Summary

From 1980 to 1986, BMW offered a special turbocharged version of its flagship 7-Series sedan called the 745i. The turbocharged six-cylinder engine was supposed to be a more economical substitute for a 4.5-liter V-12 BMW developed in the 1970s, but its fuel consumption was still high, and it performed best at very high speeds. There was also a rare South African version with the normally aspirated (non-turbocharged) 24-valve six-cylinder engine from the M635CSi coupe. The 745i was not officially exported to the U.S. and sold in relatively small numbers. It was discontinued in 1986 and replaced by the 750i and 750iL, which had a non-turbocharged V-12 engine instead of the turbocharged six.

If you’ve looked at a BMW brochure in the past decade, you’ve probably noticed that most of the company’s offerings are now turbocharged. That wasn’t always the case: When the twin-turbo E92 335i was launched back in 2006, BMW hadn’t offered a forced-induction gasoline engine in over 20 years. The company’s previous stab at spark injection turbocharging had been the E23 745i, a less-than-successful attempt to beat Mercedes-Benz at its own game.

(Author’s Note: An earlier version of this article appeared in Autoweek magazine in 2013. It has been extensively revised and greatly expanded for Ate Up With Motor.)

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

From V-12 to Turbo Six

Announced at the Frankfurt International Auto Exhibition (IAA) in September 1979 and launched in mid-1980, the first BMW 745i was the flagship of the then-three-year-old E23 7-Series line, intended to challenge the Mercedes-Benz eight-cylinder S-Class sedan on its own terms.

At the insistence of BMW CEO Eberhard von Kuenheim, BMW had been trying for years to elbow its way into Mercedes territory, but the Bavarian firm had lacked an engine capable of matching the bigger Mercedes V-8s. The 745i promised to change all that, with a turbocharged six that outmatched the most powerful version of the latest W126 Mercedes-Benz S-Class, 252 PS DIN to 240 (185 kW to 177).

The impact of its announcement was somewhat undercut, however, by the awareness that the new flagship’s Turbomotor was really something of a consolation prize, a last-minute stand-in for the 4.5-liter V-12 BMW had originally wanted to build.

Even before returning to the six-cylinder market back in 1968, BMW had been contemplating once again offering engines with more than six cylinders, although it wasn’t until 1979 that officials publicly acknowledged those efforts, perhaps mindful that the previous BMW V-8 (offered from 1954 to 1965) had contributed to the company’s brush with bankruptcy in the early sixties. After exploring the possibility of a new V-8 engine, BMW had opted for a 60-degree V-12 based on the company’s latest M20 six. Despite some exotic features, like dry sump lubrication and Bosch digital Motronic engine controls integrating electronic ignition and L-Jetronic electronic fuel injection, the V-12 had extensive commonality with the small six while offering greater smoothness, prestige, and power: 272 PS DIN (200 kW) and 269 lb-ft (365 N-m) of torque from 4,463 cc (272 cu. in.). The V-12 promised to rival even the vaunted W116 Mercedes-Benz 450SEL 6.9 in acceleration, if not in top speed.

The V-12’s downfall was fuel consumption, which was very heavy by European standards — a meager 16.8 mpg (14.0 L/100 km) DIN “normal consumption” — and flew in the face of the new U.S. Corporate Average Fuel Economy (CAFE) standards. The BMW management board had finally hedged, shelving the V-12 project in 1977.

Its demise left the BMW flagship sedan without a flagship engine. At launch, the top of the new E23 7-Series line was the 733i, with a 3,210 cc (195.9 cu. in.) six making 197 PS (145 kW) in European trim. Compared to the slightly cheaper eight-cylinder 350SE of the W116 Mercedes S-Class, the 733i had a slight edge in acceleration, but not in smoothness or refinement. The German car magazine auto motor und sport (AMS) found the BMW second-best in that league, and BMW had nothing to challenge the pricier 450SEL or 450SEL 6.9. Even the subsequent introduction of the more powerful 735i, with a 3,453 cc (210.7 cu. in.) six offering 218 PS (160 kW), wouldn’t be enough to directly match V-8 rivals.

