For the underpinnings of the new 904 Bergspyder, the Porsche engineers recycled five chassis originally laid down for a production version of the six-cylinder 904/6 Coupes. The steel platform chassis of the 904 was reinforced with cross-braces to compensate for the rigidity that had originally been provided by the coupe body. The Bergspyders were tried with both the exotic twin-cam eight-cylinder engine and a highly tuned flat six.
Three factory race cars were fitted with a flat eight-cylinder power plant derived from the 1962 804 F1 car, the 225 hp (168 kW) 1,962 cc (119.7 cu in) Type 771, which used 42 mm (1.7 in)-throat downdraft Weber carburetors. The Type 771s, however, suffered a "disturbing habit" of making their flywheels explode. The 904/8 cars had a short and relatively unlucky racing career.
In 1965, the 904’s second and final production year, some examples received a version of the 911’s 2.0-liter flat-six. This version was dubbed the 904/6 and was focused on the factory works effort by Porsche. Six of these cars were so equipped and used a chassis number of 906-0xx. Porsche built a total of six similar 904/6 Works team cars with the following chassis number assignments: 906-001, 002, 005, 006, 011, and 012.
The Porsche 904 debuted late in 1963, for the 1964 racing season. Porsche designed the 1965 Porsche 904 Carrera GTS variant to compete in the FIA-GT class at various international racing events and a street-legal version debuted in 1964 in order to comply with FIA’s Group 3 homologation regulations. When the 904 Carrera GTS debuted, it represented Porsche’s first foray into fiberglass bodywork and the last hurrah for its four-cam, four-cylinder engine.