BMW M70, M72 and M73 V12 Engines

M73 5.4 litre V12.

Chronology

There hadn't been a production V12 from any German manufacturer since the war, Mercedes had made do with large V8s. In the early 80's BMW's biggest engine was the M30 3.5, it was superb but lacked the prestige needed for an uber saloon. There had been plans for a V8 and V12 in the 70's and test units were made but not produced. So the M70 5.0 V12 was to be a big step for BMW.

When the E32 seven series was launched in 1987 it took the market by storm. It was a simply fantastic car and the range was topped by the 750i V12. Not long afterwards the futuristic E31 8 series replaced the 635CSi and only came as an 850i until the launch of the M60 V8s in 1992.

BMW developed many new technologies for the M60 V8 and M50 six cylinder engines that came after the V12. For the 1994 launch of the E38 7 series a modernised M73 V12 was released which saw an increase in capacity to 5.4 litres.

An M73TU with modified emissions systems such as a catalyst with electronic pre-heating to improve cold start performance entered production in 1998. It saw service with both Rolls Royce and BMW.

A four valve per cylinder M70 development programme was undertaken but the resultant M72 engine never entered production. It was said to offer unfavourable noise, vibration and harshness for the large saloon class. Of course a four valve M70 derivative did see the light of day, but in the Mclaren F1 super car. There was no //M8 other than a few prototypes, partly due to the early 90's recession and the likelihood of slow sales.

It's worth mentioning the BMW V16 "Goldfish" project. This took the V12 a bit further as a technology demonstrator but did not enter production.

Alpina E38 B12 5.7 version of the M73.

Design

The M70 didn't push the boat out too far technologically. It had the same 91mm cylinder spacing as the M20 six but used a new alloy block with Alusil coatings. The engine used two six cylinder distributors driven off the ends of a camshaft in the traditional Motronic format. So the fuel injection was tried a tried and tested recipe. In fact there was one Motronic DME per cylinder bank.

The engine was designed with a 60 degree vee and had two valves per cylinder. One new invention was the electronically controlled or fly-by-wire EML throttle body with one being used for each bank. This made a traction control system far easier to incorporate.

The same style of hydraulic tappets used on the M40 four cylinder engines (released the same year) were used on the V12. Presumably the thought of doing a manual valve adjustment on 24 valves every 24,000 miles was too much even for Munich's engineers! A single row timing chain drove the camshafts in a triangle configuration with the top section going directly across the vee from one bank to the other, the 70's prototype had used belt drive.

BMW had made great advances in technology in the late 80s and early 90s. In 1994 these developments gave rise to a new M73 5.4 litre V12 with lower emissions and better economy. Surprisingly it didn't use one coil per cylinder, this must have been too awkward in a two valve per cylinder engine and the small volumes didn't justify the development costs.

Internal friction was reduced by using roller bearing camshaft followers, a system developed for the M44 1.9 engine. The camshaft lobes were now made separately and shrunk thermally onto the camshaft core.

Details on the M73TU are very hard to come by. I believe it introduced VANOS on the inlet camshafts and I've heard (though I'm not sure I believe) this it had one coil per cylinder. The parts CD is very vague on this and contradicts itself. In any event it seems never have been fitted to the eight series and only to the E38 seven.

M70 5.0 V12 in an E32 750i.

Variants

Code	Size	Power bhp@rpm	Torque lb/ft@rpm	BHP/ litre	Torque/ litre	Made	Bore x Stroke	Timing	Weight (kg)	VANOS
M70	5.0	300 @ 5200	332 @ 3900	60	66	87 - 94	84x75	Chain	155	No
M73 M73TU	5.4	326 @ 5000	361 @ 3900	60	67	94 - 2002	85x79	Chain	150	Single in M73TU

 

M73 5.4 litre V12 in an E38 750i.

Problems

Starter motors are generally reliable, just as well as changing one is a nightmare and must be done from below. Access is dreadful.

If the distributor caps or rotor arms are worn the engine will be down on power and get poor fuel economy as this is how the Motronic system controls the spark timing. They should be check at 80,000 miles.

The intake manifolds join onto the heads via vibration reducing rubber couplers. These don't last forever and replacing them is a big job. If you suspect a vacuum leak pray it's not from here.

Early engines can suffer oil leaks from the upper timing cover.

The oil spray bar banjo bolts can slacken and cause problems, just like the M30.

EML faults can result from the throttle stepper motors. BMW says they can't be stripped and cleaned but they can if your really careful, I've done one myself.

If looked after these engines are not the nightmare you might imagine a V12 to be. But my preference would still be for a V8 as it's lighter. It's a mechanic's engine, if you have to pay someone to work on this for you then you're going to need very deep pockets.