One possible alternative — and one that would resurface later — was the hot DOHC 24-valve M88 six from the new M1 coupe. This was a good deal more powerful than the SOHC sixes, but it was also a peakier, noisier engine, not really appropriate for the senior executive class, and it was also rather expensive to build.

Much like their counterparts at Buick during the same period, the engineers in Munich turned instead to turbocharging the “big” M30 six, which promised to combine V-8 power with six-cylinder fuel economy and emissions. This was not entirely new territory, since BMW had been building turbocharged racing engines since 1969, but the company’s only previous turbocharged production car had been the short-lived 2002 Turbo in 1973–1974, which was a high-performance homologation model, not a flagship Autobahnkreuzer aimed at the senior S-Class Mercedes.

(Interestingly, while BMW management decided to develop diesel engines at the same time the V-12 was canceled, the company’s first M105 turbodiesel engine, a 2,443 cc (149.1 cu. in.) six produced by a joint venture between BMW and Steyr-Daimler-Puch, wouldn’t arrive until the 1983 model year, three years after the turbocharged petrol six.)

M102 Turbo Engine

Despite the suggestive badge, the turbocharged M102 engine of the initial 745i actually displaced 3,210 cc (195.9 cu. in.), just like the M30B32 six from the cheaper normally aspirated 733i and 732i. BMW justified the loftier badge based on the Fédération Internationale de l’Automobile (FIA) equivalency factor for turbocharged engine, which at that time was 1.4:1, meaning that a turbocharged 3,210 cc engine was considered to have an equivalent displacement of 4,494 cc (274.2 cu. in.) for classification purposes. At the time, this prompted a certain amount of eye-rolling from the automotive press, but it was at least more grounded than modern BMW practice, where model identification numbers that once indicated engine displacement in deciliters have become increasingly arbitrary.

To prepare it for turbocharging, the 3.2-liter six was modified with stronger connecting rod bearings, Nimonic alloy exhaust valves, and a new exhaust system. As with the normally aspirated engine, fuel delivery was by Bosch L-Jetronic electronic fuel injection. Unlike the canceled V-12, the M102 engine did not use the new Bosch Motronic integrated digital engine computer, a curious choice that to us suggests a rather short development timetable.

Boost came from a single air- and oil-cooled KKK K27 turbocharger, made by Frankenthal-based Kühnle, Kopp & Kausch (now owned by BorgWarner), which also made the turbochargers used by the Porsche 924 Turbo and Porsche 930. Maximum boost was set at 0.6 bar (8.7 psi), although BMW said peak power was achieved with slightly lower 0.5 bar (7.3 psi) boost, in the interests of better fuel economy.

Unlike the contemporary Buick 3.8-liter (231 cu. in.) turbo V-6 or the Saab Automatic Performance Control (APC) system, the M102 engine did not have knock sensors, but it did have an air-to-air intercooler to reduce the temperature of the compressed intake charge by up to 104 degrees Fahrenheit (40 degrees Celsius) before it reached the intake manifold. (Since the M102 six had L-Jetronic injection, the manifold was “dry,” with fuel injected at the intake ports.) As a further safeguard against detonation under boost, the engine’s compression ratio was reduced significantly — initially to 7.0:1, compared to 9.3:1 for the normally aspirated engine.

As with the old Oldsmobile F-85 Jetfire engine, the turbocharger controls were designed to reach maximum boost early (from as little as 2,500 rpm) and maintain peak torque output, rated at 280 lb-ft (380 N-m), through much of the rev range, up to 4,800 rpm. The redline was 6,300 rpm, but peak power — rated at 252 PS (185 kW), 20 PS (15 kW) less than the stillborn V-12 — was achieved at 5,200 rpm.

The M102 had two pressure-controlled bypass valves. A conventional wastegate opened if intake manifold pressure exceeded 0.6 bar (8.7 psi), limiting boost by allowing some exhaust gas to bypass the turbocharger. There was also an overrun control valve, which BMW called the Umluftventil, or return air valve, that opened in response to manifold vacuum, allowing some of the compressed and cooled intake air to flow from the intake pipe back into the turbocharger’s compressor inlet. Its purpose was to keep turbine speed from dropping abruptly if the driver lifted off the accelerator.

