What could I put into my 1996 BMW 840ci engine bay? Diesel?

Unfortunately there are so many inter dependant ECU's on modern cars that unless you have an increadibly thorough knowledge of the car and its ECU's and how to program them, nowadays this sort of swap is virtually impossible for the DIY mechanic. I'm sure there are companies out there that could do the job for you but the cost would no doubt far exceed the value of the car or even a factory brand new replacement engine for it.

Yep, I vote M57. Just got the one I put in my Jag running. It's an M57n from a facelift E46. They're a little more complicated wiring-wise than the earlier M57s you get in E39s and non-facelift E46s, but the power ceiling is much higher.

Early M57s need quite a lot of work to get over 250-270ish. I've seen a 330bhp one before, but that had quite a lot of work done. They are, however, easier to swap due to having much simpler electronics. Plenty of plug and play kits, and the throttle position sensors on E39s are cable-driven so you can run them off your original throttle cable.

M57ns like mine have a fair few differences. They're a full 3.0l rather than 2.9, have a different injector spray pattern (shared with all following M57s), and a more capable ECU. Electrically they're more complicated, but the solutions to get them to play nice in a standalone format have been worked through. You can get standalone loom kits that give you an ECU plug with a load of pigtails and labels about where to put them. I did the wiring myself rather than use a standalone kit, but that did take quite a lot of headscratching and trial and error as BMW still views the E46 as a 'new' car and actively works to remove public information on its workings to make people buy their proprietary maintenance systems. It's out there, just takes some google-fu. A company called House of Torque fitted an emulator to my ECU for £162 that mimics the immobiliser codes and lets you start the engine without all of the immobiliser gubbins. There's some other bits and pieces of troubleshooting in my thread (link in sig).

I've got a beefy hybrid turbo, a big intercooler, an E90 v-band manifold, 535d non-piezo injectors, a 4 bar MAP sensor, solid flywheel and E60 530d clutch disc. Hoping to hit 400hp and 650lb-ft (torque limited so I can use the regular clutch for towing). With just a plain remap they make 250bhp. Personally I think either would go rather nicely in an 8-series! They even sound quite good for a diesel.

If it were me, I'd probably go M57 . I'd probably want to keep that as a V8 or V12 myself, but I can see the appeal of an M57.

V8 diesel is an awesome idea and probably would be the one I'd want to go with over everything. However, they were I believe EU market only, and E38s have never been cheap over there, which may make an engine donor tricky. That's before I get to the customs paperwork now needed and the rigmarole around that. But at least you'd enjoy the fruit of your labours! .

I do adore an M57. Had a couple with them now, and driven a couple too. It's probably what made me own an M57 engined car today .

Best version was an E61 535d. Lots of torque everywhere in the curve, and with remaps, they are more than quick enough for what most folks need .

Don't disagree with that statement at all.

The M67 would be quite fitting. What else did they come in? (Goes off to Google).

If you don't mind mixing and matching, and breaking a little more new ground, TDV8s from Range Rovers can be had in either 3.6 or 4.4l flavours...

Personally I don't mind a smaller engined swap so long as it makes more power/torque.