24h Le Mans
24 HOURS CENTENARY – PERPETUAL INNOVATION ⎮ After the supercharger of the 1930s, the arrival of turbo during the 1970s marked a major change in the evolution of engines at the race, with 39 wins to its credit since 1976.
As early as pre-WWII, attempts were made to increase engine power by supercharging using a compressor, making it possible to boost power from 1.3 to 1.5 without having to augment engine capacity. This was covered in the regulations, but an upgrade in cubic capacity implied larger, heavier engines more difficult to integrate into a car. This increase also resulted in less agile engines, as well as an upsurge in internal friction which provoked fuel consumption.
Several manufacturers quickly adopted mechanical, screw or gear compressors. For an engine to function, the air/petrol mixture must be in fixed proportions. Thanks to the compressor, air was compressed before the carburettor, increasing the quantity sent to the engine. Therefore, more petrol was injected while respecting the necessary ratio. More air/fuel mixture always means more power.
The compressor, first seen on the Alvis 1500 in 1928, proved a huge hit, with five cars out of 17 fielded in 1930 equipped with superchargers: two Bentleys, one Mercedes SSK, one Alfa Romeo and one Lea-Francis. Bentley split its engines, with large naturally aspirated 6.5-litre 180 hp engines on the one hand and smaller 4.5-litre supercharged 175 hp engines on the other. Alfa Romeo gave the supercharger its first win in 1931.
Equivalence rules were established to limit the displacement of cars with superchargers to balance the power between supercharged cars and naturally aspirated ones. As early as 1931, a compressor engine had to have a displacement 33% less than a naturally aspirated engine.
But driving these mechanical compressors was another story. Taking from the crankshaft, gear or belt drive, or even direct drive, drew power from the engine. And the more the compressor compressed the air, the harder the car was to drive and the more it consumed a good portion of the power it saved.
It was therefore necessary to seek a free source of energy to drive the compressor. Porsche and Renault arrived at more or less the same conclusion after their respective analyses: exhaust gases could drive a small turbine whose rotating shaft drove the compressor without pulling power from the engine.
The response time was a major drawback however: the turbine continued to rotate very quickly on its inertia after a driver's foot was lifted from the accelerator. Doubling the turbocharger made it possible to produce smaller elements that responded more quickly to changes in engine speed.
Constructors also worked to reduce the temperature of the air admitted into the engine, warmed by compression, by installing air/air exchangers. And, to limit overpressure during deceleration, relief valves were installed (to the thrill of spectators).
The elimination of the Sport class (5 litres) compelled Porsche to focus on increasing engine power, limited by regulations to 3-litre atmospheric or 2.1-litre compressed (coefficient of 1.4). Aware of the limits of its air-cooled 6-cylinder flat engine (not able to exceed 3 litres), in 1974 the German manufacturer tested at Le Mans a Porsche 911 equipped with an engine reduced to 2.1 litres and fitted with a turbocharger. The car was designed with huge wheel arches, oversized rear tires and a large rear spoiler. To everyone's surprise, it set the seventh fastest time in practice before finishing second overall, just six laps behind the victorious Matra.
On the back of that result, Porsche suggested to the International Sporting Commission (predecessor to the current FIA) to create the Group 5 class, sometimes called Silhouette formula, as of 1976. The marque proceeded to build the unforgettable Porsche 935, made available to private teams as well. But Porsche also decided to construct a Group 6 car (prototype class, mainly open body at that time) especially for Le Mans. Turbo technology was definitively validated and two 6-cylinder flat engines equipped with turbo were installed in a 2.1-litre version for the Porsche 936 (Gr 6) and a 2.8-litre version for the 935 (Gr 5).
The 1976 season was a triumph: the German marque earned the title in the World Championship for Makes with the Porsche 935 and scored the overall win at the 24 Hours with the Porsche 936. Turbo technology was here to stay and it was just the beginning, with another victory for the Porsche 936 in 1977, then the Renault-Alpine A442 in 1978 and the Porsche 935 K3 in 1979. Rondeau interrupted the series in 1980.
In 1982, Group C prototypes emerged, with Porsche arriving with the 956, then the 962 C twin turbo (the engine remained a 2,650 cc flat-6). Proving its mastery of turbo technology, the manufacturer won the 24 Hours six years in a row, beating Ferrari's win record. It would take until 1988 and Jaguar's naturally aspirated 7-litre V12 to overcome the turbocharged engine.
During the 1990s, the naturally aspirated engine persevered with six more wins (Jaguar, Mazda Peugeot, McLaren and BMW) against four for the turbo engine (Porsche). However, in the 2000s, the turbocharger dominated with petrol and diesel engines (Audi and Peugeot), without leaving a crumb for naturally aspirated engines until the arrival of hybrid engines. A new technological revolution had arrived…
PHOTOS (Copyright - ACO/Archives): LE MANS (SARTHE, FRANCE), CIRCUIT DES 24 HEURES, 24 HOURS OF LE MANS. From top to bottom: the #20 Porsche 936 was the first turbocharged engine prototype to win Le Mans thanks to Jacky Ickx and Gijs van Lennep; the #9 1930 Bentley Blower supercharger seen here at the front below the radiator; in 1978, the #2 Renault-Alpine driven by Didier Pironi and Jean-Pierre Jaussaud emerged victorious after a turbo duel with Porsche; the Porsche 935 (here the winning #41 at the 1979 24 Hours) was another turbo engine car emblematic of the late 1970s; with four consecutive wins between 1982 and 1985, the 956 twin-turbo (here the #1 winner in 1982) kicked off Porsche's domination with the 962 C in 1986 and 1987; in 2006, Audi combined turbocharger and diesel technology for three wins in a row for the R10 TDI.
