V-12 Return

Neither the turbocharged six nor the twin-cam M88 engine would return for the subsequent E32 7-Series line, as BMW began having second thoughts about the 745i even before the introduction of the M106 engine. The turbo six had been expedient and cheaper to build than a V-12 (although that wasn’t necessarily something one wanted to brag about in this rarefied class), but its fuel economy benefits had proven more notional than practical, and had come with too many compromises. It had developed a certain following — the 745i accounted for about 5.6 percent of all 1977–1986 E23 7-Series production, a respectable fraction, all things considered — but it appealed most strongly to a particular subset of existing BMW fans, which we suspect was simply not enough, given Von Kuenheim’s preoccupation with matching Mercedes-Benz.

In early December 1982, BMW engineers began serious work on a new V-12 engine. Development proceeded rapidly and in great secrecy: The design was mostly finalized by June 1983, and the first prototype engines were running on test stands that October. Dubbed M70, the new V-12 was based on the architecture of the latest M20B25 small six (although the commonality was primarily in production tooling and transfer equipment rather than in actual parts), but the block was aluminum rather than cast iron, keeping dry weight to 529 lb (240 kg), some 77 lb (35 kg) lighter than the stillborn 4.5-liter V-12, but only a few pounds lighter than a cast iron Chevrolet V-8. Many of the V-12’s components were identical between both banks, with each bank having its own distributor and Bosch Motronic injection system, which was obviously more expensive on a unit basis, but helped to reduce tooling costs. Like contemporary BMW small sixes, the M70 had single overhead camshafts (chain- rather than belt-driven) and only two valves per cylinder.

The 4,988 cc (304.4 cu. in.) M70 V-12 engine, seen here in a 1992 BMW 750i, was an imposing sight, although its specifications — single overhead cam per bank, two valves per cylinder — were nothing special by the standards of its time, nor was its 60 hp/liter specific output. (Photo: “TCM24 19 – BMW” by Herranderssvensson, which was dedicated to the public domain by the copyright holder under a Creative Commons CC0 1.0 Universal (CC0 1.0) Public Domain Dedication)

The M70 shared the M20B25 six’s bore and stroke, giving a total displacement of 4,988 cc (304.4 cu. in.). While the original goal was to match the stillborn 4.5-liter’s 272 PS (200 kW), the production M70 bettered its predecessor’s output by 10 percent, giving 300 PS (220 kW, 296 SAE net horsepower). Unlike the M102 and M106 engines, even the West German version had three-way catalytic converters with oxygen sensor feedback control. In the E32 7-Series, linked to a four-speed ZF 4HP24 automatic (the sole transmission offered), the V-12’s DIN standard fuel consumption was up to 25 percent higher than the M106-powered E23 745i, but BMW claimed that the short-wheelbase E32 750i was a half-second quicker to 62 mph (100 km/h) and 0.9 seconds quicker through the standing kilometer, with a much higher (and electronically limited) top speed of 155 mph (250 km/h). The first AMS full test of the long-wheelbase 750iL returned 15.2 mpg (15.5 L/100 km) overall, significantly better than the old 745i had ever managed, and on regular unleaded gasoline rather than leaded Super.

More importantly, the M70 finally allowed BMW to offer something on its senior models that Mercedes-Benz couldn’t yet match. The latest W126 560SEL had more displacement — 5,547 cc (338.4 cu. in.) — and nearly as much power (although the 5.6-liter Mercedes engine could only match the 300 PS (220 kW) of the BMW V-12 without catalytic converters — but the first Mercedes V-12 engine wouldn’t arrive until 1991, a small but no doubt satisfying moral victory in the ongoing war between Munich and Stuttgart. Moreover, the V-12 cars’ power and snob appeal could be appreciated even by customers who would never exceed 70 mph (112 km/h), and since they posed fewer packaging problems and were catalyzed even in West Germany, they could be certified for U.S. sale without undue difficulty.

About two-thirds of V-12 E32 7-Series sedans were the long-wheelbase 750iL, like this one, with wheelbase stretched from 111 to 116 inches (2,832 to 2,947 mm) for greater rear-seat legroom. The short-wheelbase 750i was dropped in some export markets (including the U.S.) early on, but remained available in Germany through 1994. E32 cars were far more aerodynamic than the E23, with drag coefficients as low as 0.32 (0.34 on V-12 cars). (Photo: “BMW 750iL E32” by nakhon100, which is licensed under a Creative Commons Attribution 2.0 Generic license)

In a 1987 CAR comparison of ultra-high-end sedans, the acerbically erudite LJK Setright dismissed the 750iL as inferior to the 735i, “neither quieter nor sweeter … nothing more than a badge of rank among those for whom promotion is essentially stipendiary.” Nevertheless, the E32 750i and 750iL were far more commercially successful than the old 745i, accounting for 48,559 units through 1994 — 15.6 percent of E32 7-Series production. The V-12 returned for the E38 generation, now bored and stroked to 5,379 cc (328.2 cu. in.), becoming the M73. For the stylistically unfortunate E65 7-Series in 2002, BMW replaced the SOHC M73 with the all-new N73, this time a DOHC 48-valve engine displacing 5,972 cc (364.4 cu. in.).

Although most BMW diesel engines had long been turbocharged, BMW didn’t return to petrol turbocharging until 2006, beginning with the twin-turbo N54 six, first introduced in the E92 335i. Turbocharged engines were initially offered on the performance ends of the range, and gradually spread to include less-racy models. For the F01 7-Series in 2008, BMW followed the lead of Mercedes-Benz in turbocharging its latest V-12, dubbed N74, which now had twin Garrett turbos and 544 PS (400 kW). This engine returned for the subsequent G11, initially producing a whopping 610 PS DIN (449 kW) — over twice the output of the original normally aspirated M70. (A version of this engine remains in production for Rolls-Royce, although BMW finally dropped its own V-12 models in 2022.) The modern BMW lineup has become increasingly turbo-heavy, with the battery electric models now the sole exceptions.

(Photo copyright © 2024 Gregory Mazur; used with permission)

Today, more than 40 years on from the turbocharging renaissance that spawned the E23 745i, the technology has finally matured enough to pay off the promise suggested back then. With the addition of variable valve timing, direct injection, and more sophisticated engine controls, it’s now possible to produce tractable and relatively economical turbocharged street engines offering 100 or more horsepower per liter. This has made engine displacement a matter of diminishing importance, not least in the relationship between model designation and actual engine size. (Current BMW petrol models identified as “-30i” generally have 2-liter (121.9 cu. in.) turbo fours.)

BMW could have been ahead of the curve had they continued developing the turbo package beyond the M106 engine (which had made some useful strides in practicality), but it’s clear that the 745i was always more about corporate ego than efficiency or mechanical progress. With the advent of the V-12 750i, BMW finally had the prestige engine they’d wanted in the first place, and it sold well enough to make it seem like they had missed a bet with the E23. (The fact that the company clung to its 12-cylinder engines as long as it did is strong evidence that the snob appeal was not solely for the punters.)

Born of second-guessing, the E23 745i represented not so much a prelude of things to come as a novel example of a kind of car BMW doesn’t really make anymore: rare, eccentric, flawed, and as specialized as it was swift. It was not a car designed for mass appeal — even most 7-Series buyers of the time would have been happier with a 735i (particularly on U.S. roads) — but for those few who saw the point, the eccentricity was part of the trip.

NOTES ON SOURCES

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