To get the best increase for your dollar, get a increase in the differential ratio.
The ratio is on a tag on the aft face of the differential.
You can use a e30-6, e28, e24, or z3 as a donor.
Expect to pay $150 to $300 and 2 hours for the installation.
http://www.bmwe30.net/cgi-bin/datacgi/data...0104&Section=03
Later,