All M42s had small case-diffs. In my experience, all of them had been replaced at least once. My car is on her third diff - leaking output seals killed the input bearing...but was what finally annoyed the PO into selling it to me. I just tossed in what I had, turned out nicely. I couldn't read the tags, so it's possible that the donor 325i didn't have a 3.73 as originally spec'ed.
The medium case can take a lot more torque in case engine upgrades are in your future. You could also add an LSD unit to your current diff - transmission shops could likely handle a job like that. It's not like other makes that have different manufacturers for the open & LSD...I'm told the BMW diff can be converted with an overhauled LSD pack in the middle. You could also go with a cheap torsen, OBX makes a cheapie for the e30 that can have new clutch springs installed to improve durability. BMW has an OEM medium case torsen in the later Z3's that's worth a shot...put a bid for one into carpart.com & see what comes up. They're the best type from what I've heard.
The 3.73 helps a little with fuel mileage too, but it does hurt acceleration especially from a dead stop. Taller is more useful for the car's torque curve IMHO. You get more out of each gear, so you can space out your shifts more. I use the car mainly for highway driving, the taller diff really helps there. IIRC mine runs only about 2500 rpm at legal speeds and maybe 3500rpm at 75mph or so. Makes for great passing performance...the car goes from 70-90 very quickly right at the torque peak like that.
Topic: lsd (Read 4796 times)