After having withdrawn from Formula One at the end of the 1962 season, Porsche focused again on sportscar racing. The 904 debuted late in 1963, for the 1964 racing season, as a successor to the 718, which had been introduced in 1957. Porsche designed the GTS variant to compete in the FIA-GT class at various international racing events. The street-legal version debuted in 1964 in order to comply with group 3 appendix J homologation regulations requiring a certain number of road-going variants be sold by the factory. Porsche produced one-hundred and six 904s at four or five a day with a list price of US$7245 (FOB Stuttgart). Orders far exceeded the one hundred car requirement to satisfy homologation rules and more cars could have been sold. The 904 marked the beginning of a series of sportscars that culminated in the dominant 917.
The Porsche 904’s mid-engine layout was inherited from the 718, also known as the RSK (Rennsport = racing, kurz = short), the factory’s leading race car. It was powered by the 1,966 cc (120 cu in) Type 587/3, four-cam flat four-cylinder engine producing 180hp, “probably the most complex four-cylinder” ever. It drove a five-speed transmission with a standard 4.428:1 final drive, with available 4.605, 4.260, 3.636, and 3.362 ratios.
Begun as the Type 547, its development began in 1953, when the previous VW-based 1,100 cc (67 cu in) flat-four, used in the contemporary 356 and rated at 38 hp (28 kW), hit the limit of its potential. Porsche realized it needed something all-new. The brainchild of Ernst Fuhrmann, later technical director, it was hoped to achieve an “unheard of” 70 hp (52 kW) per 1 l (61 cu in), relying on hemispherical combustion chambers (which would be called hemis in the U.S.) and 46 mm (1.8 in)-throat 46IDA2 two-choke Weber carburetors to generate 112 hp (84 kW) from the 1,500 cc (92 cu in) four-cam engine. The 1.5 liter weighed 310 lb (140 kg) dry, eventually producing 180 hp (134 kW). A complex design that proved “very taxing” to build and assemble, but very durable, it was used in 34 different models, including 550 spyders, 356 Carreras, and F2/1s.
The 904 was the first Porsche to use a ladder chassis and fibreglass body, appearing more like a specialist racing car than the modified sports cars typical at the time, and was painted white. The fibreglass body was bonded to its steel chassis for extra rigidity, and achieved a drag coefficient of 0.34. While many German race cars had used unpainted aluminium bodies since the famous 1934 Silver Arrows, most 904s were painted silver, the modern German national racing color. Unusually for Porsche, the two-seater bodies were provided by contractors, which would later become standard practice among race car builders. The 904’s fibreglass body was made by spraying chopped fibreglass into a mold, the amount sprayed often varied in thickness over the shape of the car and as a result the weight of the various cars was somewhat inconsistent. Race-prepared four-cylinder 904s weighed in at approximately 1,443 pounds (655 kg) and the low weight gave the 904 the ability to accelerate to 60 mph (97 km/h) from a standstill in less than six seconds (using the standard rear gear, which was typical at Sebring) and to reach a top speed of 160 mph (260 km/h) (with the 3.362 ratio). Frontal area was only 14 sq ft (1.3 m2).
The Porsche 904 rode on coil springs (the first Porsche not to use trailing arm front and swing axle rear suspension), with unequal-length A-arms in front. The wheelbase was 90.5 in (2,300 mm) (by contrast, the Corvair’s was 108 in (2,700 mm)), track front and rear 51.7 in (1,310 mm), height 42 in (1,100 mm), and ground clearance of 4.7 in (120 mm) on 15 in (380 mm) wheels. Brakes were 275 mm (10.8 in) discs at the front and 285 mm (11.2 in) at the rear.