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    Jul 21, 2004 10:32:39 AM

    fantasma
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    fantasma
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    LSD vs ABD - pros and cons?

    The facts:
    - Automatic Brake Differential (ABD) is an integrated part of PSM and uses the brakes to emulate a limited slip diffential.
    - A mechanical LSD is available with the 997 Sports Suspension (which also includes lower ride height and different springs/dampers).

    My question is:
    What are Pros and the Cons of a mechanical LSD compared to ABD?

    On my BoxsterS without PSM (and without LSD) the inner back wheel tends to brake loose when accelerating hard out of slow corners. I understand that both ABD or LSD would prevent such behaviour, right?

    I do understand the the Sports suspension brings other benefits besides the LSD, but that is another topic.

    /fantasma

    Jul 21, 2004 11:02:36 AM

    Carlos from SpainM
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    Carlos from Spain
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    Reply to: fantasma

    Re: LSD vs ABD - pros and cons?

    Quote:
    What are Pros and the Cons of a mechanical LSD compared to ABD?

    On my BoxsterS without PSM (and without LSD) the inner back wheel tends to brake loose when accelerating hard out of slow corners. I understand that both ABD or LSD would prevent such behaviour, right?



    Hi fantasma,

    ABD and LSD both prevent the rear inner wheel from spinning freely and robbing all the torque from the outer wheel but the way they do is different. The ABD being a electronic differental that just acts via the wheel's brakes is not as sporty as the mechanical LSD, it is more intrusive and not as effective sport performance wise or at the track though its better than not having any differential.

    The reason for Porsche fitting the ABD instead of a LSD is because ABD up unitl now was not compatible with PSM, so if you had PSM you couldn't have LSD also, so after MY99 when PSM was introduced in the 996C2, the LSD option that was availbale for the C2 at the time dissapeared. Now Porsche can offer LSD along with PSM (911 40th anniv, 996TTS, 997 w/ -20mm susp) but with much lower lock up rates than usual still.

    Hope this answers your question

    Jul 21, 2004 12:04:04 PM

    airkewl
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    Reply to: Carlos from SpainM

    Re: LSD vs ABD - pros and cons?

    A clutch type rear differential will also lockup on deceleration (braking into a corner), and will help the handling of a properly set up car (mostly in a track situation). ABD & Torsen diffs won't do this.

    http://eaglewoman.racesimcentral.com/gpl/grehelp/diff.htm

    Jul 21, 2004 10:28:37 PM

    FixedWing
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    FixedWing
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    Reply to: airkewl

    Re: LSD vs ABD - pros and cons?

    Quote:
    airkewl said:
    A clutch type rear differential will also lockup on deceleration (braking into a corner), and will help the handling of a properly set up car (mostly in a track situation). ABD & Torsen diffs won't do this.




    I thought that a Torsen system would provide a degree of lock-up on overrun ???

    Stephen

    Jul 21, 2004 10:35:17 PM

    Grant
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    Reply to: FixedWing

    Re: LSD vs ABD - pros and cons?

    Quote:
    FixedWing said:
    Quote:
    airkewl said:
    A clutch type rear differential will also lockup on deceleration (braking into a corner), and will help the handling of a properly set up car (mostly in a track situation). ABD & Torsen diffs won't do this.




    I thought that a Torsen system would provide a degree of lock-up on overrun ???

    Stephen



    No, airkewl is right on that one. Torque biasing diffs only provide locking upon power application. They act as an open diff on overrun. You need a clutch-type LSD to work both ways.

    Jul 22, 2004 12:33:49 AM

    FixedWing
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    Reply to: Grant

    Re: LSD vs ABD - pros and cons?

    Quote:
    Grant said:
    airkewl is right on that one



    Good to know. Thank you Airkewl and Grant.

    Ok, so what about a Haldex differential? This one is clutched. So does that mean it is likely to have some lock-up on the overrun??

    Stephen

    Jul 22, 2004 2:21:11 AM

    W8MM
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    Reply to: FixedWing

    Re: LSD vs ABD - pros and cons?

    Quote:
    FixedWing said:
    I thought that a Torsen system would provide a degree of lock-up on overrun ???

    Stephen



    If I recall correctly, the 964 based 3.6 Turbo had a Torsen-like differential put in "backwards" so that it was "open" when on the gas, and "locked-up" when lifting off the throttle. This caused the car to want to go straight on over-run to keep the back of the car planted in those nervous situations

    Jul 22, 2004 3:30:28 AM

    Grant
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    Grant
    Loc: Denver, CO , United States
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    Reply to: W8MM

    Re: LSD vs ABD - pros and cons?

    Quote:
    W8MM said:
    Quote:
    FixedWing said:
    I thought that a Torsen system would provide a degree of lock-up on overrun ???

    Stephen



    If I recall correctly, the 964 based 3.6 Turbo had a Torsen-like differential put in "backwards" so that it was "open" when on the gas, and "locked-up" when lifting off the throttle. This caused the car to want to go straight on over-run to keep the back of the car planted in those nervous situations



    You might be right, Mike (I wouldn't typically doubt you), but Porsche has almost exclusively used the clutch type LSD's in their RWD cars. However, they do use the Torsen in the AWD models, I believe... I would be very surprised if the 964 3.6T didn't have some locking under power as those things were very torquey under boost for just one tire to cope with it. I know the 930's all used clutch LSD's.

    Some of the RWD models have assymetrical ramping on their clutch-type LSD's, so that they have a different amount of locking for powering or overrun - like the current GT2/3.
     
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