One-off prototypes aside, Citroën’s first experience with the Wankel engine came with the M35, an experimental Ami 8-like coupe that was briefly sold to the general public in the late 1960s and the early 1970s. The data gathered over the course of the M35 project was used to design a new twin-rotor engine that was earmarked for the long-awaited GS.
Citroën presented the GS in 1970, but the rotary-engined version of it didn’t make its debut until the 1973 edition of the Frankfurt Motor Show. Citroën was the second European automaker (after Germany’s NSU) to mass-produce a rotary engine, and it was the first company to mounted it transversally.
The press was initially nothing short of impressed by the Birotor, many journalist billed it as a new car in a GS body instead of merely a GS with a new engine. Interestingly, Citroën had planned on making a host of exterior modifications to differentiate the Birotor from the standard GS but the company didn’t have enough money to implement them to the production car. Consequently, the Birotor only stood out from a regular flat-four-powered GS thanks to minor updates like fender flares, five-lug 14-inch rims wrapped by 165 tires (regular GS models sat on three-lug 15-inch rims wrapped by 145 tires) and specific “Birotor” badges all around the car.
Offered only as a sedan, the Birotor was available in metallic brown and beige, though many featured a more premium-looking two-tone paint job that mixed the two colors. Contrary to many reports, the car was never offered in green or blue and the rotary engine was never fitted to the GS station wagon.
The Birotor was a fairly expensive car at the time so Citroën went to great lengths to spruce up the interior. The light brown dashboard featured a complete instrument cluster with round gauges, full carpeting came standard, the shift boot was specific to the car and the seats came with built-in headrests.
Under the hood was a water-cooled engine developed by Comotor, a joint-venture created in the 1960s by Citroën and NSU to develop and assemble rotary engines with a license granted by Felix Wankel. Built in an all-new factory located in Sarre, France, the twin-rotor mill had an adjusted displacement of 1,990 cubic centimeters, making it by far the largest engine available under the hood of a GS. With 107 horsepower at 6,500 rpm and 101 lb-ft. of torque available at just 3,000 rpm, the engine was also the most powerful unit ever offered in the GS, a car that was notoriously underpowered for all of its life.
The Wankel was designed to use about a liter of oil every 1,000 kilometers (about a quart every 620 miles) in order to ensure proper lubrication, which all but eliminated the need for routine oil changes. A button located on the center console allowed the driver to instantly check whether the engine had enough oil.
Power was sent to the front wheels via a three-speed semi-automatic transmission that was very similar to the C-Matic unit found in other members of the GS lineup. The shift lever was mounted on the floor and a torque converter replaced a conventional clutch disc. Citroën used the semi-automatic gearbox as a last resort because its engineers could not get a manual unit to work smoothly enough in time for the car’s launch. The GS wasn’t very smooth to drive in stop-and-go traffic to begin with, and Citroën’s own records indicate the Wankel engine made the issue noticeably worse.
The Wankel propelled the GS from zero to 62 mph (100 km/h) in just over 13 seconds and on to a top speed of 108 miles per hour. An audible alarm went off to alert the driver when the engine hit its red line of 7,000 rpm.
The braking system was similar to that of a regular GS, but the front brakes were mounted behind the wheels instead of inboard. All GS Birotors were equipped with the same DS-inspired hydraulic suspension system that was found in the regular GS. However, several modifications (including the addition of anti-roll bars) were made to obtain a stiffer ride and help eliminate the car’s copious amount of body lean.
Overall the GS Birotor was hands down one of the best handling and performing Citroëns of the era, but it was also one of the company’s biggest fiasco. The problems started in October of 1973 – a few months before the car’s market launch – when OPEC declared an oil embargo that sent gas prices sky-rocketing over the next few months.
Fuel economy was not one of the Birotor’s strong points and sales suffered accordingly. Auto-Journal‘s André Costa tested the car in 1974 and reported that it used up to 17 percent more gas than a fuel-injected DS23 IE; at one point Costa went down to nine miles per gallon in city traffic. The poor city mileage and the rough ride in dense traffic conditions were both largely attributed to the spark plugs fouling up.
The Birotor was also very expensive for what it was. In 1974, it carried a base price of 24,952 francs. Buyers with extra cash in their pocket could order a radio, rear seat belts (front ones were made mandatory in France in 1970), tinted windows and a manual sunroof. To put that price tag into perspective that same year an entry-level DSuper 5 cost 22,600 francs and a GS 1220 hovered in the vicinity of 15,000 francs. Wankel engine or not, many customers had a hard time accepting the idea of spending DS money on a GS.
An equally important factor that rarely gets mentioned is that France, the Birotor’s biggest target market by a long shot, implemented a speed limit on freeways mere months before the car’s launch. Buyers did not want to spend an extra 10,000 francs to get 100 horsepower when a 50-horsepower GS could cruise at legal freeway speeds just fine.
The GS Birotor was phased out in 1975 and Citroën quickly encouraged its dealers to buy back as many examples as possible in order to destroy them. Some dealers did this by offering unbeatable trade-in incentives, others bought them but didn’t destroy them and some ignored Citroën’s orders altogether. Precisely how many Birotors were built has been a topic of debate among automotive historians for decades – some claim 873 while others believe the correct number is 847. What is certain is that the number lies between 800 and 900 and that over 800 of those were built in 1974 (the rest in late 1973 and early 1975). It is estimated that only a third of the total production remains today.
Regardless of how many Birotors were built, the final tally is a far cry from Citroën’s initial goal of building 25 cars a day. It’s a real shame because Citroën was just beginning to dabble in Wankel engines and it making constant improvements to it in preparation for a rotary-engined CX. The technical specifications included a more powerful engine with fuel injection and a five-speed manual transmission, among other details. Several prototypes were built but Citroën, as cash-strapped as ever in spite of its new ownership, finally gave up all Wankel-related research 1979. A fuel-injected evolution of the GS’ twin-rotor engine did see the light of day in a fascinating prototype, but that’s a different story for a different time.
The press shots and the technical documents were kindly provided by Citroën’s archives department, and the other pictures come from our own archives.