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First concentrate on the E36 series cars for what you want. As mentioned I believe all E36 M3s came with limited slip. Also the cars equipped with the sports package did as well. This means you can often find a diff from a used or junked car. Lots cheaper, but you do run the risk of unknown mechanical problems. In any case a limited slip diff from the E36 (1992-1999?) series should bolt right in.

You can also search for these online and there are several vendors that can supply a rebuilt unit. But, the price is in the dealership range.

I am not too sure you will feel any difference after you install one. The open diff from Backdraft (again E36 series BMW) is pretty stout and can handle lots of power. I guess if you decide you want to leave two black stripes instead of one when you do a burnout, then a limited slip becomes important.

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