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Board: Porsche - 911 - 997
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May 6, 2007 6:12:29 PM
- Silver Bullet
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- Loc: Canada
- Posts: 1126, Gallery
- Registered on: Nov 24, 2004
- Reply to: jeffbco
May 7, 2007 1:03:00 AM
- frayed
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- Loc: Austin, TX , United States
- Posts: 510, Gallery
- Registered on: Oct 17, 2004
- Reply to: Silver Bullet
Re: 997 - Limited Slip Diff?
Quote:
Mithras said:
buy one from Guard. They are better than the porsche ones in many ways and I think cheaper. Loook in Panorama or Excellence for their ad.
Yes, the Guards are great.
A couple notes:
Once you add LSD, you will likely need to tune the suspension to compensate for the different handling balance(adjustable swaybars would be helpful).
The -20mm suspension is specially tuned to work with LSD - that is why you cannot order LSD from Porsche without -20mm suspension and vice versa...
Also, a serious locking ratio (above around 28%) on the Guard LSD will almost certainly interfere with the performance of PSM. Guard commonly uses locking ratios as high as 80%. This is why the LSD on the 997 with -20mm suspension has a very mild locking ratio (around 28% or less) compared to the higher locking ratio on the LSD for the GT3 (no PSM).
Re: 997 - Limited Slip Diff?
Quote:
kavaron said:
Grant, I don't understand how it will interfere with the PSM. If I turn off the antispin and put hard setting + sport button it will still interfere??
PSM is a stability system that relies on its ability to apply the brakes to each of the 4 wheels individually. With a clutch-type LSD, it is impossible to apply the brakes to one rear wheel without it also slowing the other rear wheel, so its ability to correct for yaw is compromised.
The anti-spin (traction control) function is not the problem, it's the stability function (the one that helps straighten the car when it gets sideways) that has a problem.
Re: 997 - Limited Slip Diff?
I believe the issue is ABD, active braking differential.
Anyway, adding an LSD w/o addressing the balance of suspension = Understeer city! Normally this isn't that big of a deal, as a quality coilover setup, adjustable sways and so forth can get you there. But with PSM, which incorporates ABD, ABD might not get along with an aggressive LSD. But IIRC, Cayman's with PSM have been successfully mated with an aggressive LSD, so perhaps LSD does play OK with PSM. Not sure.
The moral of the story, don't expect to crack your tranny, add and LSD, and be happy. It's going to take a full suspension overhaul.
Anyway, adding an LSD w/o addressing the balance of suspension = Understeer city! Normally this isn't that big of a deal, as a quality coilover setup, adjustable sways and so forth can get you there. But with PSM, which incorporates ABD, ABD might not get along with an aggressive LSD. But IIRC, Cayman's with PSM have been successfully mated with an aggressive LSD, so perhaps LSD does play OK with PSM. Not sure.
The moral of the story, don't expect to crack your tranny, add and LSD, and be happy. It's going to take a full suspension overhaul.
Re: 997 - Limited Slip Diff?
Quote:
frayed said:
I believe the issue is ABD, active braking differential.
No, I think this is the part of PSM that is actually benefitted by an LSD. ABD is there to brake one wheel when it is spinning all alone (inside wheel when turning).
An LSD will reduce this behavior (allowing power to go to both wheels instead of just the one with less grip) and cause ABD to need to work less.
I'm certain that the yaw correction functions of PSM are the ones that have issues with LSD. A Quaife-type torque-biasing diff will work fine with PSM (no locking under braking), but they are far less effective on the track (don't enhance stability under braking and corner-entry like a clutch-type LSD).
Re: 997 - Limited Slip Diff?
Quote:
frayed said:
That may be true. . . but note the RUF and FarnbacherLoles have both put LSDs in open diffed cars running PSM. I've not heard of any issues.
Jeff - Do you happen to know the locking ratios on those? I'd be interested to know if they're much higher than 996 40th Anniversary or 997 with -20mm suspension (only two factory models to combine LSD and PSM - both use very mild locking factors).
Re: 997 - Limited Slip Diff?
very confusing topic. I thought that if I pressed all 3 buttons (hard suspension, sport button and traction off) then the limited slip would be able to engage without interfering with all these other systems. Aren't all these systems turned off when you press these 3 buttons?
Re: 997 - Limited Slip Diff?
Quote:
kavaron said:
very confusing topic. I thought that if I pressed all 3 buttons (hard suspension, sport button and traction off) then the limited slip would be able to engage without interfering with all these other systems. Aren't all these systems turned off when you press these 3 buttons?
I think this is a nomenclature issue. Limited slip, as in a mechanical limited slip rear differential, is purely hardware. -20mm suspension have a LSD, PASM cars do not (except for the GT3 and an option on the TT). If you have a 997/997S with PASM, you do not have a limited slip differential.
With PSM off in your 997, the stability management functions are disabled until you get into ABS braking, at which time PSM comes back online to save your butt.
Hope that helps.
Re: 997 - Limited Slip Diff?
Quote:
kavaron said:
very confusing topic. I thought that if I pressed all 3 buttons (hard suspension, sport button and traction off) then the limited slip would be able to engage without interfering with all these other systems. Aren't all these systems turned off when you press these 3 buttons?
No, PSM is controlled by a separate button and it is impossible to turn it off completely. It turns itself back on after aggressive braking...
Re: 997 - Limited Slip Diff?
Quote:
kavaron said:
So, if I add an LSD on my 997S and I turn off all the aids (with the 3 buttons), then there will be no interference from the ABD, PSM except when I brake hard and the systems turn on automatically.
Since you live in Greece, you'd be much better off trading your car for one with -20mm suspension and LSD (we can't do that here in US)...
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