Removing BDR Diff

[Mark Brye]

I have been looking at changing my 3:10 open rearend for a 3:46 LSD 40% lock-up like some people on this board. I just read Whiplash's post and a couple other older threads on the topic. So I think I have the gears I want (still thinking about 3:6's) but am now looking into the actual removal.
I have not seen a write up on removing the differential from the BDR. I have looked at several DIY articles off of Bimmerforums for the E36 and my BMW Bentley Manual.
When I am under the car, it looks to me like I can just unbolt the drive shaft, CV joint bolts and the 4 big bolts holding the rearend to the chassis. Hold the shafts up by tying them so they are not dangling and disconnect a speedometer line. Is this all there really is to swap a rearend on the BDR?
Now according to the BMW bentley manual, I have to remove swaybar, and disconnect the suspension for removal. I don't want to remove any suspension components for fear of needing an alignment.
Can anyone give me an idea of what I am getting into? Is it as easy as it looks?

[Blittleton]

Mark,

I would not do it that way!!. First remove you drive shaft and half shafts. Then remove your fuel tank so you can have easy access to you two bolts holding your diff at the aft attachment. Loosen the sway bar at the aft attachment area as well. You will have to remove the one bolt that is attached at the lower front of your diff. Tie up the park brake cable so it is out of your way. Your diff will come out, it is heavy so be careful.

Mark, you don't have to pull the tank, I have done it both ways but it is so much easier if you do. When installing the bolts back in use Red Locktite.

Bill

[Mark Brye]

Thanks Bill!

I will drop the tank then. I don't mind doing a little more work if it makes it easier. Do the half shafts need to be completely removed, or can I just hang the ends of them up? I guess I need to read a little more to see how the halfshafts come out if that is needed.

Thanks,

Mark

[Blittleton]

Mark,

The half shafts stay, just zip ty them out of the way.

Bill

[Cashburn]

Mark,
Make sure you drill and tap the diff for the front mount. Stock BMW diff. will not be setup this way.

[Dennis Kelley]

Cashburn correct me if I'm wrong but you do not have to tap the diff if you get the E36 diff. If you buy the right diff it will have the mount hole already there. E30 diff did not have the front mount hole.

later

[Mark Brye]

Thanks for everyones inputs! I have done some searching online and it looks like the average price for a rebuilt LSD is anywhere from $1000-1800 or so.
Would it make sence to just have the diff that is in the car rebuilt with a new gear set? Not sure if that would be more cost effective. At this stage, I am not worried about the car being down for a couple months. The weather does not start to get nice here for a couple months.

[Trueoo7]

There are only three bolts aside from the half shafts and drive shaft that hold the diff in. (not four) Two in the rear and one in the front. I think what Cashburn was referring to was the bushing that the front bolt goes through. Thats a little tough since you have to do it in the car and its about 1 1/4" long. I didn't take the tank out or the sway bar all though I did remove the two C clamps from it and rotated it up. The half shaft Torx bolts are the ones you want to take your time with as they are easily stripped. If you can I would replace them I have part # if you need it, cost about $6.00. If you read this link and cut out all the extras like removing exhaust etc. you will have no problem.

http://www.bimmerdiy.com/diy/e36diffswap

[Cashburn]

Originally Posted by Dennis Kelley

Cashburn correct me if I'm wrong but you do not have to tap the diff if you get the E36 diff. If you buy the right diff it will have the mount hole already there. E30 diff did not have the front mount hole.

later

This varies on the early BDRs vs. the later cars. They began stepping up the size for added strength. I believe you have an early car though and Mark has a later car.

No need for the tank to come out, try and keep it to a minimum of work.

You should not have any trouble picking up a 3.46 LSD all rebuilt for under $1,000 + shipping. I would consider a more aggressive gear myself, but you probably have a preference so I won't preach.

[Dennis Kelley]

My diff has three bolts holding it in. two in back one in front. Again removing the rear diff in my car is a snap. I;ve had it out several times. I don't know if no.94 is an early car but I'm upgrading my diff to a stock replacement E36 LSD as per Reg's recommendation as to which rear end is in my car.

[Cashburn]

If you have the "old style" (to give it a name) we have one support cradle that adds rigidity sitting on our shelf new in the box. I can make you a deal on it and we now take credit cards

[SkipB]

Guys,
What do you think about a 3.91 LSD. I found one locally for not too much money. The diff is out of an E36 Automatic. Not rebuilt, but supposedly working off a wrecked car. I'm thinking 3.91 is way too low and will make 1st gear almost unusable.

I had asked about a 3.46 LSD and was told this was never a stock ratio and had to be a rebuilt. Does anyone know what LSD ratios were stock? I was also told that the Backdrafts come with a 3.15 open and not a 3.10. Not a big deal, just curious what is correct.

SkipB

[ROUSHAC]

Skip,

A lot depends on whether you will be racing and need to hold the RPM's in a certain range for max torque at all times or just use the car for street cruising.

3.91's may be a bit too low to get anything resembling traction in first gear. You might end up having to start in 2nd all the time on "vigorous" launches. Also, if it matters, it will KILL your mpg.

Do you want to make lots of pretty smoke on the street or do you want to just put the pedal down and shift when you hit redline with minimal tirespin leaving others in your rear view mirror?

I make enough power to have a reasonably good launch and good mpg with the stock gearing, so for me they are a good compromise.

Good luck with whatever you decide.

Cheers!
Dave

[Rick Parker]

3.91 was the gear ratio used in the Boss 302, Boss 429, 429SCJ's equipped with the Drag Pak option in the 70's. They worked pretty well but RPM'd pretty high while cruising on the highway.

[SkipB]

Thanks for the input. I think I'll pass on the 3.91. I am not worried about cruising as 5th gear is almost at idle at 60mph! But, 1st gear might become almost usless.

SkipB

[Cashburn]

We have a couple customers with 3.91 gears that I believe are pretty happy with them. The best gearing in my experience is a 3.73 matched to the TKO-500. The 3.64s are very nice as well.

3.46 is fairly common in stock form as a non LSD assembly.

[ROUSHAC]

But Jay, it's so much fun to be just about 3000 rpm's at 110 mph (just getting into the powerband) and remind your unsuspecting (and one who doesn't realize what a brick these things are to push through the wind at high speeds) passenger that the redline is at 6250 rpms. Not that I would EVER do that to anyone.

Dave

[Cashburn]

YMMV

I hear about a lot of things you guys "should not" be doing with these cars.

[ROUSHAC]

Jay,

Just out of curiousity, what are the guys that are using 3.91's using for engines? Seems like it would be hard to get traction through 1st and 2nd and maybe even third with 3.91's unless you are VERY good "feathering" the throttle.

Dave

[Mark Brye]

Gosh, I know how hard it is to descide on what gears to get. I have been debating between the 3:4x's and the 3:6x's for a good 3 months now. I still find myself waivering between the two gear sets. I just don't want to make
1st gear useless.
Jay,
Thanks for the tid bit on the diff about needing to drill and tap. I saw that Matrix sells a 3:73 "BDR" diff. Maybe this is already drilled/tapped. I wonder if they can drill tap any of their other gear ratio rearends?