Ferry “Butzi” Porsche was responsible for the design of the GRP body. He used some cues and the windscreen of the 718 Coupe and turned it into one of the best-looking cars ever constructed. Production of the body was outsourced to airplane manufacturer Heinkel, which was able to produce two bodies per day, twice as fast as Porsche could produce the chassis. The body was bonded on the frame, which resulted in a chassis far more rigid than the space-frame chassis used in the previous racers.

Chassis 904-006 was seen in spirited competition at the 2010 Le Mans Classic. (Photo: Wouter Melissen)

At the start of the project, the engineers planned on using Porsche's all new 2-litre flat-6 engine, which was designed for the upcoming 901/911 model. Unfortunately, the engine's development was hampered and it was not ready yet in time for the 100-example production run. As a stop gap, the four-cylinder engine from 356 Carrera 2 was uprated to produce 180 horsepower in race trim. It retained the unique shaft-driven overhead camshafts. Bolted directly on the engine was a new five-speed gearbox, which was also developed for the new road car.

Three prototypes were constructed and tested heavily in the fall of 1963. After various modifications were carried through, the car was first shown to the public in late November. Internally it was known as the 904, but it was marketed to the public as the Carrera GTS, although today it's commonly referred to as the 904.

Within two weeks, all but 21 of the 90 examples available for the public were spoken for. Production started soon after in a completely new factory constructed to produce the new 901/911 model. By April 1964, enough examples were constructed to homologate the 904 as a GT.

Chassis 906-001 was the first six-cylinder Porsche sports racer, combining the Porsche 904 chassis and body with the new type 901/911 flat six. (Photo: Wouter Melissen)

Although it took until April for the cars to be homologated, the cars were already entered in various races. At Sebring, the Lake Underwood and Briggs Cunningham 904 finished ninth overall and first in the prototype class. This was the start of a highly successful racing career, with overall victory in the Targa Florio and many class victories in such important races as the 24 Hours of Le Mans. Underlining the car's versatility was a second-place finish in the snow covered 1965 Rallye Monte Carlo.

Throughout 1964, Porsche continued to develop the 904 and provided their customers with various new parts to keep the cars on the pace. Two of the works cars were fitted with a 2-liter version of the Formula 1 racer’s eight-cylinder engine and later in the year, the six-cylinder engine finally made its introduction.

These versions were not built in sufficient quantities to be homologated, so they raced in the prototype class only. Larger ducts for cooling the bigger rear brakes is an exterior feature of the 904/6. It was eventually replaced in 1966 by the 906 prototype racer.

Today, the 904 or Carrera GTS remains as one of the finest and most successful Porsches ever constructed. It kick-started a program of race cars that would eventually result in the all-conquering 917. It also holds a unique spot in the manufacturer's history as the last dual-purpose sports car Porsche ever built.

For a gallery of photos, see Porsche 904/6.

Read more stories like this at: The Ultimate Car Page.