Differential Change from A 3.46 to A 2.93 -- Intended & Unintended Consequences
 

Y'all,

Since I'm about to take a 1400-mile trip to see interesting friends and places*, I recently changed out the 3.46 limited-slip differential (LSD) in my 535i for the 2.93 LSD out of my son's 528e (he went to a 3.25 LSD).

An intended consequence was getting better mileage for the trip -- I'm already getting ~2 MPG more in mixed commuting, and the car is much quieter on the highway, which led to an unintended consequence.

An unintended consequence was hitting 90 MPH when it sounded as if I was doing about 70 MPH! :eek: I'm also hitting the wrong gear around town, since 1st-5th now seem like 2nd-6th!

For the time being, the 2.93 will stay in, but I'll probably change it back to the 3.46 for the autocross season. Yeah, I know autocrossing ain't "eco," but it sure is fun!

-Shawn

*OK, OK -- I also installed the 2.93 since I'm installing a turbo in a couple of months.

That's what I'M talking about...

I've thought of doing that but figured I'd go for the 3.07 (a little less extreme at the low end). Of course the 6-speed would be the cherry on top, but WAY too pricey for me.

Indeed, a 3.07 ratio would have been a good compromise, but for the E23/E24/E28, that ratio only came in the large-case diff, which was Euro-only. Not only that, a medium-case E23/E24/E28/E30 diff weighs 80 lbs, but a large-case diff weighs 100 lbs. Besides, I already had the 2.93 sitting in the garage! ;)

Oh, yes, a 6-speed would be nice, but the only 6-speed that bolts up to the M30 engine is the E34 M5 6-speed, which is as rare as hen's teeth!

I see. I've been assuming that any diff that bolts to an E46 chassis, and any trans that'll bolt to an M54 engine would fit but where do you find that out fer sure?

...that and other fit details like matching half-shafts and drive shafts.

E46-to-E46 swaps should be 1-to-1 swappable. There are plenty of sources to find out for sure, such as the E46 section of Bimmerforums.com.

Shawn D: Your final drive ratio drive change had exactly the same effects as mine. A 15% reduction in ratio bought you solid MPG improvements if at the cost of autocross performance.

Overall gear ratio is a powerful tool for improvement of MPG. Too bad so many vehicles are so difficult to effect gear ratio modifications.

Thanks for the info. I'll check it out.

Both of those are very true. There are 2.79 BMW diffs out there that would fit my car, but I don't think I'd want to go that low; I do know a guy who did such a swap for a 2,000-mile trip, though.

If the fasteners aren't damaged or crudded up (i.e. undercoated or rusted), even a novice can do a diff swap on a BMW in a couple of hours. Even better, junkyard diffs go for less than $100 and are nearly always in great condition (not necessarily so for the limited-slip clutch plates, but those aren't hard to replace).

Interestingly, I'm having to drive a bit faster to get better MPG with this diff. I used to be able to do 40 MPH in a 45 zone in 5th gear, but now I have to do 50 in order to not lug the engine or hunt on/off the throttle.

When you say "lug", what rpm are you talking about?

I routinely accelerate from about 1200 rpm (30 mph in 5th gear). I have a 107 hp 4-cylinder, from Honda no less - famous for having little torque down low.

You could probably adjust your driving and easily use 5th even lower than what you are now.

40 MPH in 5th is now just under 1100 RPM and is just above the idle position on the throttle position sensor. Trying to accelerate even at 1/3 throttle from that state brings loud, low-frequency rumbling and vibration -- it's definitely lugging.

Doing the math (1200*40/30), you're doing 1600 RPM at 40 MPH, which means my 535i has about 30% taller final gearing (drivetrain & tire radius combined) than your Honda.

I can't really adjust driving to go any lower in 5th than that.

Ok, that's cool. I'm envious of your gears now. If you continue the math, I'm at 2500 rpm at 60 mph and 3000 at 70 - BAD for mileage.

Good to see another automotive competitor. I also autocrossed many years ago. Went on to racing old 2002's and from there to Miata now. Autocrossing and racing are great fun:D

At the races we changed those 2002 diffs in about 30 minutes. BMW has a great setup for diff changes.

I took my 91 318i (M42) from the stock 4.10 diff down to a 3.64. It make the car much more enjoyable to drive on the street because there was less shifting required and quieter on the highway. What I didn't expect is it became quicker to 60 (no more 2-3 shift) and actually more fun to autoX because I never run into the rev limiter or have to use 3rd gear anymore. With a 7200rpm limit, second gear can get me to almost 65mph. I think its the ideal ratio for this car although I wouldn't mind a 3.73LSD either.
Stock as it left the factory
http://farm1.static.flickr.com/231/5...dc947c06_o.jpg

As I have it now with slightly shorter 195/60 autoX tires. The graph doesn't go far enough but with a chip it revs to 7200rpm now.
http://farm1.static.flickr.com/220/5...857d3774_o.jpg

That's a great result. With the 3.46, I can stay in 2nd during an autocross, but if I left the 2.93 in, I'd have to shift down to 1st.

Hehe, it must be nice having some torque to play with. With the 1.8 engine I can't avoid down shifting to first in slow corners. That said, I managed second out of 13 cars in GS at my last autoX. It was a fast 'drift' course so I spent the whole time in second gear mostly out of control desperately trying to avoid the cones. Quick E36 rack without a PS pump was a handful but I can't wait to do it again.
http://farm4.static.flickr.com/3602/...9291ca84_o.jpg
Still managed 33mpg on the way home.