Alfa Romeo 6c 1750 SS

Mille Miglia cars

Alfa Romeo 6C is a series of cars, presented in 1925, which in various versions and evolutions was produced by Alfa Romeo in the Portello factories from 1927 to 1953.

The acronym "6C", which stands for 6 cylinders, describes the fractioning of the engine with which all the models were equipped, albeit with substantial differences between the series, starting from the displacement, continuously increased from the original 1500 cm3 up to 2500 cm3.

The historical context and the "6C" family
The chassis of an Alfa Romeo 6C 1500 Mille Miglia Speciale with twin-cam engine at the Automobile Museum in Turin

In the autumn of 1924, the echo of the triumphs of the Alfa Romeo P2 had not yet died down, when Vittorio Jano received from the management of Alfa Romeo the task of designing an "economic light car with brilliant performance" intended to replace the "RM" model.

The 6C designed by Jano to achieve the desired brilliant performance exploited the power-to-weight ratio rather than an absolute powerful engine.

On a light and resistant steel chassis were mounted rigid axle suspensions on semi-elliptic leaf springs front and rear and a revolutionary six-cylinder in-line engine with valves and overhead camshaft of only 1500 cm3 with 44 HP. Later the displacement was increased to 1750 and then 1900 cm3 and were used twin-cam heads, volumetric compressors and ancillary components in light alloy to increase performance.

In the mid-1930s the cars of the 6C series were completely redesigned by modernizing the chassis introducing independent four-wheel suspension for the first time ever in Europe and mounting a new twin-cam engine of 2300 cm3 of displacement, increased a few years later to 2500 cm3.

The naming of the models followed a very precise pattern for the entire duration of production: 6C, or the number of cylinders, the approximate value of the displacement that followed the progressive increases, the indication of one of the three trim levels and the consequent level of power available, the type of body mounted and possibly the signature of the coachbuilder who had created it.

Throughout production, the cars were sold either as a simple chassis to be bodied as desired or, subsequently, as complete cars with different body versions. For this reason, the 6Cs of all series were equipped with bodies of the most disparate types such as torpedoes, closed sedans, spiders, coupés, cabriolets, berlinettas and even executive sedans. The cars of all series have very different styles from each other both because they were made by different coachbuilders, and therefore designers, and because they were built over a very long period of time (1927-1950) in which the way of understanding cars also changed a lot. On all cars, from the beginning to the end of production, the steering wheel was placed on the right, as was normal at the time.

The strong point of all the cars in the series, but especially of the first models, was the advantageous power-to-weight ratio that made them competitive and winning competitors in all types of races; from road races such as the Mille Miglia to endurance races on the track such as the 24 Hours of Spa-Francorchamps.

The 6C 1500 or "NR"
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The first chassis, called "NR" in homage to the engineer Nicola Romeo, owner of the company, and only after Romeo left the company renamed "6C 1500", was presented at the Milan Motor Show in April 1925 and, in the autumn, also at the Paris and London motor shows. Due to delays in preparing the Portello factory for production, the first complete car was not delivered to customers until 1927.

The 6C 1500 Normale
A 1929 Alfa Romeo 6C 1500 Normal Drophead Coupé James Young Ltd., Young was a very active coachbuilder on Alfa Romeo chassis in England

The new car used a simple but light and strong chassis with pressed steel side members and cross members with a "C" section, a suspension system with rigid axles at the front and rear, semi-elliptic leaf springs and friction shock absorbers and mechanically operated drum brakes with foot control on the four 19" spoked wheels. It was available in a short wheelbase version of 2900 mm and a long wheelbase version of 3100 mm. The factory offered a closed body with four or six seats as standard, or a chassis to be bodied as the customer desired.

The new car was equipped with a six-cylinder engine in a totally new line designed with such advanced criteria that it remained in production, albeit updated, until 1933 and obtained numerous victories in races of all kinds. The first model was equipped with an aluminum crankcase, monoblock and (fixed) cast iron cylinder head with overhead intake and exhaust valves controlled by a camshaft, also overhead, driven by a shaft 

camshaft, always overhead, driven by a vertical idler shaft with double bevel gear, placed behind the engine block. The exact displacement was 1487 cm3 (bore x stroke: 62 x 82 mm), just under 1.5 litres, hence the name 6C 1500. This engine developed 44 HP at 4200 rpm, an exceptionally high power and rotation speed in relation to the displacement and year of production thanks to the reduced unit displacement (250 cm³) which had increased the thermal output by approximately 200% compared to the standards of the time.[2] The engine was combined with a four-speed gearbox with an integrated handbrake that acted on the transmission.