A Heavy-Drinking BMW Flagship

BMW was keen to emphasize that the turbocharged 745i was an executive car, not a sport sedan. There were no hood scoops, cooling vents, racing stripes, call-out decals, or special instruments (to the annoyance of some critics, who wanted a boost gauge). Although there was an available sport suspension with a hydro-pneumatic self-leveling system, low-profile Michelin TRX tires, and metric alloy wheels, it was an optional extra. Standard 745i fare was the same relatively soft suspension and 205/70VR14 tires as other 7-Series cars, although 745i did come standard with a Bosch four-channel antilock braking system (ABS), a new addition to the 7-Series line.

Another telling point was that the 745i turbo was available only with automatic, initially the ZF 3HP22, a three-speed Simpson gearset transmission optional on other 7-Series models. BMW built a handful of pilot production turbo cars with five-speed manual gearboxes, but that option was never offered in production. The company offered several rationalizations for standardizing the automatic — its torque converter helped to compensate for the turbocharged engine’s weaker torque between idle and the arrival of boost, and power-shifting a relatively wide-ratio automatic kept the turbo on the boil through the rev range — but we assume the principal reason for not offering a five-speed was that it would have risked sending the wrong message about the car’s intended mission.

Standardizing the automatic suggested an interest in adapting the 745i for American sales, but while BMW said the turbocharger arrangement was compatible with three-way catalytic converters and lambda feedback control, the E23 745i was never officially certified for U.S. sales (and was never offered with a catalytic converter even in Germany). Nonetheless, there were some gray-market importers, who combined the turbo package with the emissions equipment from the U.S. 733i, reducing output by about 20 horsepower (15 kW) and creating some uncomfortable ambiguities with regard to warranty coverage; BMW was reluctant to accept responsibility for gray-market cars. The E23 745i would also be a rare sight in the UK and Australia, since the turbocharger and its bulky intercooler installation precluded right-hand-drive (RHD) conversion.

In any case, the turbocharger package was really optimized for the unrestricted Autobahn. Until the tachometer needle approached 3,000 rpm, the 745i was actually a bit more sluggish than a normally aspirated 732i or 735i, and it was hampered by the transmission’s reticence to downshift at part-throttle. BMW claimed 0–62 mph (0–100 km/h) in 7.8 seconds, which was good but hardly spectacular for the era (and 0.4 seconds slower than the abortive V-12 car). Once the tachometer needle crossed 3,000 rpm, however, the 745i would soon leave its normally aspirated brethren for dead: From 62 mph (100 km/h), the 745i would reach 112 mph (180 km/h) more than 10 seconds quicker than a 735i Automatic. Manually holding second gear as long as possible also gave the 745i an edge in highway passing over the latest W126 Mercedes-Benz 500SE, which had the advantage at lower speeds. The BMW turbo couldn’t match the top speed of the taller-geared Mercedes, which in AMS tests topped out at 144 mph (232 km/h) to the Bavarian car’s 140 mph (226 km/h), although the latter was still 2 mph (4 km/h) faster than the BMW factory claim.

This impressive high-speed performance had its costs. The first and most obvious was fuel consumption: Even on paper, the M102 engine was only marginally more economical than the 4.5-liter V-12, with DIN normal consumption of 17.4 mpg (13.5 L/100 km) against the 12-cylinder engine’s 16.8 mpg (14.0 L/100 km). In the real world, it was easy to push fuel economy below 12 mpg (20 L/100 km), closer to a 5.3-liter (326 cu. in.) Jaguar XJ-12 than a normally aspirated 7-Series. The first AMS full road test of the 745i returned a grim overall test average of 10.9 mpg (21.6 L/100 km) on German low-lead premium fuel, dipping to a mere 10.4 mpg (22.5 L/100 km) in 100 mph (160 km/h) Autobahn cruising, and as low as 8.9 mpg (26.5 L/100 km) with a lot of full-throttle acceleration. By comparison, the taller-geared, significantly more aerodynamic Mercedes 500SE managed a comparatively thrifty 13.1 mpg (17.9 L/100 km) on the Autobahn, although the 500SE was a bit thirstier than the 745i in urban driving. Tellingly, BMW increased the size of the turbocharged car’s fuel tank from 23 to 27 U.S. gallons (85 to 100 liters) to compensate for its thirst.

