When BMW engineers were given the task of replacing the successful 2800 and 3.0CSi coupes, the engineering team had to work predominantly with parts and components that BMW was already making. This meant that the scope for an all-new model simply wasn't an option. The existing 2626mm wheelbase of the 5-series saloon was chosen as the floorplan for the new CSi, while the engine and brakes were to the specifications found on the larger 7-series. However, for a hybrid, the 6-series looked pretty damn good....
There were several factors that influenced the design of the 6-series. It had to retain BMW's sporting heritage, be civilised, elegant, and comply with US emission and safety guidelines. The latter proved to be the most difficult task. Interestingly, it was also suggested that the car was intended to last BMW for a decade without the need for major design revisions. To enable it to meet crash laws, the engineers had to do away with the previous model's pillarless design - with its slim rear pillars and relatively lightweight construction. The new 6-series had many box section body and chassis components to give the necessary added strength - but it also considerably increased the overall weight. The new 2 door weighed in at around 1450 kilograms.
The lines of the 6-series were aimed at keeping the unmistakable BMW look, while also providing improved visibility (through the use of more glass area) and the aforementioned increased body strength. Paul Bracq was the chief stylist at the time, and he drew upon the 'hockey stick' styling of the sculpted rear pillar of the 5-series saloon to emphasize the heritage of the 6-series. Another major BMW styling link was the by-now traditional quad headlights and double-kidney grille.
It wasn't until around the mid 80s that BMW scored their own wind tunnel, meaning many of their earlier cars suffered from aerodynamic shortcomings. While some versions of 6-series did receive a lower front spoiler and some aero considerations, the main body shape was dictated by the requirements already discussed. The result was a range of cars with a Cd that is rumoured to be around 0.40. However, the 6-series did have a massive 21% less overall aerodynamic lift than the old 3.0CSi model... although comparisons with the awesome 'Batmobile' 3.0CSL are probably less favourable!
The 6-series was initially earmarked for a 2.5 six cylinder engine, but this idea was dropped at the very last stages of pre-production. Instead, a range of larger six cylinder engines was offered. These all sat in the bay at a 30 degree angle, which aided designers in achieving a relatively low bonnet line.
The smallest engine was the 2788cc injected engine of the 628CSi, that from 1979 went only to the UK. This used Bosch L-Jetronic injection, a 9.3:1 compression ratio and the common chain driven SOHC arrangement to deliver 137kW at 5800 rpm. The next size up was the 1976-79 630CS's Solex carburetor fed 2955cc engine with a lower 9.0:1 compression ratio. It developed slightly more torque and made one extra horsepower, bringing it up to 138kW at 5800 revs. This engine accelerated the car to 100 km/h in 9 seconds on its way to a top speed of 211 km/h. The US and the UK also both received a 3210cc L-Jetronic injected engine that went into the 633Csi. In its most powerful form, it made 150kW at 5500 rpm, but it did at one stage drop to 131kW in the US. The best of these cars jumped to 100 km/h in around 8.5 seconds.
Later in the car's life it became available with a larger Bosch Motronic injected 163kW 3.5 litre engine - the 635CSi. This engine was previously a "race only" engine designed by M-sport. Not co-incidentally, after the fitting of this engine, the 6-series started to win numerous Group A and Touring Car races all around the world. So given the new widespread acceptance of the car, the services of M-sport were once again called upon to up the ante.
They released a version called the M635CSi which came complete with 24-valve high performance head, enabling the delivery of 213kW - enough to push the M635CSi to a top speed of 240 km/h... BMW also toyed with the idea of a turbocharged engine, as found in the later 745i saloon. However, there were problems (such as finding room to locate the intercooler in the tighter nosecone of the 6-series) and this and various other hassles meant a turbo 6-series was never produced - although it certainly would have been nice!
The fact that BMW had to use existing parts was evident in the rear end of the cooking coupe, which used the same (optional LSD) final drive and axles as the 5-series. But the potent 213kW M635CSi required a heavier duty item, with the next size up gearset from the 735i used. However, this fitment required modification of the floor.
The chassis package saw lots of development at BMW's then-new Ismaning proving ground. BMW engineers specified that the car was to be a driver's car first and foremost, but it also had to be easy and fun to drive for the average folk. It had to feel "light and lively", which was very difficult, given the amount of weight the car had to haul around to comply to US safety regulations! One point is that engineers matched the 6-series' springs and dampers to the natural frequency of the seats.
BMW's engineers had to ensure that the 6 cylinder spec brakes and suspension would go in without any extra tooling (incidentally, the 5-series was still four cylinder only at the time of the 6-series design). The brakes that were used were of the same dimensions as those on the 528i - at 280mm and 272mm front and rear - but the 6-series' parts were vented all 'round. Handling wise, BMW retained the traditional front engine/rear wheel drive layout that gave it inherent oversteer, but enabled drivers to access superb chassis feedback. The layout retained BMW's traditional MacPherson struts at the front with a trailing arm rear arrangement. A front swaybar was introduced to give less body roll, and a large diameter rear 'bar was also used. One of the ways that BMW got the 6-series to handle so well was to pay particular attention to reducing changes in track and camber during cornering, and the use of anti-dive geometry. On the extra high performance side, the M-version was equipped with stiffer Bilstein shocks and revised springs. Fourteen and 15.35 inch (ie 390mm TRX) wheels were available on the car depending on the model, its date of manufacture and country of release.
The dashboard was one of the focal points of the car. It used BMW's new single plane viewing panel that gave no reflections, and a unique "test" computer that monitored brake pad thickness, water and oil level, brake light operation and various other factors. It also had the clever service interval LED indicator. Materials used throughout the interior were of the highest quality, with BMW's own plastics and soft leather seats on most vehicles. To further create the luxury feel, a total of 41kg of sound deadening was applied.
On its official release in 1976, the car was marketed as having the sportiness of a Porsche and the quality and practicality of a Mercedes. However one of its main stumbling blocks was its price - in Australia, especially when compared with locally-produced vehicles. Nonetheless, BMW celebrated their largest time of growth during this era, when they also participated in Formula 1 with their turbocharged engines. Certainly, this F1 exposure couldn't have hurt the popularity of BMW's sports cars. Sales of the 6-series peaked with the 633CSi with 21,889 units, while the shorter-lived and non-USA delivered 635CSi came close with 19,087 vehicles. In total, 50,912 models were sold between the 6's release in 1976 and demise in 1989.
Running changes to the cars (other than the choice of engine) included reduced weight though improved design, a new front spoiler, a change to solid rear discs, an optional 5-speed manual 'box, extended rear bumpers and an engine bay undertray. Goodies like ABS and other improved electronics were also added. So, essentially, BMW's initial goal of keeping the same basic design over the lifespan of the car was met.