BMW M6 - 30 years on
Luxury of sportiness:
The first 6 Series Coupé.
The red wedge shape was a sensation. In 1972, BMW launched an ultra-flat showcar that seemed to have come back from the future. And this idea wasn’t actually too far off the mark: The BMW turbo was the design precursor of the
6 Series Coupé that was only launched four years later.
The futuristic sports car – fitted with a 200 bhp turbo four-cylinder engine
and with a top speed of 250 km/h – was given such a positive public reception
that BMW was encouraged to develop a completely new luxury coupé.
The first 5 Series marked the start of the new BMW design and it turned into a bestseller within a very short space of time. In 1975, the 3 Series replaced
the 02 Series and also generated rising sales figures. And now it was the turn of the CS models. Although they had run in victory after victory in motor sport, customers were making increasingly vociferous demands for more luxury, comfort and elegance. These demands couldn’t be met by these models.
The 5 Series delivered the engineering foundation.
BMW responded to the demands and commissioned the development department to originate some appropriate concepts. The specifications were clearly outlined: A 2+2 seater that was supposed to meet the aesthetic
and technical expectations of the customers. The technical basis was soon established. A great deal of development work had gone into the chassis
and floor pan of the 5 Series and they were therefore able to meet even sophisticated requirements. The chassis engineers could be certain that a fast and powerful coupé would be well-equipped on this basis.
This left the final contours to be decided. BMW had already had extremely good experience with having a proposal developed in-house and commissioning an outside design from an independent designer. The Board of Management therefore asked the internal styling team led by Paul Bracq to produce a design for the project coded E24 and also commissioned his Italian colleague Giorgio Giugiaro. However, in contrast to the design of his independent colleague, Bracq kept closely to the lines of the turbo. The engine bonnet had a scoop that came to an end in the typical kidney-shaped grille. Half-concealed twin headlamps and the lines of the windows at the side were other characteristic features that were also reflected in the 3 Series and
5 Series. This convinced the Board of Management, and Bracq’s design was accepted. This coupé was to be manufactured without change for
a period of thirteen years, longer than any BMW design before and since.
Exemplary passive safety.
However, the new coupé was not only supposed to be fast and beautiful.
It was also to be particularly safe. On these grounds alone, it was high time to replace the previous model. The use of digital metrology and the results
from increasingly detailed crash tests placed designers in a position where they were able to design a body with exemplary passive safety. Although the deformability defined for the engine bonnet, the telescoping safety steering columns, and energy absorption by the side members were invisible to customers, they quickly resulted in the reputation of the 6 Series Coupé as
a particularly safe automobile. These priorities even took precedence
over design. Instead of a continuous side window surface, the coupé had
B-columns supporting the roof in the centre. Although this meant that the side glass area couldn’t be opened completely, the matt-black paintwork of the members concealed this so ingeniously that nobody took serious exception
to it.
They make BMW distinct: six-cylinder engines.
There’s no question about it: The power unit was definitely superior.
The design engineers were able to use an ultramodern six-cylinder engine that was a benchmark for engine design for several decades. The in-line
six-cylinder engine was completely redesigned in 1968 with the designation M06, based on the design principles of the successful four-cylinder -
crossflow cylinder head with overhead camshaft and V-shaped overhead valves above the “three-sphere swirl pan” combustion chamber.
This geometry created a strategic turbulence in the mixture while at the same concentrating volume at the spark plugs. The result was a very effective
but soft combustion process. This, together with a forged crankshaft resting on seven bearings with two balancing weights on each crank pin, resulted
in extremely smooth running characteristics. The BMW six-cylinder,
often described as running like a turbine, achieved a new high point.
Two models to start with: 630 CS and 633 CSi.
Two new engine versions were produced on this basis. In line with other
BMW engines, these were installed in the 6 Series Coupé in 1976 inclined at an angle of 30 degrees. Firstly, the M68 with a capacity of 3 litres and
twin two-stage carburettors. This engine generated 185 bhp at 5,800 rpm and already gave the BMW 630 CS – the basic model in the new series – a top speed of 210 km/h. The engineers designed the M69 as the second version with a capacity of 3 210 cubic centimetres, electronic injection and transistor ignition. The 633 CSi reached a top speed of 215 km/h, delivering 197 bhp at 5,500 rpm, and thanks to the future-oriented Bosch L-jetronic engine control, it only consumed 10 litres to cover 100 km, 1.4 litres less than the three-litre engine with its lower capacity.
