Ser­vice Bay

In­side BMW’s V12 which pro­vided the power for a range of lux­ury sa­loons and stylish coupes.

Classics Monthly - - Con­tents - WORDS ANDREW EV­ERETT

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.

BMW launched its V12 in the 1987 ‘E32’ 7-Se­ries, beat­ing Mercedes to the game by four years.

V12 was also mas­saged by Alpina for use in its re­work­ing of the 8-Se­ries (above left) and the 7-Se­ries.

The M70 was es­sen­tially two sep­a­rate en­gines with two con­trol ECUs and two air­boxes.

BMW Mo­tor­sport built a 500-plus bhp V12 for the still­born M8 pro­to­type.

Much com­po­nen­try was based around pro­duc­tion parts from other en­gines.

The plain 850i wasn’t well re­ceived as a 6-Se­ries re­place­ment.

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