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The Porsche 904 Story & History
Porsche’s Type 904, officially called the Carrera GTS because Porsche and Peugeot were in dispute over numeric designations with “0” in them, succeeded the RSK Type 718 as the last sports-racing iteration of the 356 series. Developed after Porsche left Formula One in 1962, the 904 (as it soon became popularly known) was also the last full-competition Porsche that could be readily driven on the street. This is the full story.
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.
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.
PORSCHE 904 CARRERA GTS
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. 
Porsche 904 Bergspyder
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.