1976: Premiere in Geneva.
Four months after production of the coupé from the 1960s came to an end, BMW launched the two new 6 Series automobiles at the Geneva Motor Show in 1976. The change in generation and the new direction that had been
taken were obvious. The new coupé was clearly a very different car from the CS. Although it still had the low roof of its predecessor, it was wider and
also significantly longer with an external length of 4.75 metres. The growth was particularly evident in the interior. The driver and front passenger were able to enjoy more space, and the journey for the passengers in the back of the 2+2 seater was also significantly more comfortable.
More luxurious and spacious interior.
The sophisticated claims of the 6 Series were also evident in the interior.
The instrument panel was ergonomically designed around the driver in the centre of the car, in the same way as in the 3 Series. The steering wheel
and the driver’s seat were height-adjustable as standard. This meant that very small and exceptionally tall customers alike were able to enjoy the experience of sheer driving pleasure tailored to their individual requirements. The big glass surfaces offered an outstanding overview and conveyed a generous impression of space. They also offered the benefits of tinted windows as standard to protect the occupants against being unduly affected by sunshine.
Launchpad into the era of electronics.
The era of electronics in the automobile was not only evident in engine control, but also gave the driver new information. BMW provided the 6 Series with the Check Control System for the first time. This system represented
a pioneering development for the entire automotive industry. BMW engineers had developed sensors that monitored a range of vehicle functions.
At the touch of a button, drivers were able to get information via LEDs on the fluid levels for oil, brake fluid, coolant and washer water. The system
also monitored the operation of brake pads, brake lights and taillights.
Often selected: Automatic as special equipment.
This comprehensive range of equipment showed that the two new coupés were clearly aiming for the top echelon geared towards comfort. Servo-assisted steering and brakes were included as standard. Customers also had the option of ordering leather upholstery and sliding roof, air-conditioning or automatic transmission with three drive levels. The last option met the expectations of many customers. Around one third of the 630 CS automobiles manufactured were ordered with selector instead of switch levers. Naturally, this doesn’t mean that BMW had lost sight of those sections of its clientele who were more interested in sporty performance. Both coupés were supplied with a five-speed gearbox as an alternative, as well as a limited-slip differential.
This was particularly appropriate in the case of the 630 CS that was provided with tighter tuning for the springs and shock absorbers from the outset. The fact is that not many of the new coupés rolled off the BMW assembly lines in the standard version, although this in itself was impressive, with the car positioned within the upper price ranges. The 630 CS was supplied from DM 40,600, and a price tag of DM 43,100 was attached to the 633 CSi.
Start of production at Karmann.
The success of the 3 Series and 5 Series had already meant that BMW was operating virtually at the limit of its capacity in the mid-1970s. The company didn’t want to run any risks with the new coupé and keep the executive clientele waiting with extended delivery times. The coachbuilding company Karmann was therefore commissioned to manufacture the body-in-white for the 6 Series during the first two years of production. The coachbuilders also fitted the cars with technology delivered from Munich. The concept proved to be successful. During these initial two years, sales climbed to around
11,000 vehicles, of which around 1800 were exported to the USA where the
6 Series forged the lasting reputation of the BMW brand at the top of the range.
US version with catalytic converter as early as 1976.
BMW was aiming for the overseas market right from the start. Designers therefore paid close attention to the development of emission regulations abroad. Soon after production commenced, the company was therefore
in a position to react to demand in the USA with a special 630 CSi. This was because its injection system permitted the operation of a fully-controlled three-way catalytic converter. The coupé was therefore in conformity with the emission regulations in 49 US states. A specially equipped version was produced for the State of California, which already introduced stricter limits
in 1976.
The two new luxury coupés gained new customers looking for more comfort, while existing CS drivers continued to make increasingly strident demands
for a replacement model that would at least be equal to its predecessor as far as sportiness was concerned. The point was that although the discontinued 3.0 CSI was more Spartan, it was faster than its successor. They didn’t have to wait for long. In 1978, BMW launched the 635 CSi as a new top-range model. The six-cylinder under the bonnet was a direct descendant of the engine
that had powered the CSL touring car to numerous victories since 1973.
BMW 635 CSi: Six-cylinder with even more “punch”.
