Physically to fit the S52 into the E34 is fairly simple. You only need to remove the E36 engine mount arms and install E34 M50 engine mount arms and swap on the M50 oil pan. To swap the oil pan also requires swapping the oil pickup tube from the E36 rear sump style to the E34 M50 front sump oil pickup tube
Above is the stock S52 rear-sump pickup tube setup and the below image is with the E34 pickup tube installed
However, that rear mount bracket for the S52 pickup tube now interferes with the E34 oil pan. There are two ways to remedy this - you can remove the crank main bolts on that crank journal and remove the bracket completely (which requires replacing those bolts and following the correct torque angle procedure with new bolts for re-installation).
Or you can mask off the entire engine to protect against shrapnel and cut off the bracket, leaving a little nub of it in place while allowing ample clearance for the E34 pan. This is what I did - I didn't want to loosen a crank main and then re-torque it out of sequence. I also didn't want to loosen and replace all the crank main bolts just to remove a bracket. The bracket was not particularly strong steel, a steady hand and a small rotary tool with a metal cutting disc made quick work of it. I also rounded the edges nicely just in case anyone is poking around there in the future
Electronically this is a very simple swap, because you convert the S52 to OBD1 and run it like an M50 VANOS, with a chip tune to account for the S52's larger displacement and different injectors. If you are starting with a VANOS M50 car and installing an S50, you almost have no work to do at all in this regard. Just swap the E34 M50 VANOS harness onto the S50, install the S50 DME and you are good to go electronics-wise. If you are going to re-use your M50 VANOS harness (or removing it from a different engine yourself) I recommend labelling the harness when you remove it from the M50
If you are starting with a non-VANOS M50 E34 and installing an S50, you need to source an M50 VANOS harness and should also source the DME box bracket that holds the fuses, power cable and harness on the M50 VANOS cars. Otherwise some of these 12v wires have nowhere to go as shown below
You may also need to extend a few ground wires as seen in the below image
If you are installing an S52 then follow the above steps and also OBD1 swap the S52 as detailed below
M52/S52 OBD1 Swap
1. M50 VANOS DME (part # ends in 413) 2. M50 VANOS engine harness (use an E34 harness if swapping into an E34)
3. M50 VANOS crank position sensor (leave the OBD2 crank sensor in place to seal the bellhousing)
4. M50 VANOS cam position sensor
5. M50 VANOS VANOS solenoid (may or may not be required, just make sure it is the correct length to reach the engine harness plug)
7. M50 VANOS HFM (part # ends in 502)
8. M50 VANOS or M50 non-VANOS exhaust oxygen sensors
9. M50 VANOS or M50 non-VANOS knock sensors
10. OBD2 single to OBD1 dual harness coolant temp sensor adapter (you can also use your stock parts to make a custom harness for this)
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