Service Bay
Inside BMW’s V12 which provided the power for a range of luxury saloons and stylish coupes.
This year marks 30 years since BMW’s first automotive V12 engine was launched, 1987 marking the arrival of the 750iL with the M70 engine. This was BMW’s first production postwar V12 engine but the firm had already considered and developed various V12 options during the 1970s, some of which were considered for use in the first-generation 7-Series before being replaced by the turbocharged straight six 745i.
Early V12 prototypes were basically two ‘small six’ M20 engines spliced together and BMW also built the larger M30 into V12 units for testing. The M30-based V12 was too big and too heavy however, and the smaller unit was not considered suitable due to the wide cylinder head and the need to adjust the tappets – simple on the six-cylinder but hard work on a V12 when both inlet manifolds would need to be removed first.
By the early to mid eighties when the M70 was starting to be developed, the M20 was already getting old and development work was starting on its 24-valve M50 successor. However, BMW was also developing a new M40 engine to replace the ancient M10 four cylinder OHC unit that had been in production since 1962.
This was to be a smaller, lighter power unit that was quieter, more powerful and much better on fuel and it would used a piston crown and combustion chamber/port design that was very efficient as well as finger rockers and hydraulic tappets. It also used a belt-driven camshaft and an oil pump integral with the front cover, lightweight connecting rods and pistons. All in all, a state of the art OHC engine for the 1980s.
It’s often said that the M70
V12 is two M20 engines spliced together, but this isn’t true. Apart from the distributor caps and a few bolts, there is nothing in common between the two engines and the M70 is in fact based on the M40 design.
BMW wanted to beat Mercedes Benz to build the first postwar German V12 passenger car and outdo the Jaguar V12 and this they did by four years. But BMW knew it would never be a big seller and as the 750iL was being launched in late 1987, a new generation of lightweight all-alloy V8 32-valve engines was being developed so the V12 was designed as a relatively low-tech unit based heavily on high-volume parts from other engines in production at the time.
In fact, the V12 engine was built using parts (block, crank, heads, cams) which were machined on tooling for the M40 fourcylinder and a number of parts are from the M40 itself: connecting rods, valves and rocker arms are all the same. The pistons are a similar design but made from a different material and with slightly offset gudgeon pins meaning the pistons are different from side to side.
Unlike the M40 though, the M70 uses an alloy block with four-bolt main bearing caps. This was cast in Alusil, a silicon alloy where the block is cast and the cylinders bored, honed and finished but not sleeved. The boring process reveals tiny silicon crystals on which the special pistons and rings run. The steel crankshaft is counterweighted and the M40-style cylinder heads bolted down with stretch bolts. The overhead camshaft on each banks sits in plain bearings and is driven by a roller chain from the crankshaft, and it’s a simplex chain rather than a heavier duplex.
The crankshaft sprocket drives not only the cam chain, but a chain to drive the oil pump which unlike the M40, is bolted to the underside of the block at the front. M40-style valves are opened by rocker fingers and hydraulic tappets and the cams are lubricated by a long oil spray bar bolted to the camshaft caps and fed by pressurised oil.
The lubrication system is conventional, but the oil filter housing is remotely mounted with reinforced rubber supply and return pipes. Cooling is taken care of by a combined water pump and thermostat with the thermostat opening at 85°C.
The electrical and engine management system was state of the art at the time. The M70 was the first BMW to use fly-by-wire throttles without cables and certain cars used two alternators. Bosch Motronic 1.7 was used, and two set-ups were used, one for each bank. That means two sets of plug leads, two distributor caps, two crank sensors and so on, and the the idea was that if one bank went down, you’d still have 2.5 litres and 150 bhp to haul the car along and get you home. As launched in the 1987 750iL (L for
Lang or long wheelbase), the M70 gave 300 bhp at 5200 rpm, and torque was rated at 332 lbf.ft at 4100 rpm. The bore and stroke of 84 mm and 75 mm gave a swept volume
of 4988cc and the compression ratio was a low 8.8:1.
The 750iL and the standard wheelbase 750i were both 155 mph cars and whilst sales volumes were low, BMW hoped to increase the production of the engine dramatically with the 1990 launch of the new 850i. For this expensive new coupe, BMW also offered a six-speed manual gearbox alternative but the same 5-litre V12 with the same power output.