One could argue that fuel consumption was a trivial point for the intended clientele of these relatively rare and very expensive cars. At launch in June 1980, base price of the 745i in West Germany was DM 54,200, 8,600 marks more than the 735i and 73 marks more than a Mercedes 500SE. The 745i was not quite as audaciously priced as it appeared at first blush, thanks in large part to its standard antilock brakes (ABS was an extra DM 2,370 on other 7-Series models, and around 2,500 marks on the Mercedes S-Class), but it was still a costly prestige model intended to compete in the upper tiers of the European executive market. On the other hand, given that fuel consumption had been the principal reason for dropping the V-12 in favor of the turbo six, the fact that the turbocharged M102 engine was thirstier than a 4,973 cc (303.4 cu. in.) V-8 was none too impressive, and the Mercedes M117 V-8 was both more flexible and more refined than the BMW M102 six.

The turbocharged engine brought some other compromises as well. Much as it relished Autobahn speeds, the M102-powered 745i wasn’t much for slowing down: The overrun recirculation valve effectively eliminated engine braking, so lifting off the throttle at speed gave a disorienting sense of limitless momentum. ABS improved braking control, but not swept area or brake cooling, which were no different than on lesser 7-Series models, and could be overtaxed with the higher velocities on offer.

As for handling, BMW in the early eighties still clung to MacPherson struts in front (with the company’s novel double-pivot lateral links, giving a dynamically variable scrub radius) and semi-trailing arms in back. Steering was more precise and handling more neutral than was customary for luxury sedans of this era, but the 20-degree semi-trailing arms made it easy to bring the tail out with the throttle, either by applying too much power when existing a turn, by lifting off the accelerator after entering too fast, or by triggering a mid-corner automatic downshift. Contemporary attitudes about this behavior were mixed: American drivers who fancied themselves highly skilled found it delightful, since it was flattering if properly managed, while British testers, faced with generally higher speeds on narrower, wetter roads, regarded it as worryingly skittish; German reviewers tended to fall somewhere between these extremes. There was little serious debate, however, that the penchant for tail-happiness could get an unprepared or unwary driver into a lot of trouble.

Some testers, like Paul Frère, felt the early 745i was less prone to trailing throttle oversteer than its naturally aspirated relatives, since the overrun air return valve blunted the torque reaction that would otherwise be caused by lifting off the accelerator. However, semi-trailing arm suspensions did not suffer nonlinear power delivery gladly, and the transition from off-boost to on-boost power could make the turbocharged car a handful, especially in the wet. The Swiss magazine Automobil-Revue (AR) found that the presence or lack of a limited-slip differential made a significant difference; with an open differential, a sudden kickdown was more likely to set the inside rear wheel spinning than kick the tail out, but the opposite was true with limited-slip. The sport suspension and TRX package offered on the 745i didn’t change the car’s basic dynamics, but they did usefully expand the limits of adhesion and made the rear end’s sideways-seeking tendencies easier to manage short of the limit.

Ride was not the E23’s strongest suit, in any case. Even with the softer standard suspension, it wasn’t terribly absorbent at around-town speeds, and some testers felt it was under-damped at higher speeds. The TRX package had better high-speed damping, but was even stiffer than the standard setup at low speeds. AMS was also none too keen on the front seats for longer drives, although their testers thought the 7-Series had a more comfortable back seat than did the contemporary S-Class. Wind noise was another sour point, a sign of an aging platform for which aerodynamics had ranked relatively low on the original list of design priorities.

In sum, the 745i was a narrowly focused car, aimed at the kind of well-heeled, aggressive driver who would weigh the fuel costs of a road trip against the price of an airline ticket rather than the fuel economy of some milder sedan; who was more concerned with maintaining high trip average speeds than with wind noise or around-town ride comfort; and who might actually learn to program the car’s complicated trip computer, a standard 745i feature optional on other senior BMW models. The turbocharged 745i wasn’t really an all-arounder like the senior S-Class models, and it didn’t do much to challenge Mercedes leadership in this class, in either sales volume or prestige.