Differential parts compatibility - DISCUSSION

[cooljess76]

Originally Posted by spidertri

Cool, that's what I was hoping you'd say. My car is manual. I've been playing with the gear ratio calculator and I'm still deciding if I'll be able to handle the new highway rpms.

It's gonna rev pretty high, I think Bluebimma ran a 4.10 for a while. Marv17 ran his stock 4.44 for a couple days after swapping in an S52 and ZF 5speed. He said the tach was pegged out at highway speeds and the gears were so short the car was practically undriveable, lol. You might consider swapping the ring and pinion gears between the two or swap the LSD into your current diff. It's a pretty tedious task though.

[spidertri]

That's an interesting idea, swapping the ring and pinion gears. Stock is 38:11 and the 4.10 is 41:10. I could go with a 41:11 or 38:10, which would give a 3.72 or 3.80, respectively. I'll have to look into that.

[Bluebimma]

4.10 is not undriveable, ive done it for the past 2 years. I still get 24 city, 32 highway with the m50.

[sam can man]

yay gay how it is so great that every thing bmw interchances,,, sory for the misspelling bmws are so great, you can customise to the whay u want

[wolferj-RIP]

I just completed the final drive swap...

Used e30 325is med case diff with 2.93 LSD, output flanges, e30 M3 half shafts.

[DaveKern]

I've got a couple questions for you diff swap experts. Here's a little background first:

I'm building a rally car - i.e. competition only
I'm swapping to a MkIII Supra rear diff b/c ring & pinions are just $140 new...even high ratios
I'm going to be running a 4.56 ring & pinion
I need to have custom adapter rings made where the axles bolt to the diff, and need to know how thick to have them cut.

I see that when folks upgrade from the small to the medium diff that using E30 6-cyl axles is needed. Can someone tell me how much wider the medium diffs are? How much shorter are the axles?

I have measured the width of the small diff at the axle mounting surface and they are 10.25" wide.

Thank you,

Dave

[spidertri]

I just measured my E30 3.73 and it was 11" wide. I don't have the E30 axles yet so I can't help there.

Edit: Got the axles, they are 22 3/4".

[Fuzzy Nutbucket]

Ok so pulled the differential and axle shafts out of my ti so I can swap in an e30 2.93 LSD unit and e30 6cyl. axle shafts. (for my s50b30 swap) How ever the length from the face of the input flange to the center of the front mount holes is about a 1/2 inch or so longer then the small case diff that was in there, after fighting with it for a half hour or so I gave up for the night, and figured I'd see if anyone here has any input, help or ideas for me. I'm totally stumped here, and on a side note, can i swap speed sensors so that the plug will work or should i just by a plug with a pig tail and splice it in place? Any and all input, ideas, or help is appreciated!
Thanks!!

[wolferj-RIP]

FNB-

I did the exact swap you are doing. It was a very tight fit, and I, too thought the e30 case was longer, but after triple checking and swapping the input flange, it fit. Ended up loosening the driveshaft center bearing so there was some play, and that helped get it in place. Also needed to make a little rear clearance room at the back of the diff by whacking a small cross beam a couple of times.

[Fuzzy Nutbucket]

Hey Wolferj,
Thanks, what did it take to swap the input flanges? Looking at it I figured that it would fit fine as far as the chassis cross bit was concerned as long as the drive shaft was like, 1/2in. shorter. but i was just eyeballing that. What about where the torque mount (big "ear")on the rear cover mounts up, did that have to get bent out any or no?
Thanks again!

[wolferj-RIP]

Originally Posted by Fuzzy Nutbucket

Hey Wolferj,
Thanks, what did it take to swap the input flanges? Looking at it I figured that it would fit fine as far as the chassis cross bit was concerned as long as the drive shaft was like, 1/2in. shorter. but i was just eyeballing that. What about where the torque mount (big "ear")on the rear cover mounts up, did that have to get bent out any or no?
Thanks again!

Well, the input flanges turned out to be the same. I, like you, thought that when I looked at them, the med case was longer and the flange was the culprit. However, there is no difference in overall length for the input section. My issue is that the trans sits back a little farther due to the 6 cyl swap, so an adjustment at the central joint did the trick. We did expand the width of the ear mount, but it was just to give a little more manuver room to get the diff in place. The chassis cross is only an issue for 95 models (like mine) to make a little room for clearance for the top cover bolts, not the mounting bolts.

[Fuzzy Nutbucket]

Well I know that the trans. is sitting back farther due to the s50, like you said, and that the DS did suck up a little but I'm not sure there was enough length left for it to shortn' any more. Plus that was infront of the center bearing, can the bearing mount move, I don't recall if it was slotted or not...

[318ti808]

I am looking at buying a 1995 318ti with a 1999 s52 modified with a 95 intake. It is running the stock trans and drive line as well as the stock rear end. Would like input on the rear end. I have J!ms swap manual which highly recommends changing out the diff as it could go at any time. I also emailed him and he recommends the same. I am purchasing the car from Sacramento, Calif and shipping to Hawaii so getting parts in Hawaii is not easy. The PP seller is willing to help out by swapping in a M3 coup rear setup or the appropriate diff from a 325i. I am looking at $355 for diff and install vs $1500 for the full upgrade. The car purchase price is $7800 (has suspension mods struts and shocks all around. He is throwing in M3 front suspension and brakes that I will install). What is the experience of running the stock rear end? I have seen a couple of recent posts that indicated they have run the stock rear end for a year. Any other input would be greatly appreciated.

What happens to the rear end that blows up, too much pressure and blows the rear cover? grinds up the gears due to too much torque?

Any comments/ adivse would be appreciated.

Any one with swap knowledge in the Sacramento area that would be willing to go look at the car for me?

I had the local dealer do a pre-inspection purchase, which noted some minor repairs such as trans mounting and guibo replacement, but other than that it looks good mechanically. Dealer was not able to hook it up to their computer however as that was not functioning on the car, bummer. I am basically purchasing sight unseen, although I had a good buddy check it out for me for general body and interior looks and condition. The ti body has 185,000 but is in good shape. The motor has under 23,000 miles and 75% of the reason I want the car. My dream car is a 1998 or 1999 M3 convertible, so this motor could fill part of that dream. I already have the hardtop for the convertible sitting in my sister's garage in Sacramento.
Norman

[cooljess76]

Any number of things can cause diff failure and the results can be as minor as howling, chatter or grinding and as major as the thing exploding, locking up causing you to wreck, bent or snapped driveshaft, damaged transmission, ujoint damage, axle damage, subframe damage, or worst case scenerio... pole vaulting your car off the highway, lol. The ring, pinion and planetary(spider) gears are much smaller in the stock small case diffs and will not hold up to the added torque of the 6cyl engine. Bearings could wear out causing things to lose their alignment, metal can fatigue from overheating or low oil levels, teeth can snap off of the gears, shrapnel can get sucked into the ring, pinion or planetary gears. There's a reason why BMW put medium case diffs in their 6cyl cars. While it may hold up for a while, failure is inevitable. Think of it as a ticking time bomb. When it goes, you might get lucky and catch it in the early stages, or it might just cause you to wreck.

[318ti808]

Jess
thanks for your input.... the howling of the gears as warning is good; the balance - aaah I will definitely at the minimal have the 325i rear end swapped before I have it driven to the dock to ship.