PORSCHE TYPE 956 Porsche’s dominance of international prototype sports car racing in the early 1970s faced new challenges as other manufacturers upped their game.

The incredible five-liter, 12-cylinder normally aspirated 917s had been regulated out of existence, replaced by the open-topped 936 that utilized a 2.65-liter turbocharged flat-six, originally created for Porsche’s short-lived Indianapolis project.

This engine, designated the 935/76, gave Porsche a third consecutive victory at Le Mans in 1981.

Then, at the end of the 1981 racing season, the rules changed.

The FIA replaced Groups 1 through 7 with new categories: A, B, and C, with Prototypes falling into the latter.

Although rules for these new Prototypes were loose, there were limits on their size and fuel consumption.

The new regulations also demanded that the forward-most section of their undersides be flat, as to limit their ground-effects capability.

Within two weeks of the announcement, Porsche began design work on...