The 1928 24 Hours of Le Mans saw the advent of forced induction in the shape of a compressor, bringing a clear improvement in available power. This addition thus introduced a distinction between conventional, naturally aspirated engines, and forced induction or supercharged engines.

An engine runs on an air/fuel mixture with a specific ratio. To generate more power, the quantity of fuel admitted must be increased, and therefore the amount of air too. The easiest way to achieve this is by increasing the displacement but restrictions on the cubic capacity of engines written into race regulations limit the scope of action. Superchargers overcame this issue by increasing the volume of air drawn into the engine by raising pressure from 0.8 to 1.5 bar. More fuel could therefore be fed in while keeping the richness of the mixture at a reasonable level, thus boosting power.

However, superchargers must be powered in order to compress the air. The versions designed in the 1920s and ’30s used the rotary motion of the crankshaft to drive a screw compressor or tooth compressor (with “tooth” rotors compressing the air inside the supercharger). This device was particularly noticeable at the front of Bentleys, at the end of the crankshaft, under the radiator.

The advantage of superchargers was that they could be used with any size of engine. French specialist Cozette used the technology with the Tracta’s 998 cm3 engine. It was estimated that supercharging resulted in 30% higher displacement than the basic geometric displacement. Boosted by this technology, Bentley, Mercedes and Stutz all generated 200 horsepower by the end of the 1920s.

From the supercharger to the turbo

As superchargers are active even at low engine speeds, they are particularly effective on acceleration. However, as the engine ‘s crankshaft powers the supercharger, it causes a loss of horsepower. They were therefore abandoned after the Second World War in favour of larger engines coupled with fuel injection systems.

By the mid-seventies, however, Porsche and Renault had designed a system to use waste exhaust gas energy with a small turbine to drive a small blade-type compressor. The turbocharger first appeared at the 24 Hours of Le Mans in 1974 on a Porsche, before the 1976-78 period produced a series of turbocharged face-offs between the German manufacturer and Renault.

Generations of engineers have worked to overcome the main drawback of this technology: the response time on accelerating. This is because a large amount of hot gas is needed to drive the turbine. However, on decelerating into a turn, the driver releases the throttle, reducing the quantity of exhaust gases and the pressure. Therefore, when accelerating out of the turn, the turbine is running at low speed and little air is compressed. The turbocharger only becomes active again when engine speed has built back up.

Super- and turbocharger technology has helped limit the cubic capacity of engines and therefore the volume to be integrated into the chassis. The compact size of the latest generation of engines is highly revealing of the progress accomplished in this respect. Whereas they were initially used almost exclusively with enormous truck engines, turbo systems now feature on 1500 cm3 to 2-litre models – or even smaller engines on production cars. Meanwhile, the ACO and the FIA continue to work on the equivalences between naturally aspirated and super- or turbocharged engines, with multiplication factors ranging from 1.4 to 2 according to technological developments. This coefficient indicates that a 1500 cm3 turbocharged engine with a multiplication factor of 2 corresponds to a 3000 cm3 engine.

Night time at Le Mans has always held a special kind of magic. The advent of the turbo has added extra sparkle with the rear of the cars lit up by red-hot turbos and exhaust pipes spitting out flames as they decelerate into the bends.

PHOTOS: LE MANS (SARTHE, FRANCE), CIRCUIT DES 24 HEURES, 24 HOURS OF LE MANS – FROM TOP TO BOTTOM (© ACO archives). Three pictures capturing the transition from the supercharger to the turbocharger: in 1930, the supercharger on the Bentley Blower of Henry Birkin/Jean Chassagne can be seen here below the radiator; in 1928, Robert Bloch/Edouard Brisson finished second in the Stutz DV16 Blackhawk: the best result achieved by a US manufacturer until Ford’s maiden win in 1966; in 1976, the Porsche 936 of Jacky Ickx (at the wheel) and Gijs van Lennep became the first turbocharged car to win the 24 Hours of Le Mans.