You can’t beat the French when it comes to radical automotive engineering, as anyone who’s driven a Citroën DS will attest. But even that famously avant-garde marque surprised its own fans when it married hydro-pneumatic engineering with Italian power to create a futuristic GT coupé in the early ʼ70s.

Most of us have our own reasons for being addicted to old cars. Maybe that Golf GTi in your garage was your wall poster favourite as a teenager, or you simply fell for the aesthetic qualities of Jaguar’s Mk2. Or the glorious sound of its straight-six. Then again it could be the chrome exuberance of a 1950s Yank and the simplicity of its mechanicals. Me? What I cherish most is the unique driving experience of each marque – something that’s increasingly been watered down with modern cars. Step out of the hard ride of a Big Healey into a wafting piece of Detroit iron and your fillings will remain in but you won’t get round a corner in a hurry. Or stop all that easily. Get the keys to a ʼ50s Italian and you’ll revel in the tactility of its delicate handling, while fretting over the equally delicate bodywork, and a stint in one of Stuttgart’s rear-engined marvels will thrill, thanks to the howl of its air-cooled flat-six and astonishing rear-end grip.

Fancy something radically different? Then slide behind the wheel of something French, such as a Citroën. Or, for an utterly unique experience, one with a Maserati engine under the bonnet, as in Citroën’s SM. This futuristic coupé was once the pride of the French brand following a tie-up with Maserati. And it is as radical to drive as it looks, thanks to its use of wafting-on-air hydro-pneumatic suspension, variable ride height, ultra-sharp steering and a race-derived quad-cam V6.

Never heard of it? Well SM stands for Série Maserati and the link came about after the French carmaker bought Maserati in the late 1960s. By then France’s once-illustrious reputation for exotic GTs was waning, with Facel Vega about to fizzle out and luxury brands such as Talbot-Lago, Voisin and Bugatti mere names in history books. Citroën was on a roll after shredding the rulebook with its pioneering DS saloon and felt it could make its mark with a sophisticated Continental-crossing GT.

But the marque’s management wasn’t keen on an evolution of the curvaceous DS. Instead, it called on aeronautics designer Robert Opron to pen the company’s path into the angular 1970s. What’s more, the new model needed to offer a step-change in performance from the marque’s four-cylinder mainstay that had evolved from the Traction-Avant. Cue the benefits of its purchase of Maserati, which allowed Citroën to tap into the marque’s band of talented engineers led by Giulio Alfieri.

He was tasked with creating a 2.7-litre V6 engine – the capacity being dictated by France’s vehicle taxation categorisation. Developed in just six months, the V6 featured a 90-degree block and a flat-plane crankshaft as it was effectively a cut-down version of the V8 developed for the Maserati Merak. In SM form, the unit boasted triple twin-choke Solex carburettors and was good for 127kW at 5500rpm.

The Italian power unit was married to standard Citroën thinking: a five-speed, front-mounted transaxle (ahead of the engine, Audi-style) driving the front wheels with hydro-pneumatically controlled suspension all round (wishbones up front; trailing arms at the rear). Braking was by inboard discs up front and conventional discs at the rear – also controlled by the car’s hydraulic system with a pump running via shaft drive and a large reserve tank holding the crucial LHM fluid. The same system also powered the car’s rack-and-pinion steering, but more on that later.

All very leading edge for 1970 but even more so were Opron’s angular lights and styling flourishes: the wide glass nose (the SM’s trademark) housed a rack of six headlights (the middle set of which point around corners as they’re connected to the steering) and the number plate, while the rear was all angular stainless steel with the wheels partially encased in spats.

When the SM made its debut at the 1970 Geneva show, its avant-garde mechanical thinking attracted praise from the press, although some of it was guarded: Motor magazine called it ‘a triumph of design complexity over production engineering’… clearly the SM’s technical specification looked mildly terrifying for anyone accustomed to conventional car making.

Quite frankly, on aesthetics alone, the SM must have looked like it was from Mars parked next to anything humble such as a Coke bottle Cortina or Rover at the time. Still, it was a hit and some 5 000 rolled (or glided) off the lines at the company’s famous Quai de Javel factory in the first year alone as the SM became the must-have for sophisticated European playboys. The cars’ owners dreaming about high-speed weekend blasts to the French Riviera with a Bridget Bardot lookalike ensconced on the passenger seat and an unfiltered Gauloise in their left hand…

Fast-forward 50 years and the SM is an intriguing proposition. In some sense, it looks like it’s waiting for a future that’s yet to arrive. But get behind the wheel and the interior will have you time travelling in the other direction to an era of bell-bottomed jeans, big hair and orange and mustard soft furnishings.

Just as with a DS, starting and driving an SM involves a bit of theatre as the suspension needs time to rise up, particularly if the car’s been parked overnight when the vast coupé will be on its bum, so to speak. Move off too quickly and you’ll likely be making an immediate detour to your local Speedy exhaust branch.

