Feb 28, 2012 update to the BMW 2002tii project car

Project 2002tii: Limited-Slip Differentials

The car was back up on jackstands as we replaced the differential.
It was a greasy job, but Per replaced the differential in a couple of hours.
The replacement differential got a full serving of Red Line Oil's 75w90NS.

F Street Prepared machines don’t pack a lot of horsepower, so we want every little bit to be used effectively.

One of the most effective modifications allowed in our autocross class is the addition of a locking or limited-slip differential. F Street Prepared machines don’t pack a lot of horsepower, so we want every little bit to be used effectively.

We replaced the differential with a factory limited-slip unit that we had extra shims added to. The breakaway torque is in the 60-to-70-pound range with these extra plates and should be a good starting point.

Thanks to our experience in autocross, we’ve learned that the proper differential oil is critical for performance—but not in the way you think. Most people recommend using a 75w90 oil that contains a friction or limited-slip additive to prevent the differential from making noise or chattering while turning.

We would recommend the exact opposite for a car that sees track use. We want the plates to slip and slide as little as possible, and for things to be forced to break free and create the most locking action possible from the differential. The solution, then, is to use Red Line Oil’s 75w90NS, which has no LSD additive. True, the differential makes noise with this oil, but we’ll be hard pressed to light up the inside wheel while on track.

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