When we started running our V8 cars on the track, one of the things that most impressed us was the way the cars would put down the power. While the standard Torsen differentials in Miatas do a good job at distributing torque and being subtle about it, they have a hard time dealing with really high power levels. They also turn into an open diff if you pick up a wheel, killing your drive altogether. The clutch-pack diffs in the GM differentials we were using made a big difference.
OS Giken's Super Lock LSD is a clutch pack type differential that's a direct replacement for the Miata Torsen. It uses a high number of clutches for a very strong lock and a long life - in fact, they claim they've never had to rebuild one.
How well do they work? Well, we installed one in our Targa Miata test bed and sent Keith out to compare times on our local track. This is a car and track he knows very well, and we didn`t change the suspension setup at all. The result: He immediately cut almost a second off his personal best in the car on a one-minute course. From the driver`s seat, he reported that he could get on the power much earlier and get a stronger drive out of all corners. He's a convert.
Due to the design of the differential, you may experience a bit more turn-in understeer, so your ideal chassis setup will change slightly. The standard setup is as a 1.5-way differential, but it can be changed to 1-way or 2-way depending on your needs. You can also change the amount of lockup based on the clutches and even how quickly it locks. It's an excellent tuning tool for extracting maximum performance.
This version is a direct replacement for the differential in the 2004-05 Mazdaspeed. It won't work with the smaller halfshafts used on other Miatas. This is just the differential, it does not include a carrier or a ring and pinion. Using the special 80w-250 OS Giken differential fluid is strongly recommended for best operation.