Citroën GS Birotor: The history of the Wankel rotary engine that changed everything

In the rich and tumultuous history of the French automotive industry, the brand with the double chevron has always stood out for its unique propensity to challenge established conventions and explore technical paths deemed impossible by its competitors. This new article in our series dedicated to "Citroën's Wildest Gambles" looks back at a particularly daring episode, one whose financial repercussions nearly sealed the company's fate forever. In the early 1970s, the introduction of the Citroën GS Birotor embodied the culmination of a total technological ambition, centered around a revolutionary mechanical architecture. Yet, what was intended to be the technological showcase for a new era of smoothness and performance transformed into a major industrial trap, proving that a major technical innovation, no matter how brilliant, can prove fatal when confronted with an unpredictable global geopolitical and economic upheaval.

Technical genesis: From the M35 to the strategic choice of the GS

Citroën's interest in alternative powertrains did not begin with the design of the GS, but was part of a long-term strategy initiated during the previous decade. During the 1960s, the brand's management became convinced that the rotary piston engine, developed under the patent of German engineer Felix Wankel, represented the definitive future of automotive propulsion due to its remarkable compactness, its total absence of vibration, and its outstanding operational smoothness. To realize this vision, Citroën partnered with German manufacturer NSU in 1967 to found the Comotor joint venture, tasked with designing and mass-producing these new engines from an ultra-modern factory built in Altforweiler.

Before installing this prestigious powerplant under the hood of a mass-production vehicle, the manufacturer decided to conduct a unique, full-scale experiment with the Citroën M35. This experimental coupe, of which only 267 units were produced, was entrusted to selected high-mileage customers to evaluate the engine's reliability under real-world driving conditions. The feedback from this rolling laboratory proved mixed, highlighting particularly high fuel consumption as well as premature wear of the rotor apex seals, leading to numerous replacements under warranty.

Despite these warning signs and increasingly heavy development costs, management persisted in its desire to industrialize this technology. While it might have seemed more logical to reserve this high-end engine for the luxurious SM or the upcoming CX then under development, Citroën made the strategic choice to install it in the GS. Launched in 1970, the GS was a modern, lightweight saloon featuring an exceptional aerodynamic coefficient and a hydropneumatic suspension widely praised for its comfort. It represented the ideal platform to democratize the rotary engine and demonstrate its technical superiority, a major project that received the internal designation of project GZ.

• 

• 

• 

• 

• 

• 

• 
• 
• 
• 
• 
• 

A deep metamorphosis: the technical anatomy of the GS Birotor

Contrary to a persistent misconception, the GS Birotor was not simply a standard GS fitted with a different engine. The brand's engineers had to carry out a major structural and aesthetic overhaul to adapt the car to the requirements of its new powerplant. Aesthetically, the model was immediately distinguished by widened tracks that required subtly flared wings, giving it a significantly more aggressive stance on the road. The wheels featured specific five-bolt rims, while the front end benefited from aerodynamic modifications, and the cabin received a much more premium treatment, characterized by a specific dashboard and reinforced sports seats.

On a purely mechanical level, the heart of this transformation lay in the Comotor 624 engine. This twin-rotor unit displayed an equivalent displacement of 995 cc and developed an impressive 107 hp DIN for the era, combined with 140 Nm of torque. To channel this unprecedented power to the front wheels, the GS Birotor abandoned the classic manual gearbox in favor of a three-speed semi-automatic transmission, equipped with a torque converter and an automated clutch, ensuring a remarkably smooth driving experience.

The running gear and braking system were resized to handle the dynamic stresses, notably including the adoption of ventilated front brake discs. Global performance radically transformed the family saloon's behavior: the top speed reached 175 km/h, a particularly high figure in 1973 that allowed the GS Birotor to compete with cars of much larger engine displacements, while maintaining a contained weight of 950 kg and an imperial ride comfort.

The 1973 Shock: a commercial launch against the tide of history

The history of the GS Birotor was permanently disrupted when its official commercial launch took place in the autumn of 1973. By a tragic irony of the calendar, this launch coincided almost to the day with the outbreak of the first oil shock, a major geopolitical event that instantly plunged the global economy into crisis and abruptly ended the era of cheap fuel. Overnight, the priorities of motorists and European governments reversed, placing fuel efficiency at the absolute top of purchasing criteria.

In this new economic paradigm, the intrinsic weaknesses of the Wankel rotary engine became prohibitive. The GS Birotor suffered from particularly excessive fuel consumption, ranging between 12 and 20 liters per 100 kilometers depending on driving style, an appetite comparable to that of luxury luxury saloons powered by eight-cylinder engines. Added to this were structurally high oil consumption and significant pollutant emissions, which came into direct conflict with emerging environmental regulations. Citroën found itself in the untenable position of marketing an avant-garde vehicle whose main selling point, the refinement of a high-performance engine, clashed head-on with a Europe that was introducing speed limits and searching by all means to save energy.

The mechanics of failure and the mystery of the destroyed survivors

Beyond the energy crisis, the GS Birotor also faced a major commercial obstacle: its pricing structure. Due to the complexity of its manufacturing process and the massive investments poured into the Comotor factory, its retail price soared to unprecedented heights, positioning it alongside a top-of-the-range DS and directly threatening the market segment of the upcoming CX. For customers, the financial effort required proved difficult to justify for a body shape derived from the standard GS line, despite the level of technical sophistication on offer.

Faced with this disastrous economic situation, production collapsed immediately, and the manufacturer made the decision to halt the project entirely by 1975. Production records sealed the commercial failure, showing that only 846 or 847 units left the assembly lines, making this model one of the rarest production cars in the brand's history.

The epilogue of this industrial adventure took an even more unusual turn in 1977. Anxious to eliminate the colossal logistical costs associated with managing specific spare parts and maintaining an after-sales network trained in this complex machinery, the newly formed PSA Group orchestrated a systematic buyback campaign. GS Birotor owners were offered extremely advantageous financial terms to trade their cars for a brand-new CX. The vehicles recovered by the dealer network underwent a radical procedure: they were neutralized, dismantled, and officially destroyed under bailiff supervision to ensure that no examples could return to the market.

Fortunately, a handful of passionate collectors and customers refused to yield to the temptation of this trade-in offer, allowing around 200 units to escape organized destruction. Today, experts estimate that only 50 to 60 vehicles remain in perfect working order or complete condition worldwide, which explains the exceptional enthusiasm and high valuations commanded by this model on the classic car market.

In the final analysis, the Citroën GS Birotor remains an eloquent testament to the manufacturer's unique ability to dare and push the boundaries of automotive engineering. This undeniable technical audacity unfortunately occurred at the worst possible moment in contemporary economic history, forcing the brand to commit huge sums of money to contain the fallout of this failure, notably through the buyback and destruction program. Accumulated with the massive investments required for the SM program, the costly buyout of Maserati, and the simultaneous development of the CX, the deep financial losses tied to the Comotor venture directly precipitated Citroën's bankruptcy in 1974, leading inevitably to its acquisition by Peugeot to form the PSA Group. However, reducing the GS Birotor to a simple industrial failure would be a major historical mistake. It embodies a genuine technological breakthrough and a fascinating vision of the future, which simply saw its wings clipped by a global energy context that had changed forever.

Source des photos : https://www.citromuseum.com/2025/02/06/gs-birotor-de-1974/#bwg59/1469