The new engine was dubbed the M90. It had a bore-stroke ratio of 93.4 to
84 millimetres, making it a distinctly short-stroke engine with a correspondingly lively performance. Originally designed as a six-cylinder with a capacity of 2.5 litres, it didn’t really have any space for a capacity of 3.5 litres, particularly given the large bore diameter. The 3.2 litre engine had already reached the limits. The engine designers therefore resorted to the unusual measure of casting the cylinder linings together.
If you picked the right moment and changed gear quickly enough,
the 218 bhp delivered at 5,200 rpm were capable of sprinting from zero to 100 km/h within the space of 7.3 seconds. Even the 3.0 CSL with its lightweight bodywork had never been able to achieve this feat. One test driver commented that “the 3.5 litre engine powers into the red zone with even
more agility than its companion with lower capacity, and with the same silky smoothness of a turbine”. He continued, “And with the ‘punch’ to hand at any time, the speedy coupé evokes memories of the legendary reputation earned by the dynamism of the types 3.0 Si and CSi. These models exerted a defining influence on BMW’s image for performance at the start of the 1970s. The six-cylinder engine built into the 635 starts performing even in the low-rev range as though the aim was not to have to change gear.” BMW left it to customers to choose between a sporty five-speed gearbox or one tuned to comfort.
And with a top speed of 220 km/h, there was only a select handful of competitors on the autobahn.
“Rediscovered sportiness.”
“There’s no doubt about it”, was the verdict of the motoring press, “BMW in Munich is deadly serious about rediscovering sportiness.” It wasn’t simply
the badge that marked out the new top model. A low-slung rear front apron joined forces with a spoiler at the edge of the boot lid to increase the lift by around 15 percent. Combined with chassis tuning this had a marked effect on driving safety at high speeds. “However, it’s not simply the sensational engine transmission unit in the new BMW that impresses driving aficionados.
The 635 chassis tailored to the sporty character is almost equally impressive.”
The sporty appearance was influential in the 635 CSi achieving the “overtaking prestige” that remains famous to this day. A recent comment: “Paul Bracq designed a timeless classic. The finely balanced proportions
and the typical series front end provide an immediate reminder of the title music for Jaws. Seeing the 635 CSi looming in one of the rear mirrors must provide a similar feeling to that experienced by water skiers if they suddenly sense a shark behind them.”
Streamlining the range with the debut of the 628 CSi.
The age of the carburettor in the BMW top range came to an end in 1979. The 628 CSi replaced the three litre. It had just one bhp less and was just as fast with the big advantage that it was much more fuel-efficient. However,
the relatively modest price difference between the 628 CSi and the more powerful sister models made those more attractive. In 1980, the 628 CSi cost DM 46,000, the 633 CSi was DM 48,700, and the 635 CSi was at the top
of the range with a price tag of DM 51,900. The result was that the 635 CSi became a bestseller. In 1980 alone, BMW sold 2,100 vehicles – almost three times as many top coupés as the basic versions.
The 633 CSi was in the middle of this range, ignored by customers, and was removed from price lists in Germany in model year 1982. However, customers in export markets like the USA and Japan remained faithful and the
3.2 litre continued to enjoy great popularity there. In 1983 alone, more than 3,000 633 CSi automobiles were sold, half of them with automatic transmission.
Ongoing technical development.
BMW kept the timeless contours of the 6 Series unchanged. But the top-range coupé was constantly being developed beneath the bodywork.
The 6 Series and the 7 Series were among the first cars to be fitted with ABS at the end of the 1970s, and the analogue engine control was replaced
by digital engine electronics at the same time. Signs of advance visible to the driver were the onboard computer and the service-interval display in a
revised instrument panel from 1982. But the chassis was also revised.
As in the 7 Series, the double-joint front axle and improved rear-axle bearing system now delivered precise driving pleasure.
The year before, major engine tuners had already started to explore the sporting potential of the 635 CSi. Drivers like Dieter Quester and
Hans-Joachim Stuck, Marc Surer and Gerhard Berger moved over to the new coupé – and were soon successful. Quester won the 1983 European
Touring Car Championship in a 635 CSi. In the same year, the racing version of the BMW top sports car also won the 24 Hour Race at Spa-Francorchamps. This success was repeated in 1985 and 1986. But at that time, the sporting world was focused on Formula 1. Nelson Piquet had just risen to become World Champion in a Brabham BMW.
