745 Differential Story,Email between John and Matt (archive)

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Here is the low down on the differentials. I have learned the hard way on this due to mis-information from the "experts" at the dealership, salvage yard and others. I have my E23 3.46 LSD I ordered in and I spent this evening changing it out but stopped in the middle. As soon as I pulled the 745 2.92 LSD out of my car I was in for a
shock. The e23 3.46 out of an 85 735 is a little piece of junk compared to what came out of my car. Although the 3.46 gears would be ideal for the 745, the diff. from an e23 735 is a light duty part compared to the larger HD diff. that came out of my 745. I cannot think that all the guys that have changed theirs out for one of these did not catch this major delima unless the 735's have a big and small rear end that I an not aware of and have never seen. I just checked my 735 and its the same little thing as the one I just got in. If others took out their 2.92 diffs. and put a 3.46 out of a 735 in, then they have weakend and downgraded their cars in a major way. Its like changing out a 12 bolt posi on a GM car and putting in a vega rear end with quicker gears. Although the 735 diff bolted up in my car just as the 2.92 came out, I am not going to even try is as I am certain that I would rip this thing apart. The differences are;

1. 745 rear end weighs about 30 LBS more than 735
2. 745 rear end case is about 4" taller which means it has a larger ring gear.
3. 745 rear has hardend steel output flanges, 735 has little cast
4. 745 rear has a larger input (pinion) dia. than the 735

This is just from a basic look on the outside and I am not going to even bother to compare the insides as I know what I need to know now, and I would not use this 735 diff. in my car even if it makes it quicker.

I have encolsed some pictures of the 2 rear ends side by side and need your help before I move on. The 745 rear end is on the left side of the dolly and the and 735 on the right. I need to know if the 745 rear end on my dolly is the same thing visually that is in your M6 and the one you just orderd for your 745 from BMW south. I have a strong feeling now against the advice of others that the 745 shares the same rear end as an M5/M6 just with 2.92 gears. This would make total since to me and I should have known as I own both a 735 and a 745, that every part in the driveline
of a 745 is far superior to a 735. I almost bet there are vast differences in the ZF22HP other than just the torque converter too. If this rear end on my dolly is the same as your M6, then I probably am going to be stuck with 3.73 gears unless there are 3.46's made for this larger rear
and I bet there are not. The ratios for the large rear end that I am now certain of are 2.92, 3.73, 3.93. If you or anyone knows of any others, please let me know on this
too as it might just make since to by a ring and pinion and put in my rear end.

I am also going to post this on the forum to see if anyone else knows the real deal on this subject as I should have know this, but everyone including Bavarian Auto Recycling, and a 20 year technician that I know assured me that the rear end of all e23s of the same year are the exact same things with different gear ratios. This is far from the
case.

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