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For My Next Trick…


There are good ideas and there are bad ideas – and then there are those ideas that are so bad they come full circle and become good ideas. The corollary is also true: Really exceptional ideas can turn bad, if pushed beyond the bounds of good sense.

“Great, Jonathan. Very profound,” you mutter.

Further, it’s okay to disagree with your friends, even those closest to you. Friends will give you all sorts of advice, ranging from marriage details to a career change, to having that one last drink before hitting on the waitress in the bar. They are your friends, after all, and they generally have your best interests at heart – or at least their best interests, which are usually peripherally connected to yours.

“Fabulous. What exactly are you on about now?”


I want to build a BMW E23 7 Series (1977–1987) with a BMW M73 5.4-liter V12 from an E38 7 Series (1995–2001). Roughly half of the people I’ve floated this plan by think it’s a bad idea, including a former employer and close friend – a guy whose opinion I hugely value, and who shall remain nameless, except to say that he is the sole proprietor of Bill Arnold BMW Repair in San Rafael, California. My cohort here at TTS, Reed Hitchcock, thinks I’m smoking something funny, either in leaf or crystalline format.

The other people think it’s a great idea, but each is the type of person who would stuff a BMW 4.4-liter V8 into a Bavaria, or an M30 3.5-liter Big Six into an E30 3 Series over the course of a weekend, using nothing but hand tools and ambition. One of them has even offered to drive a clean, but sunburned, 1983 745i from Phoenix to Minnesota for nothing but the cost of a bus ticket home. That’s dedication, kids.

Yes, I understand that there are many challenges with a project like this. While I think I have most of it worked out, there will still be details – the answers to which will not be known until the wrenches meet the bolts, as it were. The first, and probably most obvious, question is whether or not the V12 will physically fit in the E23 engine compartment. I’m confident that it will. Why not? This V12 is nothing more than two SOHC inline sixes fused together, and thus won’t be much longer than either one of them. The V12 is also narrower than a BMW V8 (a 60-degree vee versus 90), and there is at least one V8-powered E23 running around.

Old Timey BMW V12

Old Timey BMW V8

BMW actually built V8- and V12-powered prototypes in the late 1970s. These are BMW’s factory photos. The V12 engine looks massive compared to M70 five-liter that came later in the E32 7 Series.

The battery and its tray will need to be relocated to the trunk. No E23 has a traditional vacuum-assisted brake booster, so there won’t be a fitment issue there. M73 exhaust manifolds are specific to the engine, and the driver-side pieces will have to be routed around the steering box. Again, it will help that the V12 is relatively narrow.


BMW V12 powerplants are let down largely by their overly-complicated engine-management systems, complete with multiple computers and motor-driven throttle bodies. The odds of getting this disaster salad to talk to an analog-era BMW chassis is slim. So why not just do away with it all and use a stand-alone system like MegaSquirt 3?

The basic V12 engine – the hardware, if you will – really isn’t that complicated. It doesn’t have VANOS, BMW’s adjustable cam timing system. It doesn’t even have coil-on-plug ignition. You need a handful of sensors and something to control the ignition and idle, and that’s it. One challenge will be finding the suitable mechanical, cable-driven BMW throttle bodies to replace the electronic versions.

Of course, the V12 will need to route its power through a proper manual transmission. The only manual that easily bolts up is the BMW S6S 560G (Getrag Type D) six-speed from a BMW E31 8 Series. They are not cheap, and they are not easily obtained. However, there exist several custom adapters made from a V12 automatic-transmission bellhousing, which allow the use of a good old-fashioned Getrag 265 five-speed. It uses a clutch-slave cylinder from a Tremec T56 six-speed transmission, and a clutch adapted from the General Motors parts found bolted to a re-drilled BMW V8 or V12 flywheel. While it won’t be easy to create this adapter, the only real alternative is to spend $3,500 or more on the bolt-up option.

BMW E23 Interior

With any major surgery such as this, there are always a million details to consider and work through: motor mounts; welding up the EGR ports on the exhaust manifolds, swapping in an M70 lower-front timing cover and pickup wheel so you can use a front-mounted crankshaft position sensor rather than the stock M73 flywheel sensors, determining which throttle bodies are going to fit (and then making them work with the E23 throttle cable), getting MegaSquirt 3 to talk to an ancient E23 gauge cluster, figuring out how to make the M73 power-steering pump run the E23’s hydro-accumulator power-braking system. Then there’s the air-conditioning system….

I’m sure that by now at least some of you are wondering why. Why put all this effort into a big old pig like an E23? Why not just turbo an M30 to 400 horsepower and be done with it? What are you going to do with this creation after you’re done?

Easy. Everybody does the M30 turbo thing. I’ve always liked the V12. And I’ll drive the hell out of it!


10 thoughts on “For My Next Trick… Leave a comment

  1. This has got to be (by far) the most insane BMW based project I have ever heard of. The tears that will be shed, amd the cubic a$$loads of money that will be burned would make a NASA contractor blush. Only someone who hates his time and treasure would even consider this

    Having said this, however, I like it. Count me as a big F’ing YES. Daddy like

  2. I think its a great idea. I’ve seen pics of a prototype M70 in an E23 and for a minute I thought it was just a regular E32 750. I want to do this swap with my extra M70 into to a euro E23. Seems it would be a little complex wiring wise but I’d just use my complete 750il harnesses. I wish somebody already accomplished this, it would defiantly be helpful with some insight and provide a great amount of motivation!

  3. I think it would be more complicated with the stock E32 DMEs and all the inputs, including the electronic gas pedal. The harness for the M70 isn’t that scary, though. Converting to manual throttle bodies and an aftermarket ECU would be easier, I think, if not actually easy.

  4. Pre 2010, I believe the Getrag S6S 560G transmissions were shipped from Germany for $8099 USD +$200 core, not $3500. Around 2010, there was a short time where they were ‘blowing out’ the N.O.S. trannies for around $2500, but that didn’t last long.

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