With the engine of the M1: M 635 CSi.
BMW found a special way to enhance the pleasure of the victory for its customers. The 1983 Frankfurt Motor Show featured a 6 Series that uniquely combined the luxury of the big coupé with racing car engineering. The power pack was called the M 635 CSi. The engine under the bonnet was the four-valve engine of the former M1 super sports car. The engine was designated the M88 and was based on the six-cylinder volume-production power unit.
It had the four-valve cylinder head of the racing cars. This cylinder head was split in two. The lower part formed the combustion chamber and the water compartment. The top section housed the bearings for the camshafts and the tappets. The mixture was supplied through three twin throttle-valve assemblies with six independent throttle valves measuring 46 millimetres in diameter into two inlet channels each which measured 26 millimetres in diameter per cylinder. In fact, the new digital engine electronics enabled the engine with 286 bhp to generate 9 horsepower more than in the M1.
The six-cylinder ran smoothly and evenly over the entire range of engine speeds, but had a tendency to modesty at the bottom end. This changed immediately the rev counter went over the 5,000 rev mark. The M88 then propelled the M635 CSi forward to the limiter engine speed of 6800 revs so fast that even hardened test drivers started to rave: “And anyone who is caught up by the almost hair-raising engine speeds is abruptly reminded by the
limiter at 6,800 m–1 that limits to the torque of an engine are not just set by the limits of valve operation. The unavoidable critical torsional vibration of
the crankshaft in the in-line six-cylinder engine precludes even higher values.”
“250 km/h aren’t exactly chicken feed.”
Masterful driving performance. The engine catapulted car and driver to
a speed of 100 km/h within the space of 6.4 seconds, and it wasn’t until the speed of 255 km/h had been reached that air resistance put an end to further acceleration. “It you don’t happen to be travelling on New Year ’s Eve, you’re hardly likely to be able to enjoy the potential for power over a long period of time because of rising traffic volume”, was a regret expressed by one noble automobile journal. “After all, speeds of some 250 km/h aren’t exactly chicken feed. Some Formula 1 cars only achieve slightly in excess of this speed –
and that’s on closed roads.” BMW didn’t provide reserved lanes for the press launch of the M635 CSi, but the route on the autobahn from Munich
to Garmisch was an astute choice. In those days it was still possible to speed along this highway for long stretches with the speedometer needle at
255 km/h.
The specialists at Motorsport GmbH had done sterling work. The chassis
and brakes were modified so that they were able to operate at high driving speeds. The chassis was 11 millimetres lower than in the 635 CSi. Gas shock absorbers on all four wheels delivered superb road-holding. On the front
axle, four-piston fixed-calliper disc brakes operated enlarged brake discs with ventilation. “Smooth directional stability and only low wind sensitivity
set no limits for the maximum speed”, was the verdict of the professionals.
1985: First four-speed automatic with selectable gearshift program.
Meanwhile, the 6 Series had been on the market for seven years, but it wasn’t seven years old. It didn’t reach that age at any point during the time it
was being manufactured, because technical updating continued unabated.
In 1985 BMW supplied a four-speed automatic with program preselection – also in the 6 Series. Depending on the gearshift position, the computer changed the drive levels to emphasize comfort and fuel efficiency, or sporty dynamism. This was a first. In the same year, the coupé with a catalytic converter was launched in Europe, a 635 CSi with initially 185 bhp and later 211 bhp. In 1986, the M635 CSi was launched with exhaust emission
control, but the top-performance engine lost 27 horsepower because of the increased backpressure present in the high-performance exhaust system.
The technical finale followed two years later. In 1988, the BMW engineers had developed the first electronically controlled chassis to volume production – and at the same time made it available as an option for the 6 Series. The first generation of the Electronic Damper Control EDC was already using a range
of parameters to calculate the optimum damping force for the momentary driving style, making appropriate adjustments via controllable shock absorbers.
On 6 April 1989, the last E24 rolled off the production line after 86,216 cars had been manufactured – it was salmon silver. And if you look carefully,
you’ll see that the 6 Series is still alive and well on the roads. Not because
it has become a rarity. But because it’s never aged.
On the left is the M635 belonging to your webmaster. Chassis number 509 from 524, built in January 1989, and first registered in january 1990, it shares its stable with a 1988 635Csi Highline.
Oh what a joy!