Once the SM is up and running you can set the ride height using a ratchet lever left of the seat but there seems little point as it’s hardly a car for tackling rough with although, if you’re an SM virgin, your first few kilometres are likely to be rough. Seriously rough. The steering and braking systems are both uber sensitive with the latter controlled by a large rubber button that would honestly look more at home below your mother’s Singer sewing machine.

That floor button needs the merest hint of movement to trigger strong retardation while the steering’s high gearing (just two turns lock to lock) and chronically strong self-centring ability need subtle inputs to make direction changes. Overdo the inputs on either – or accidentally let go of the large mono-spoke steering wheel – and you’ll end up see-sawing ungraciously across the tarmac as the soft suspension amplifies the coupé’s lack of composure. Get it horribly wrong at speed and I suspect you could be admiring the Armco barrier up close. But get the hang of the need for precise, economical inputs and the SM will reward and inspire with surefooted progress that’s surprisingly rapid: 0-100km/h in nine seconds and a top end nudging 220km/h was heady stuff for the time when a lot of the competition needed a V8 with twice the cubic inches to beat those numbers.

What makes the steering particularly tricky to master is the severity of the wheel’s self-centring which isn’t dictated by the suspension geometry (the castor) but by the system’s design. While the concept of centralised hydraulic power for all the controls was already tried and tested in the DS, the steering arrangement in the SM was new. Referred to as DIRAVI – which is an acronym for direction à rappel asservi literally meaning ‘steering with controlled return’ – its key design feature is that it doesn’t have a direct mechanical link to the wheels which means unnecessary road feedback is ironed out. It was designed to harness and tame the power output of the V6 in a front-wheel-drive format but it also found its way into the Maserati Quattroporte II and Khamsin.

Easier to adapt to is the SM’s gear change which has a surprisingly direct, mechanical feel (considering the location of the ‘box) with a gear lever spouting out of a chromed gate that looks like an ashtray from a ’70s night club. Other equally period touches include the vertically mounted radio cassette in the centre console and the horizontal stitched panels to the velour-covered seats which just seems very Day of the Jackal.

On the move the alloy V6 has plenty of grunt and moving deftly through the gears translates that into rapid progress. And it’s at speed where this GT coupé comes into its own as it increasingly beguiles you with its smooth and rapid progress, with only the wind noise a clue to the fact you’re piloting something that was built in 1970 and not 1990 or later. In cruise, it will glide comfortably at 140-150km/h if you’re prepared to break the limit, while its braking ability inspires confidence to do so – unlike many other early ʼ70s classics where an ABS-spec’d modern slamming on anchors ahead could spell disaster.

The SM was only ever offered in left-hand drive (although there were four right-hooker prototypes) and that meant it never officially made it to our shores. More than 12 000 were built and a lot of them went across the Atlantic although the car’s frontal treatment fouled Federal safety laws so, on US deliveries, the glass-encased nose gave way to conventional fixed headlights. On the plus side, engine capacity was bumped up to three litres to maintain performance with the auto option, which was popular with buyers Stateside, although I can’t imagine quite what the average US mechanic made of the sight of hydraulic sphere-and-pipe-festooned longitudinally mounted but transaxle-harnessed engine that greeted him after lifting the ‘hood’…

The SM had plenty of fans including motorcycle champion Mike Hailwood and Fiat boss Gianni Agnelli, who fell for the car’s styling. Cold War leader Leonid Brezhnev had one tucked away in the Kremlin garage and Uganda’s Idi Amin was known for coveting a pair. The SM has also been a favourite of successive French prime ministers who used a stretched open-top four-door, coach-built version for state occasions and which is still in the French government garage.

So how come this sophisticated and hugely capable GT coupé was only in production for five years? Too radical? In hindsight possibly, as it didn’t inspire radical offerings from rival makes and, in reality, it was without peers if you consider that Mercedes-Benz’s SLC, Alfa Romeo’s Montreal and Jaguar’s V12 E-Type were the two-door GT alternatives in the new car guides of the time. Citroën would arguably have made a bigger mark with its SM had corporate politics not got in the way. When Peugeot acquired the famous chevron brand in 1974, it couldn’t stomach a product so obviously powered by another marque and also wanted to position Citroën as a mainstream saloon car maker so Opron’s futuristic coupé got the chop a year on.

Today SMs are collectible having been the classic of choice – certainly in European capitals – of architects and advertising creatives although less so than a Mercedes Pagoda was a few years ago, the latter proving to be a lot more conventional to maintain. And that’s what normally scares potential owners of SMs and its four-door sibling, the DS. But find a mechanic who is familiar with – or better still – specialises in them and you could be rewarded with a truly unique driving experience while living out your ’70s spy-thriller fantasies. It’ll be classic life like you’ve never known it.