Due to the extremely high cost, the 850i was not a sales success and a more powerful unit was needed to spur sales on. This was to be the 1992 850CSi, a car developed partly by BMW Motorsport with a special hand-built 5.6-litre version of the M70 engine codenamed S70 B56.
This unit sported 5576cc with the capacity increase achieved by increasing the bore and stroke to 86mm and 80 mm, the compression ratio being bumped up to 9.8:1 and the Bosch management system being revised.
Power was increased to 380 bhp at 5300 rpm and torque was now 406 lbf.ft at 4000 rpm. But that wasn’t all. The camshafts were a different profile, but the head castings and valves remained as before. Just over 1500 850CSi cars were built and this V12 remains one of the lowest-production BMW car engines built.
For 1994, the ‘E32’ 7 Series was replaced by the all new ‘E38’ model and with it came a new 5.4-litre version of the V12 engine. Now coded M73, the 5.4 was fitted into the 8 Series from January 1994 and it was extensively revised.
BMW renamed the slow-selling 850i as the 850Ci in 1993 to bring the car in line with the new 4-litre V8-powered 840Ci but earlier Ci models retained the 5-litre V12. The bore and stroke were not (oddly) standardised with the 5.6 engine but were new at 85 mm (bore) and 79 mm (stroke) to give 5379cc, with the forged steel crank using a new design of thrust washers first seen on the M60 V8 engine.
The compression ratio was a nice high 10:1 and the power rated at a useful 326 bhp at 5000 rpm, and the torque increased to a very handy 361 lbf.ft.
However, the M73 incorporated a lot of improvements over the M70. For a start, the finger rockers were replaced by a new roller rocker system as developed for the BMW M43 four-cylinder, the unit that replaced the M40 in late 1993. BMW had experienced wear problems on the M40 engine (but rarely on the M70) and so the M43 used roller rockers that are impervious to wear and a major improvement with benefits to economy and refinement.
The crankshaft position was now monitored on the flywheel and not the front pulley. Oil cooling jets for the pistons were also used. Also, the old Bosch 1.7 system was never that great and was replaced by the far better and more reliable Bosch Motronic M5.2 system with twin flywheelmounted TDC sensors and knock sensors in the cylinder heads.
Smaller diameter camshaft bearings reduced friction and softer double valve springs did the same, and solid steel exhaust valves replaced the sodium filled valves used on the later M70 unit – sintered metal camshafts now replaced the solid steel items.
On these, sintered metal lobes are made, finished and then pressed onto a steel hollow shaft and the result is a lighter cam with better wear properties. Finally, secondary air injection was used for emissions and steel tubular exhaust manifolds that heated up faster than the cast iron manifolds on the earlier engine.
In this final form, the M73 was used in the 850Ci until July 1997, and in the E38 750i until July 2001. By this time the M43 four-pot with which it shared parts and machine tools was leaving production and from January 2002, a new N73 6-litre V12 engine went into production based on the concepts and machine tooling used to produce the new-generation ‘Valvetronic’ four-cylinder engines. The 5.6-litre 850CSi left production in October 1996 and didn’t receive the M73 round of improvement, keeping the original M70-type block and rocker fingers to the end.
STRENGTHS AND WEAKNESSES
The BMW V12 is a tough old bird, being very unstressed and mechanically simple: there’s no variable valve timing or multi-valve technology for example. The cooling system is generally reliable and it’s almost unheard of for an engine to suffer head gasket failure or crankshaft problems. Camshaft wear is possible on the M70 and S70 and if you hear the ‘clack-clack-clack’ of a worn cam lobe, you need to remove the inlet manifold and cam cover that side and investigate. A new cam isn’t hard to fit once the timing chain has been detensioned.
Oil leaks are the main bugbear, with the sump gasket being a favourite. The sump is a two-layer alloy job and it’s the sump to engine block gasket that can fail. The official repair is to remove the gearbox to access the rear sump bolts but you can drill goodly sized holes into the bellhousing to access them with a 10mm socket on a wobbly extension.
Early examples of the 750i and 750iL were known for engine management problems, with the electronic throttle bodies being particularly troublesome. These issues were largely ironed out by 1990/91 and the M73 is generally very reliable with few problems. Changing the 12 spark plugs on any V12 is never a five-minute job though. Water pumps can suffer bearing failure and whilst it’s fairly easy on an M70 and S70, the bigger crank pulley damper on the M73 means that this has to come off before the pump can be replaced.