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The mid-engine sports car that could have been: the Mercedes-Benz C111

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Mercedes-Benz C111 variants on display at the Mercedes-Benz Museum in Stuttgart. Photos courtesy Mercedes-Benz unless otherwise noted.

Mercedes-Benz seriously began exploring the possibility of building a mid-engine sports car during the 1960s. In 1965, designers Giorgio Battistella and Paul Bracq jointly penned a mid-engine coupe envisioned to slot a notch above the SL in the company’s lineup. Known as the SL X internally, the low-slung coupe never progressed beyond the prototype stage, and it was never fitted with an engine.

Mercedes-Benz SL-X design study

The Mercedes-Benz SL-X design study. Photo by Ronan Glon.

A mock-up of the SL X that’s rarely seen today is stunning to look at. Its design is characterized by bulging fenders, tall wheel arches, pop-up headlights, a gently-sloping roof line, and an oversized three-pointed star emblem proudly affixed to the low nose. If produced, the SL X would still be remembered as a masterpiece of Mercedes-Benz design, even nearly five decades after its introduction.

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SL-X design study photos by Ronan Glon.

While the SL X project was shelved, a small team made up of engineers and designers was assigned to work on a new sports car known as the C101 as early as 1967, according to the company’s archives department. Mercedes remembers that it never asked the group to build a successor to the 300 SLR or to the 113 SL; the car it was working on wasn’t intended to ever see the inside of a showroom. Instead, it was a rolling laboratory that could be used to test out new features and ideas.

Mercedes-Benz C111 I

Mercedes-Benz C111 I Mercedes-Benz C111 I Mercedes-Benz C111 I Mercedes-Benz C111 I Mercedes-Benz C111 I Mercedes-Benz C111 I

The C111 version I.

Mercedes nonetheless presented the C101 to the public during the 1969 edition of the Frankfurt Auto Show, though, as Karl Ludvigsen explained, it had to change the name to C111 because Peugeot had trademarked every three-digit nameplate with a zero in the middle. Enthusiasts were awestruck. The C111 stole the show with an eye-catching orange paint job, gullwing doors, and a highly-aerodynamic body made out of fiberglass, a construction technique that saved a considerable amount of weight. The body was bonded to a steel chassis. And while early prototypes looked like an overgrown Fiat 850 Spider put on steroids, the example shown in Frankfurt was designed with input from Bruno Sacco so it was quite a head-turner.

4 rotor Wankel engine

4 rotor Wankel engine

The four-rotor Wankel engine, which debuted in the C111 II.

Perhaps the most surprising aspect of the C111 was its Wankel engine, however. Rotary engines were becoming increasingly common – if not popular – at the time thanks largely to companies like NSU, Citroën, and Mazda, but a Wankel-powered sports car was considered groundbreaking. Mercedes had been experimenting with Wankel engines since buying a license from Felix Wankel, its inventor, in 1961, and the engine in question was a direct-injected, tri-rotor unit with a displacement of 600cc per chamber. It was tuned to generate 280 horsepower, which was enough to send the coupe from zero to 62 mph in five seconds flat and on to a top speed of 161 mph. Brakes borrowed from the 300 SEL 6.3 safely brought the coupe down to a stop.

C111 I and II

C111 I (L) and C111 II (R)

When a production car debuts at an auto show, it generally arrives in showroom several months later. The C111 was different; it wasn’t bound for production, and Mercedes was constantly making changes to it. An evolution of the coupe dubbed C111-II was unveiled just a few months later during the 1970 edition of the Geneva Auto Show.

Mercedes made an array of revisions to the styling, partly to improve visibility, but bigger updates were found under the hood. Power for the C111-II came from a quad-rotor Wankel engine rated at 350 horsepower. That was enough grunt to send the coupe from zero to 62 mph in 4.8 seconds and on to a top speed of 186 mph.

Four-rotor Wankel

Four-rotor Wankel, as used in the C111 II.

Mercedes explains it managed to solve the reliability issues typically associated with the Wankel engine, but no amount of engineering prowess could reduce its unreasonably high fuel consumption.

“Our four-rotor engine with gasoline injection represented the optimum of what could be reached with this engine concept. The multi-rotor design called for peripheral ports for the intake-air and exhaust-gas ducts. We were able to solve the difficult problems in engine cooling and engine mechanics by technical means. But the main problem of the concept, its low thermodynamic degree of efficiency, remained. Due to the elongated, not exactly compact combustion chambers, fuel economy was poor, resulting in high fuel consumption and unacceptably high pollutant emissions. These drawbacks were inherent in the design principle,” remembers Dr. Kurt Obländer, the head of engine testing for the C111 project.

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C111 II.

Mercedes insisted the C111-II was to remain an experimental prototype, a high-profile showcase of future tech that could ultimately trickle down to the company’s regular-production models. That didn’t stop a handful of determined customers from mailing the company blank checks in a bid to change executives’ minds.

There are more than a few ways to explain Mercedes’ reluctance to add the C111-II to its lineup. Notably, the company had come to terms with the idea that the Wankel engine was impractical at best, and it put an end to all rotary-related development in 1971.


Three generations of C111s, including the initial prototype (background), the C111 I (center) and the C111 II (right).

The ever-stricter fuel economy and emissions regulations in the United States convinced Mercedes to build more efficient cars, so it shifted its focus to diesel. Specifically, the company saw a great deal of potential in the 3.0-liter straight-five (called OM 617 internally) that it was developing for the w115-series sedan. The naturally-aspirated version of the engine that would debut under the hood of the 240D 3.0 1974 made just 80 horsepower; engineers bumped its output to 190 horses by fitting it with a turbocharger and an intercooler. While a Wankel-engined sports coupe surprised enthusiasts, an oil-burning version of it left them speechless.

C111 II D

C111 II D tests at the Nardo track in Italy.

The rolling laboratory was again re-named, this time to C111-IID. Mercedes shipped it down to the Nardò circuit in southern Italy to demonstrate its potential. Four drivers managed to set 16 world speed records over the course of 60 hours; 13 of them were for cars powered by a diesel engine, and three of them were general records. Even more impressive was that the C111-IID logged an average speed of 156 mph. Naysayers who protested that the idea of a diesel sports car was an oxymoron were silenced.

C111 II D

C111 II D

C111 II D.

Mercedes was still not ready to add a range-topping, mid-engine model to its lineup, so the C111-IID never received the proverbial green light for production. However, the lessons learned during its development later helped engineers as they designed the turbodiesel straight-five engine that debuted under the hood of the w116-series 300 SD in 1978. The engine was offered in the 126-series 300 SD starting in 1980, and in the US-spec version of the w123-series 300D the following year.