrust to just 50% of the total obtainable if the chassis were set up correctly. The Torque Biasing differential will continue to bias at the 80% rate, but since only 10% thrust will be available on the weak wheel, the better wheel will still have 4 times the thrust of the inside wheel, or 40% of potential. 10% + 40% = 50% or half the possible thrust.

    If the chassis is too stiff then the way torque is biased can introduce power oversteer.

    As with plate type diffs; ATB's are not just simple fit and forget devices but need to be considered in terms of the whole set up.

    I don't feel that you can just say fit Type A for this application and fit Type B for another application.

    I believe that you need to define the reason for fitting an LSD type device and the change in characteristic you are looking for and make a choice on this basis.

    I know several people that have installed basic KAAZ 1.5 way plate diffs to their road cars and are entirely happy. These diffs have light preload and modest ramps and are very well engineered at a decent price.

    Whichever decision you make will involve a significant investment once the unit is installed in the car and does deserve some analysis.
    Last edited by Joe901; 01-07-2012 at 02:35 AM.

  9. #9
    Senior Member uai's Avatar
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    If you want the best go to http://drexler-drivetrain.com/ very Pricey but the best LSD money can buy.
    Cheers

    Uli
    Last edited by uai; 01-07-2012 at 06:04 AM.

  10. #10
    There are many different LSD's available most are plate type and a couple are ATB - It would be interseting to compare them all.

    Drexler Diffs are clealry well made but can you really justify the significant price that they charge ?

    Plate Type:

    Original ZF - rebuilt with Aftermarket Plates

    ATS - Japan

    ATB

    Quaife - UK

    Wavetrac - USA

    There are other mnaufacturers such as Cusco and TranX whoc make LSD's but have nothing for Porsche 911s.

    Drenth - Netherlands

    Gripper - UK

    Bacco Romano - Italy

    Colotti - Italy

    OS Giken - Japan

    KAAZ - Japan

    Modena Engineering - Australia

    Carbonetic - USA

    Guard - USA

    X Trac - UK

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