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BMW 635 CSi

 

Chic, svelte and 30

Unquestionably one of the most elegant coupes ever built, BMW’s classic six-cylinder 6-Series has celebrated its 30th birthday.
The lithe two-door, which became a mainstay of Group A Touring Car racing in the early 1980s, grew out of the red wedge, an ultra-flat show car that caused a sensation in 1972. The futuristic sports car, fitted with a 200bhp turbocharged four-cylinder engine and with a top speed of 250km/h, was given such a positive public reception that BMW was encouraged to develop a completely new luxury coupe – the 6 Series.
Three decades have passed since the elegant 6 Series Coupe debuted at the 1976 Geneva Motor Show. The range replaced the ultra successful CS models, which had run in victory after victory in motorsport, but had customers increasingly demanding more luxury, comfort and elegance. The 5 Series delivered the engineering foundation for the 6, and BMW commissioned its development department to originate some appropriate concepts. The specifications were clear: a two-plus-two seater designed to meet potential customers’ aesthetic and technical expectations. The technical basis was soon established.
A great deal of development work had gone into the 5 Series’ chassis and floor plan, and BMW engineers were certain that it was good enough to be the basis of a high-performance coupe. BMW commissioned its in-house styling team led by Paul Bracq to produce a design for the project coded E24 and also commissioned his Italian colleague Giorgio Giugiaro. Unlike Giugiaro, Bracq kept closely to the lines of the turbo show car. The engine bonnet had a scoop that came to an end in the trademark kidney-shaped grille. Half-concealed twin headlights, and the lines of the windows at the side were other characteristic features that were also reflected in the 3 Series and 5 Series. The BMW board accepted Bracq’s design, which went on to be manufactured without change for 13 years – longer than any BMW design before or since.
BMW had also said the new coupe had to be particularly safe. Computer-based engineering and data gathered from increasingly detailed crash tests put designers in a position to design a body with high levels of passive safety. Although the deformability defined for the 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 Coupe as a particularly safe automobile.
Those priorities even took precedence over styling. Instead of a continuous side window surface, the coupe 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 the pillars so well that nobody took serious exception to it.
The inline six-cylinder engine was completely redesigned in 1968 with the designation M06, and used design principles from BMW’s successful four-cylinders. They included a crossflow cylinder head with overhead camshaft and V-shaped overhead valves above the three-sphere swirl pan combustion chambers. The combustion chamber 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.
Initially there were two 6 Series models, the 630 CS and 633 CSi. They used different versions of the inline six, each inclined at an angle of 30 degrees. The 3.0-litre M68 ran twin two-stage carburettors, and generated 185bhp at 5800rpm, giving the 630 CS a 210km/h top speed. The 3210cc M69 has electronic fuel-injection and transistor ignition. It pushed the 633 CSi to 215km/h, and developed 197bhp at 5500rpm.
BMW launched the two new 6 Series automobiles at the Geneva Motor Show in 1976. Although it still had the low roof of its predecessor – the CS – the 6 was wider and significantly longer, at 4750mm. The growth was particularly evident in the interior. The driver and front passenger had more space, and rear seat passengers were more comfortable. The instrument panel was ergonomically designed around the driver in the centre of the car, in the same way the 3-Series was. The steering wheel and the driver’s seat were height-adjustable. The big glass surfaces offered excellent visibility and conveyed an impression of space.
The era of automotive electronics gave the driver new sources of information. BMW provided the 6 Series with the Check Control System for the first time. It used sensors that monitored a range of vehicle functions. At the touch of a button, drivers could get information via LEDs on the fluid levels for oil, brake fluid, coolant and washer water. The system also monitored the brake pads, brake lights and taillights.
Customers could order leather upholstery, a sliding sunroof, air-conditioning or automatic transmission with three drive level. Around one third of the 630 CSs manufactured were ordered with the auto. Both coupes could be had with a five-speed manual gearbox, and a limited-slip differential.  Not many of the new coupes were built as the standard version.
The success of the 3-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 coupe and didn’t want to keep customers waiting with extended delivery times.
It commissioned coachbuilder Karmann to make the body-in-white for the 6-Series during the first two years of production. Karmann also fitted the cars with technology delivered from Munich. The strategy worked. During the first two years, sales climbed to around 11,000 vehicles, of which around 1800 were exported to the US where the 6 Series forged the lasting reputation of the BMW brand at the top of the range.
BMW aimed at the overseas market right from the start. Designers paid close attention to the development of emission regulations in the US. BMW built a special American market 630 CSi whose injection system permitted the operation of a fully-controlled three-way catalytic converter, meeting emission regulations in 49 States. A specially equipped version was produced for California, which had introduced stricter limits in 1976.
The two new luxury coupes gained new customers looking for more comfort, but existing CS drivers continued to demand a replacement model that would at least be equal to its predecessor in sporting terms. For, although the discontinued 3.0 CSi was more Spartan, it was faster than the 6.
In 1978, BMW launched the 635 CSi as a new top-range model. Its six-cylinder motor was a direct descendant of the engine that had powered the racing CSL Touring Car to numerous victories since 1973. The M90 engine had a bore/stroke ratio of 93.4 to 84mm, making it a distinctly short-stroke engine with a correspondingly lively performance. Originally designed as a 2.5-litre six, six-cylinder with a capacity of 2.5 litres, the block didn’t really have 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 218bhp delivered at 5200rpm were capable of pushing the car from zero to 100km/h in 7.3 seconds. Even the 3.0 CSL with its lightweight bodywork had never been able to achieve that. BMW left customers to choose between a sporty five-speed gearbox or one tuned to comfort. And with a top speed of 220km/h, the car had few competitors on the autobahn. The age of the carburettor in top level BMWs ended in 1979 when the 628 CSi replaced the three-litre. It had just one horsepower less, and used less petrol. However, the relatively modest price difference between the 628 CSi and the more powerful sister models made those more attractive, and the 635 CSi became a bestseller.
In 1980, BMW sold 2100 vehicles, almost three times as many top level coupes 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 1982. However, customers in export markets like the US and Japan remained faithful and the 3.2 -litre continued to enjoy great popu­larity there.
BMW kept the timeless contours of the 6-Series unchanged. But the cars 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 more precise driving. 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 coupe – and were soon successful. Quester won the 1983 European Touring Car Championship in a 635 CSi. In the same year, a 635 also won the 24 Hour Race at Spa-Francorchamps, Belgium, repeating the success in 1985 and 1986.
BMW continued to update the 6-Series. In 1985 it got a four-speed automatic with program preselection. Depending on the gearshift position, the computer changed the drive levels to emphasise comfort and fuel efficiency, or sporty dynamism.  The final technical changes came in 1988 with the first electronically controlled chassis in volume production – and made it available as an option for the 6 Series.
On April 6, 1989, the last E24 rolled off the production line after 86,216 cars had been manufactured. It was painted salmon silver.

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