Does anyone know how compatible a motor from an 1988 750iL (12cyl) is with a 633CSi chassis? I currently have the motor from an '87 535 in it, but have a chance to get the 750 at a good price.

TIA,
Ian

Lots of work for no power and no possible power upgrades! The V-12 can not be rebuilt without a lot of cost, if it is bored, it must be sleeved, stroker cranks are very expensive ($4000) camshafts are expensive ($1000 ea) If left stock, it is only 300 HP, 1988 engines have smaller exhaust valves and have a tendency to break head bolts.

Transmission choices are only 3, a 6 speed from a E31 850 and that will require a lot of tunnel surgery, the automatic that came with the 750, or a Tremec 6 speed with a custom adapter that has to be made by someone, I started one but never finished it.

Go here for my fit check a few years ago, gave up on it and went to the LS6
http://www.mwrench.com/m6v8/v12install/

Would the transmission from the 750 fit into the 633? (the hole align and stuff) I would not discard the 3.5 motor, I was just wanting a toy so I can rebuild it without loosing transportation. I would like to upgrade the 3.5 motor with a turbo kit, but I don't quite have the money right now, and I will need a motor to use while the it is being rebuilt. The 5 liter was offered to me at a reasonable price (actually the whole 750iL, but the brakes need redone and there is damage to the rear passenger door) Also, will the CPU be compatible with the chassis if I use the one from the 750 or will the current 633CSi cpu suffice? Currently the 633 is manual.

There is nothing that is compatible between the 750 and the 633. The 750 has two of everything. Electronics are not compatible, nothing crosses, wrong idea to use the V-12 for anything in the 633 if you are not going to stay with it!

It will cost you more to put the V-12 in the 633 then to do a turbo kit on the 3.5 engine.

HTH

as much as im really interested in this conversation, check it out guys.

Engine Conversions Major projects involving upgrading BMWs to accomodate non-BMW engine swaps.

Even though everyone has advised against it, I am going to do it. I think it will be fun. I will be sure to take many pictures and maybe some video to document this. I hope I don't screw anything up (don't comment on that last bit, that wouldn't be very nice).
I am going to keep the 12 cyl motor permanently.