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It all started with a ‘red wedge’ – an ultra-flat showcar which caused a
sensation in 1972. 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
coupe. Enter the BMW 6 Series.
Three decades have passed since the elegant 6 Series Coupé made its debut to
motoring audiences at the 1976 Geneva Motor Show. The range replaced the ultra
successful CS models, which had run in victory after victory in motor sport, but
had customers making increasingly vociferous demands for more luxury, comfort
and elegance.
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.
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