Unveiled in the spring of 1976 at the Geneva Motor Show, the original 6-Series perfectly encapsulates the intentions that this German motoring manufacturer had moving into the ‘Eighties and beyond. Just fifteen years earlier, BMW was simply trying to stay afloat by still producing bubblecars such as the Isetta for a market crying for cheap basic transport. However, with the 6-Series BMW was laying the foundations of what would result in it being known as the premium car maker that it is today.

The ‘Neue Klasse’ cars had proven to be pivotal for BMW, while the six-cylinder 2000C/CS – its first mass-produced luxury coupé – had shown it was capable of producing highly desirable machines that were a cut above the rest. Three years later, BMW gave this slightly awkward looking coupé a facelift that meant it now had a package combining good looks with terrific performance and taut, driver-orientated handling. From this car spawned the homologation CS ‘Lightweight’ – otherwise known as the 3.0CSL – which shaved kilos by adopting aluminium panels, plus a stripped out interior. It’s slightly bored-out 3.0-litre engine produced around 206bhp, which was good for a 0-60mph time of 7.5 seconds.

The racing version of the 3.0CSL was even more impressive, sporting a dramatic rear wing that was responsible for its ‘Batmobile’ nickname. It won a total of six European Touring Car championships between 1973 and 1979 and was the first BMW to be fitted with the marque’s 24-valve in-line six-cylinder engine, which would later power the legendary M1 supercar.

With all this in mind, the E24 6-Series came as something of a surprise to the buying public in 1976, as its clean, straight lines were markedly different from the comparably outrageous 3.0CSL. An energy crisis had struck just a few years before the E24’s launch and this was reflected in the car’s styling – yes, the 6-Series was unashamedly aimed an affluent section of society but at a time when big cars were no longer in vogue the 6-Series was designed to look much more discreet.

Built for BMW by Karmann, though with assembly was soon taken in-house, the E24 6-Series was based around the platform of the original E12 5-Series and boasted a range of six-cylinder engines that were now fuel-injected – with the exception of the 2985cc 630CS. Although its looks differed from its predecessors, the driving recipe remained the same: Sharp steering, keen dynamics and strong performance.

In 1983, BMW introduced to the world the second generation of its 6-Series – the same year Brazillian driver Nelson Piquet won the Formula One Championship in his BMW-powered Brabham. With increased interest in BMW’s coupé, the company’s motorsport division was inspired to create a ‘supercoupé’, achieved by fitting the E24 with the 286bhp 3.5-litre M-Power engine from its mid-engined M1 supercar.

True to its E24 roots, however, the M635CSi would not shout its performance capabilities from the rooftops in the same way as the 3.0CSL did. M Badging is used sparingly both inside and out and almost all body panels are shared with the normal 6-Series. Under the skin, there are stiffer springs, specially-tuned Bilstein shocks and a chunkier anti-roll bar at the front. Additional stopping power came in the form of thicker and larger diameter discs at the front with four-piston callipers – the same as fitted to the M1.

The M635CSi made its debut at the 1983 Frankfurt Motor Show, with cars going on sale the following year, and instantly became one of the fastest cars of its era. It was capable of a top speed of 158mph and a 0-60mph time of just 6.5 seconds. What marks the M635CSi out though is that while it is performance figures are undoubtedly impressive (and it is remarkably composed at speed), it is a car suited to real-world situations and the reality of British roads.

When you need to show self-restraint and stay off the throttle, it’s comfortable and the straight-six is able to happily burble at low speeds – that is until the driver’s discipline cracks at the sight of an open stretch of road. But even when this does happen this powerful engine is also capable of being amazingly frugal.

In September 1984 Paul Frère of Road & Track magazine wrote of the M635CSi: “This is much better than any BMW I’ve driven before. It’s fast, civilized and the engine is beautifully smooth. And the car feels quite stable at high speeds. It is an excellent high-speed touring car.”

Much like the Volkswagen Golf GTI or the Porsche 930 Turbo of its era, it very quickly established itself as a must-have machine – the ultimate version of an already desirable car.

That would perhaps explain the price: The 6-Series could never be described as an inexpensive, though the launch price of £32,195 for the ‘M’-version was frankly obscene – especially when you consider that the second generation M535i started with a price tag of £13,745, and the E28 M5 was priced £21,805 (launched the same year as the M635CSi in October).

The M635CSi’s pricetag was truly eye-watering in the ‘Eighties though today it’s a much more affordable prospect. The very best examples currently sell for between £15,000 and £18,000, while those out there that aren’t perfect but still presentable can be had from £8000 or so. You really need to exercise caution here as less than perfect rustproofing left several areas of this BMW’s bodywork vulnerable to tinworm – especially the front wings. The M635CSi is a car that you really need to buy on condition rather than with a specific specification in mind.

At last count, according to How Many Left which collates data from the DVLA, just under 100 M635CSi cars are taxed with around 150 again on SORN. According to the BMW M Registry around 500 M635CSis were produced to European specification (without a catalyst) in right-hand drive of the some 5800 cars produced – in case you were wondering, a total of 86,000 E24 6-Series BMWs were produced between 1976 and 1989.