(Yeah, I realize this is not a math forum...lol...differentials...triple nerd score ) Hey everyone, just had a quick question about upgrading the differential on my car. As far as I know, it's still the factory unit, which begs the question of what the final drive ratio is. I've heard talk about different final drive ratios, and wanted to know what is best for my situation. I do a decent amount of highway driving, and the revs get pretty high at speed (around 2500 at 60 mph). Also, does this tie into limited-slip units as well? That'll be a direction in which I want to head, too. Any insights?
Hmm. Depends... do you want quicker acceleration or better fuel economy? I've got a 2.93LSD, and currently rev 3000 doing 90! Of course I've heard people who think 2500 is high for 60 and I completely agree, so it all comes down to what you want.
Usually the 325i got a 3.73, but some convertibles got a 4.10. 3.73 is great for a 325i, but if you must get your revs down, a 3.64 or a 3.45 from a 6 series are good. I had a 3.25 on my 327i, but that is a bit tall. Any of these ratios are available in LS.
To generalize I would say German cars rev higher than their American counter parts. I would say 2,500 rpms at 60 is not high at all. Although when comparing to American cars it is high. Is your 325i an auto or manual trans. I would say you don't want to go much lower numerically than a 3.73. It will feel like a bit of a dog around town. But then again maybe you shouldn't listen to me. My M3 has a 4.10 and my 318i has a 3.91 You should be able to find an LSD in the ratio you want. Just change the oil before you install it. I got a LSD for my 318i and it was shipped to me with no oil in it. I was sure glad that I checked and was planning to put new oil in it anyway! GR
Your car should have a 3.73, an "S" on the plate on the differential will identify if it is an LSD or not. I think Auto 'verts got the 4.10. Remember not to get a 318 diff, they are a small case, and all other E30 diff's including the E30 M3 are the medium case diff. I swapped my dying 3.73 out for a 4.10 LSD out of an ix. 1st and 2nd are absolutely nuts now, hehe. I'll eventually go back a 3.73 once I can afford to get one a nice rebuild with more lock up then the stock 25%. The 4.10 is going to bump me into SM from DSP in SCCA which sucks and also means I have to grab third above ~52-53. 80mph = 3500 RPMs.
Z31maniac has some good points. The E30 M10 and M42 motored cars have the small diff. The rest of the E30s came with the medium diff. As Paul mentioned you can get a diff from E28 and E24s just make sure it is the medium diff. Some of these cars came with a large diff that won't fit an E30. If you do get a diff from and E28 you will have swap out the half shaft mounting points. I forget the correct name of these pieces. I have never done this, but from what I understand it is pretty straight forward. Just take the ones from your diff over to the new diff. GR
Thanks for all the tips, guys! Looks like I've got some research to do...hehehe. Also, as an aside, I plan on replacing my clutch over my winter break. The shop that I take my baby to wanted upwards of $700 (incl. labor) to replace JUST the pressure plate, so then I found an entire clutch kit online (BavAuto) for around $240--this includes the pressure plate, disc, throw-out bearing, alignment tool, pivot pin, and all sorts of gaskets. Does this sound like a good deal? Keep in mind, I'm armed with a Bentley manual and a Mr. Fix-It grandpa who's really excited to tear my car apart...lol
Hey Elkoholic: I just noticed your signature. I live here in Philly area too. Are you a member of the Delval chapter? I think they have their banquet coming up in Jan or Feb. This past summer they had a picnic, not as good of a turn out as past years. Almost no older cars, I was disappointed. GR
Clutch is lots of work, but if you can find a diff that you want, you can save a little time by installing it once you have the driveshaft out and the exhaust down. These are not necessary to change the diff, but sure do make life easier. I recommend a bell housing extension (very long 1/2 extension on the wrench end with a 3/8 fitting on the socket end for better clearance) and a high quality (with warranty in case you frag it) universal joint. You will need some E Torx sockets as well. An impact wrench is nice to have, too. Your diff should have a tag on it telling you the ratio, but if it is missing or you want to check that it is accurate, get the car on jack stands, put it in neutral, mark the input and output flanges, and count the number of turns of the input shaft it takes to get the output shaft to go around once. Otherwise, you can do some math. To see if it is limited slip, try to turn the rear wheels in different directions by hand. If it is easy, you have an open rear. If it is only kind of difficult, you have a worn out LS, and if it is pretty darn hard, you have a good LS.
Grant, I'm actually a member of the Nittany Bimmers Central PA chapter, only because I keep the car at home in Lancaster. Thinking of changing that, though, since I spend over 85% of my time in Philly... Paul, thanks so much for the tips--they'll definitely come in handy!!!
I use to be a member of the Nittany Bimmers when I lived in Lancaster a number of years ago. I didn't do a whole lot with that chapter. If you have never gone, the Carlisle Import show every May is a great show to go to. I highly recommend it. Saturday is usually the best day. Last year there was a pretty good turn out of BMWs. Maybe this year we should rally a big group. The Audi and Saab people always seems to be having more fun. They stay late and BBQ, while the BMW crew is usually gone by 5pm. Here is a link. http://www.carsatcarlisle.com/ce/events/import/ GR
I believe you have to swap the diff cover and the output flanges from the E30 diff. If you plan on tracking the car, the finned diff cover would be a great idea to help the diff last even longer. The fins hang down below diff to get in the airflow and also allows an extra 1.5L of capacity I believe.
Yes, the output flanges, that is what I was trying to describe. I forgot about the diff cover. Again this is all easy to do. The finned cover is a good idea, but expensive. GR
For $250 it seems like a pretty good deal. But I'm still not going to buy one until I pony up the cash to get a fresh LSD rebuilt.
For anyone dropping a diff off the car (or a rear subframe for that matter) - check for any rust on the metal brake line sections running to/through the rear of the car. Easier to r&r those w/ the diff out, and access is way, way, easier if the rear subframe's out.
I've got a 4.10 on mine and it's lots of fun around town and just plain ol' accelerating anywhere. The high rpms at higher speeds (3741 @ 80mph with my wheel/tire dimensions) gives me nice passing power in top gear. I don't drive very many miles with mine so I don't mind the pisspoor mileage it gets.
Haha, better get my checkbook ready... I probably won't get a lot of this taken care of at the same time, but it's still cool to have a laundry list of items to restore, so thanks guys! And Grant--that sounds like an awesome idea, I'm actually relatively close to Carlisle. Now, it just depends on where I get a job for my co-op...hopefully not too far from my car...lol. Let's work on getting some people together as it gets closer...
