Torsen Limited-Slip Differentials
in a First Gen
   by David Lane
   dlane@peabody.jhu.edu

   For those not familiar with the Torsen differential, it is standard on all 3rd gens, and I believe it is available on some of the special versions of the Miata. By way of background: during a sharp turn, a standard (non-limited slip) differential can allow the inside wheel -- which has less traction -- to spin, while the outside wheel gets little, if any, power. A limited slip design makes sure that both wheels are driven to some extent at all times. The standard 1st gen limited slip differential (LSD) uses multiple clutch plates to ensure that the wheel with the most traction always gets some power. Eventually (especially under race or autocross conditions) the clutches wear out, and the limited slip function is lost. Mazda has a special race version of the clutch type LSD, but it is not recommended for the street because of its lock-up characteristics.

   There are two Torsens available from Mazdatrix and Mazda Comp parts, which should fit 84-85 1st gens, and non-turbo 2nd gens. I can never keep the two Torsens straight. One is supposed to be stronger, the other (I think) is less complex. Torsen LSDs use gears and, in some cases, thrust washers to keep power to the wheel that has the traction. It is said to be "torque sensing," and, to quote the Mazdatrix catalog, "It is able to transmit far more torque to the wheel that still has traction than any other type of differential."

   Be advised that Bruce Wentzel has one in an '86 ITS car, and reports that it is slipping after 3 seasons. Bruce prefers the clutch type competition LSD for durability. Last I heard, Mazda Comp was working on a kit to replace the thrust washers if that happens. I had already bought the unit, and I am not a rabid autocrosser -- more of an occasional thrill seeker -- so I went ahead with it.

   The following comments relate to a GSL-SE only. I have no experience with any other generation, although I suspect the units are more of a direct drop-in for the 2-gens.

   Either Torsen will fit directly into the differential housing, but they are a little smaller than the stock LSDs. Therefore, the axle splines do not engage fully. The OEM spline contact is 25mm. Contact with the Torsen is reduced to about 15mm. Since my car has an aftermarket turbo, we didn't think it was a good idea leave it that way, although Mazda Comp said one guy was racing with a stock engine and a Torsen. To deal with the problem, the axles must be moved inward. This is accomplished by removing the spacers adjacent to the wheel bearings, which results in spline contact of 23mm.

   Unfortunately, this little trick results in the axle hubs at the other end also moving inward, which takes the brake disks with them, so the disks need to be shimmed outward to recenter themselves in the caliper. We used cut-down rear disks from a junked car, plus some washers around the studs for fine tuning. And, since the brakes are now farther out on the wheel studs, you don't have enough threads left to secure the wheels, so you need longer studs, and for that matter, longer fasteners to hold the brake disks to the hubs when the wheels are off.

   I think that about covers it. While Donnie was in there, they did the baffle modification recommended by Mazdatrix to keep diff fluid from sloshing away from the center during hard cornering. Mark Schroeder recommended Redline Lightweight, Shockproof gear oil for it, and I followed his advice.

   For what it's worth, I have been pushing the car pretty hard, and it has certainly lost every hint of 1st gen rear end twitchiness -- even with 300 horsepower. I really love the thing, and just hope it all holds together.

[RotaryRckt wonders why David didn't have custom axle shafts built by a local rear end shop. Many even guarantee their shafts not to break. Considering the cost already involved (Torsen differentials are not cheap), it would seem like a more elegant solution. At any rate, it's something to keep in mind if you're considering this procedure.]