Question: Rear end bushings for turbo LS

[amklein]

I have been working on a turbo LS swap into my 740i. I bought it non-running but I was able to get it daily drivable without too much effort. I am trying to avoid having the car un-drivable for long periods of time.

One piece of preparation that can be done well in advance of the motor swap is a rear end swap. I picked up a low mile e60 M5 differential and axle shafts and I will be stuffing them into a rear subframe that I picked up. The 3.64 ratio should be not-awful paired with a tr-6060.

I really want to keep this vehicle reasonably luxurious, so I don't want to just hard mount everything. Has anyone had issues with rubber subframe and diff bushings for higher horsepower builds? I plan on replacing all the bushings while I am in there. Lemforder subframe bushings are only around $22 a piece for the e38, but I know they were never intended to isolate a rear axle seeing this much torque.

[JimLev]

I’d box in the front diff mount to strengthen it. It can/will break the welds on high HP upgrades.
Which LS are you using? I’m using an LS3 for my hotrod project, not a BMW.

[mrallande5020]

What rear subframe did you go with?

[amklein]

I am planning to use a regular e38 rear subframe.

For the motor, I am using an LC9. The DOD lifters go bad often enough it is easy to find sub-100k long blocks for sale for $500-1000. Its a fully aluminum 5.3L with all the gen 4 upgrades, so lightweight and durable.

Currently the plan is to keep both rubber bushings in the back connected to the diff cover and use two forward rubber bushings, so four total.

The reasonable thing to do probably would have been to find a 210mm diff out of an e38 and stick the e60 m5 guts in it, but I decided to try something different. I pulled up come ebay pictures of the diff cover off the 210mm e38 differentials and designed a hangar bracket for the diff cover.

Capture.jpg

The mounts on the diff covers twist the differential in the subframe. I measured the "twist" of the e38 diff I had at around 1.25 degrees, so I milled the m5 diff cover bolting face (not the sealing surface!) and spotfaced the hangar bracket accordingly. Here is the m5 diff with the hanger bracket next to an e38 diff.
PXL_20210422_193146876.jpg


Interestingly, judging by the two mounts on the nose of the e60 m5 differential, it is twisted the opposite way that the e38 diffs are. I have no idea why that would be, but again, the best direction I could come up with was to emulate the mounting position of the 1995 e38 differentials. I think I will just lop off the existing subframe mount and weld on two more to match the m5 diff's odd angle.
PXL_20210422_200447711.jpg

[mrallande5020]

Oh this is dope, sounds like you have a really nice plan and it should work no problem. The cover hangar bracket was a really nice idea.