Is there a bolt in Diff Swap ?
 

I would like to change gear ratios in my 97 318ti to something more highway friendly like a 297 or something there a bouts, But I would just like to swap the rear and not the axles too.
Is there a direct bolt in Rear diff that will work for me,?
I am new here and new to BMWs, So please be direct,
Like, I need a E-?? rear and it will swap right in,
Thanks all!
Peter

z3 is a direct swap, you can technically use an e30 diff too but it seems random who has fitment issues with half shafts and who doesn't.

Fitment issues like what?
Shaft length or flange bolt pattern?
I would like to gather what I need and in the next 30 days have a more highway friendly rear,
Peter

So here is basic information about the diffs:

318Ti's use a small case diff

4cyl E30's use a small case diff but you wont find a highway friendly ratio there

6cyl e30's use a medium case diff which is a direct bolt on. You can find a 2.93 on ETA's

Most e28's use a medium case diff which is not a direct bolt on. You would need to take the cover and flanges off an e30 diff for it to work. You can find 3.25's on 528's

z3 4cyl use a small case, direct bolt on, not sure on ratios but i believe there isnt a highway friendly option there.

z3 6cyl are a medium case, direct bolt on. You can get a 3.15 ratio on any of the manuals

Here is where this fitment issues come into play. Because you are putting a medium case diff in, the width is slightly larger than the small case diff. Some people seem to have no issues using the Ti half shafts, while others either cant get them to fit or they explode rather quickly. If you want to be safe pick up a pair of e30 half shafts with the diff and you will be set.

I found a guy locally to me with two med diffs. What is need to do the swap and pred maybe you can get one and i will get one

http://denver.craigslist.org/pts/3114147158.html

As Uber E30 mentioned, You'll need e30 halfshafts and flange bolts/plates. Plus you'll need to shim your ABS sensors(I used a washer on each one to raise them about 1/16th-1/8th of an inch.

I used a E30 diff with my stock ti axles flanges with no problems. Went from a 3.45 to a 2.97 ratio. Much better at highway speeds now. It is slower around in the stop and go but these cars are not know for speed anyway. BTW I now get 33-34 mpg!

Looking at the Ti flanges VS the E30 flanges I see that the Ti splines won't fully engage, While It looks like they will work I would be afraid that I would wind up stripping out the Ti spline shaft.
I am looking for a reasonable priced pair of E30 axles.

Just some food for thought, I've been considering a ratio change from stock 3.45 lsd to 3.25 lsd and have gathered all the pieces except the time.
I see Derek stating 33-34 mpg after a change to a 2.97. I am achieving solid high 33-35 mpg with the 3.45 lsd. So I'm wondering if this swap is worth the time.
There are factors to consider and to compare; (my DD) '97 w/ sunroof/ active pkg., most of my driving is a highway (I-95) commute (160 rndtrip), nearly flat Florida, cruise control @ 32-3300 rpm (approx 66-68 mph), tire pressure @ 36psi, (those oh so lovely but light) 15" 10 spoke Style 27 rims, if I have to cut the muggy humid S. Florida air with a knife I'll run the a/c. The only mods are a COP with stock plugs and my version of the "fogged" airbox with std. Mahle filter (no K&N here). Redline fluids in the gearbox and diff. Amsoil 5-40 Euro in the engine. I don't have rook racks or carry excess stuff. Stock suspension with Bilstein Sports, 19mm rear sway. I run mid-grade 89 fuel, Hess or Chevron.
There it is. Apples to apples, oranges to oranges. Just for comparisons. I, like others, would like to knock a few revs off the tach easing the torture on the engine and drivetrain even if it is only 2-300 rpms (more with a 2.97) for the sake of longevity and (maybe) some improved fuel economy.
At this point if your thoughts of a pure fuel economy improvement is the goal where time invested and the expense are possible factors it may not wash out. If like me you enjoy spinning wrenches for therapy and don't mind trying a little experimentation for real world results I say go for it BUT let the rest of us know what they are under what conditions. I believe there are other Ti drivers that are interested as well. If I choose to go ahead with a swap I will post after I have some numbers.
Derek and others, this is not in anyway meant as a negative. I think the sharing of info and help among the members/ visitors on this forum is huge. I personally have been helped out of a jam or two and saved from getting into a few as well.

excellent description of your conditions! help a lot to know these details of what's going on with your car. so many times people post with so little info that it's hard to give advice to.

myself I find the 3.45 short on take off, and was looking for a 3.25 locally to add to my parts collection.

now my question , and I have used the search without the right answer,
I have a 97 sport

does the Z3M 3.23 diff fit easy in our cars?

I would think so , and it would be a medium diff so I would need the e30 stuff for it to work, is this correct?

Answer is yes to both.

If I choose to go ahead with the swap I'll be using an E28 3.25 lsd (on the shelf) E30 output flanges and refreshed axles. The Z3M 3.23 will work but may be a little harder to find. As Jess always reminds, "Don't forget to shim the sensors!"

Also, if you source a Z3 diff, it's gonna have Z3 output flanges which might have a different bolt pattern than e30 output flanges. Depending on length, you may or may not be able to use Z3 halfshafts. Otherwise, you'll need to run 6cyl e30 flanges and e30 halfshafts. I don't have much knowledge about Z3 halfshafts, but I'd imagine that they probably have larger ABS tone rings like the e30 halfshafts, so as Russ mentioned, you'll likely need to shim your ABS sensors.

There are medium case 3.46 lsd in E28's and there are Viscous and non Viscous 3.64 and 2.79 in E30's as well.

E30's had a wide range of ratio's available.

For the OP's intended purposes none of the viscous diffs would work (only 3.64, 3.73, 3.91 and 4.10 ratios). Although as you mentioned there are a lot more diff ratios than I mentioned above, I was only talking about ratios better for highway MPG than the stock 318ti diff.