The first example delivered to customers and used for advertising photos was a torpedo made by Carrozzeria Nord Italia that reached 110 km/h during testing in the winter of 1927.

The 6C 1500 Sport and Super Sport
As befits an Alfa Romeo, the newly born 6C 1500 was also used in competitions and in 1928 the more specialized Sport version was launched with 54 HP of power at 4400 rpm, 10 more than the 1500 Normale, followed shortly after by the even more powerful Super Sport.

The Sport had a new removable and no longer fixed cylinder head with two camshafts, still controlled by a shaft and two bevel gears, with valves inclined at 90° to each other, hemispherical combustion chambers and increased compression ratio. It was not the first time ever that an Alfa Romeo exploited these technical solutions but for the first time they were applied to a production car, albeit numerically limited. The petrol tank was also moved behind the rear axle and in its place a smaller tank was mounted to feed the double-barrel carburetor by gravity.

On this mechanics, 2-seater sports bodies were often mounted, very often made by Stabilimenti Farina e Castagna with the Weymann patent, a wooden frame with elastic joints covered by panels of pergamoide or imitation leather, which was lighter than traditional bodies and allowed them to reach 130 km/h.

The sporting debut of the 6C 1500 Sport took place on 6 June 1927 on the newly created Modena Circuit when the Sport was still a prototype equipped with the experimental twin-cam engine. At the start there were two cars lined up, one for Attilio Marinoni and one for Enzo Ferrari, who won the 360 ​​km race ahead of his teammate in 3.20'25" at an average speed of 107.601 km/h.

The Super Sport is the even more powerful version of the Sport, still with a naturally aspirated engine but with an increased compression ratio and a different double-barrel carburetor that developed 60 HP at 4800 rpm and allowed the lightweight spiders to reach 135 km/h. The 6C 1500 Super Sport Spider Zagato of 1928 was the forefather of the Alfa Romeo spiders, intended as open two-seater sports cars, which would have famous heirs such as the Giulietta Spider and Duetto.

The 6C 1500 Super Sport Compressore and Super Sport Compressore "Testa Fissa"
After the Sport and the Super Sport, two new versions were built specifically designed for competition: the Super Sport Compressore and the Super Sport Compressore "Testa Fissa" Fixed". The Super Sport Compressore was based on the Super Sport but also featured a Roots lobe supercharger system that increased power to 76 HP at 4800 rpm and allowed the cars to reach 140 km/h.

This model is also known as the Mille Miglia Speciale because it was with the first of these lightweight Zagato-bodied Spiders driven by Giuseppe Campari and Giulio Ramponi, the only one with a compressor entered, that Alfa Romeo recorded the first of eleven victories in the Mille Miglia in 1928.

The Achilles heel of the Super Sport Compressore, like other cars of the period, was the quality of the materials the gaskets were made of, meaning that it was not possible to increase the compression ratio beyond a certain limit, otherwise the cylinder head gasket would burn during intense racing use, causing the car to retire. To overcome this problem, Vittorio Jano developed a version of the engine with a compressor and fixed cylinder head capable of 84 HP at 5500 rpm, which, since there was no cylinder head gasket, could not lose compression or burn the gasket itself. The cars on which these engines were mounted, known as 6C 1500 Super Sport Compressore "Testa Fissa", were all spiders capable of reaching 155 km/h.

In addition to the one in the 1928 Freccia Rossa, many other victories arrived for the 6C 1500 in various sports configurations including the 24 Hours of Spa, with Boris Ivanowski and Attilio Marinoni at the wheel, and the circuits of Alessandria and Modena, with Enzo Ferrari at the wheel, and numerous placings including two second places by Giuseppe Campari at the Targa Florio and the 24 Hours of Le Mans. Obviously the sporting career did not end in 1928 but continued for several years in the hands of 

private pilots.

Before it was replaced by its heir with increased displacement, the 6C 1750, 1,075 road-going examples of the 6C 1500 model were produced, of which 171 Sport, 15 Super Sport, 10 Super Sport Compressore and 6 Super Sport Compressore "Testa Fissa".