I changed from a 3.25 to a 3.73ls and am pleased with the useable low-end torque. The U.S. 635 is not built for off the line competition, obviously. I noticed much more improvement with the diff. swap than with the chip. Don't buy a diff. from a big name tuner becuase you will pay twice what it's worth. Check with Bimmerworld.com or some other BMW garages. Chris '85 635 5spd.