The Limited Slip Differential (LSD) can aid traction by locking up the left and right (rear) tires when it senses excessive slip.  For U.S models, the 1986-1988 Zenki FC's all run stock clutch-type LSD's; the 1989-1991 Kouki FC's all run viscous-type LSD's.  The Zenki clutch-type LSD's are known to wear out within 30,000 - 50,000 miles.

The clutch-type LSD uses clutch packs of alternating friction and steel discs.  The discs can wear out of tolerance and cause the LSD to stop functioning properly.  Only rebuilding the LSD to replace the discs can fix the problem.

The viscous-type LSD typially uses a sealed viscous fluid coupler within the LSD.  The advantage is it had no discs to wear out like the clutch-type LSD.  The downside of the V-LSD is that at very low slip rates, the V-LSD does not engage at all and acts like an open diff; to get the V-LSD to engage securely, you need very high slip rates between the left and right side wheels.

Cusco, KAAZ, OS Giken, and Mazdaspeed are popular aftermarket clutch-type LSD's available for your FC3S.
Mazda Motorsports -
Cusco Japan (English) -
Cusco Japan (Japanese) -
OS Giken (English) -
OS Giken (Japanese) -
Active Traction Service (ATS) -

Another option for an LSD is the Torsen-type LSD.  Torsen stands for "Torque Sensing" and patented by the Zexel-Gleason Group.  This LSD is also known as a helical gear LSD, as it uses a complicated set of helical, worm gears for lock-up.  This is a rather delicate LSD design that's only suited for use up to 400 lbs/ft of torque; anything more torque, and it destroys itself.  Stock FD3S use this type of LSD's.  Aside for it's power limitations, this is one of the best LSD's on a high-speed track.  The Torsen LSD exhibits a very predictable lock-up ramp that makes for very easy driving.

A table on the different lock-up characteristics of the different types of LSDs available...

Clutch-type LSD's are also categorized either 1-way, 1.5-way or 2-way.  This describes whether the clutch-type LSD locks up either upon acceleration or deceleration (throttle lift-off).  A 1-way LSD will engage only in acceleration.  A 2-way LSD will engage in both acceleration or deceleration equally.  A 1.5-way LSD engages both in acceleration and deceleration, but it engages only partially on decel.  Hardcore drifters prefer the 2-way LSD, as you get full lock-up on throttle lift, allowing initiating a drift just just by pumping the throttle.  A 1.5-way is easier to control for grip driving, as a 2-way LSD can cause the vehicle to spin suddenly from the decel lock-up from the 2-way design.  A 2-way LSD is recommended for drifting, but if the decel lock-up is causing some problems, you can step down to a 1.5-way for better control.

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