RSAErick

I have recently purchased a 93 RS America with the optional 40% limited slip differential (or LSD as it is so hallucinogenically known). I know that this is a performance "upgrade", but I don't have the understanding of what a limited slip differential actually does to improve performance. In addtion, I understand that ALL 911's of this generation have an LSD, but not the upgraded 40% version. What does the 40% LSD do that the standard LSD doesn't, and why is that "better"? Thanks for helping me to understand this system better!

Cupcar

The Limited Slip Differential (LSD) is an option on the RSA that replaces a so called "open" differential not another lesser type of LSD. A differential is required on any car in order to allow the outside rear wheel in a corner to turn faster than the inside wheel. This is accomplished by coupling the drive axels to the driven ring and pinion gears in the open differential with bevel gears. The problem with an open differential is that all the torque can be applied to the wheel with the least traction. You have probably seen this occurr in the situation where one rear wheel is on ice and the other is on pavement, the wheel on ice spins and the other wheel remains still. BTW Porsche cures this problem in the Automatic Brake Diferential (ABD) equipped car by applying the brake to the spinning wheel, diverting torque to the wheel on pavement.

In the LSD there is a clutch pack that allows a minimum torque difference to be applied to the wheel with the least traction. In the 40% differential a minimum of 40% of the available drive torque at the faster turning wheel is applied to the slower turning wheel. This prevents the situation described above when one wheel is on ice the other on pavement i.e. the wheel on the pavement gets torque so the car will drive away instead of standing still. The LSD works well in a race track situation when the car tends to spin the unloaded inside rear wheel on corner exit. ABD, on the other hand, works well to help move the car from stopped in a low traction condition, but not so well in the situation on the race track. Give me a good ol' LSD! <img src="graemlins/yltype.gif" border="0" alt="[typing]" /> <img src="graemlins/burnout.gif" border="0" alt="[burnout]" />

Russell Gardner

Speaking of LSD's...has anyone tried a Quaife? I have a '93 RS America with an open diff and am exploring my options.
Russ

JonSeigel

Very informative answer from Cupcar. While we're on the subject, my '93 C2 has an option called "Locking Differential" (option # 220). Where does that fit in?

Thanks,

Bill Gregory

[quote]<strong> my '93 C2 has an option called "Locking Differential" (option # 220). Where does that fit in?</strong><hr></blockquote>

Option M220 is the 40% limited slip that Cupcar described.

Bill Gregory

[quote]<strong>Speaking of LSD's...has anyone tried a Quaife? I have a '93 RS America with an open diff and am exploring my options.
</strong><hr></blockquote>

Russ,

Go to the Porsche Owners Club website at <a href="http://www.porscheclub.com," target="_blank">www.porscheclub.com,</a> and on the "express menu" choose Velocity Magazine, and once there view the February 2001 issue, looking for the article "Slipping, Slipping, Spinning" on page 10. An excellent article on Porsche limited slips and variants.

JonSeigel

Thanks Bill, for the excellent citation to that article.

Adrian

DearRSAErick,
I do not know who told you that ALL 911s of this generation came with LSDs as standard but this is not correct. The LSD is an option on all normally aspirated C2s including the RSA. The ROW RS, RS 3.8, RSR and RSR 3.8 came with LSDs and the Turbos of course. In the C4 maintenance manual it says it has a LSD but this is a bit of a push. The C4 has traction control which simulates a LSD when activated,
Ciao,
Adrian
911C4