What's the DIFFerence?

As anyone who has ever driven behind me at a track in the past year or so can attest, my car always tended to, ahem, spew a touch of differential fluid.

This was not really BMW's fault, the differential I had in the car was not the "good for fuel economy but not so good for the track" 3.15 unit the MZ3 came with but an 80's vintage 3.46 unit from a 6 series and, to be honest, it looked like a piece raised from the Titanic when it was put in.

That's not inherently a bad thing, it's just that the unit probably had 200K miles on it. The gear ratio is a good one for the car and lap times at all the tracks I visited dropped considerably. However, heavy use soon took its toll and what started as nuisance became a smelly stream of burnt brown crap covering the back of my car and other people's windshields. Not good.


For a while I made the mistake of trying to cure the leak, the symptom of the problem, rather than trying to figure out why fluid was coming out in the first place. Mistake of course.

Earlier this year I went to Watkins Glen with Group52 and was forced to quit after only one session, the first time I ha a mechanical interrupt a track day in seven years. The differential got extremely loud and I was black flagged for the nasty stuff spewing from the back of my car.

It was silly to have worried about venting when in fact the differential was running so hot that the plastic speed sensor mechanism completely melted! You can see in the picture what remained of the part. The damage inside the case was not immediately obvious so we figured the problem was with the front bearing.

I limped home from the Glen with a a whine so loud it made telephone conversations impossible, not much choice but to get a rebuild. Dan Fitzgerald's Diffsonline.com was the most highly recommended and a few calls later I had a newly rebuilt unit on the way. The new differential in not just a stock factory LSD unit but a much more specialized ramped variable lock build. So what's the difference?



This past week end most of the Axis was at Lime Rock with NASA and I was most curious to find out how the new differential compared. It had been quite a while since I had driven at the Connecticut club but the last time I was there my best lap times were in the mid 1:02 range, this past Saturday my best was a 1:00.7. It was not all about the differential of course, the track has been repaved, tires might be different, I got better, lots of factors but car did behave in a different manner.

The first thing I noticed was more stability under threshold braking with the rear feeling much sell squirrelly. The next sensation is that mid-corner the car does not want to oversteer as much when lifting. This surprised me, I would have thought the opposite. I used more aggressive lifts or left foot braking to rotate the car in Big Bend, that worked.

The best part though was corner exit, way more thrust and much more control, very nice.

You can find some video clips after the jump. Next up we'll have some much more exciting racing footage from Lime Rock so stay tuned.