The engine is the 547, aka Carrera four cam, designed in 1952-53 by Fuhrmann for the 550 Spyder. It is not a Hirt engine and was never referred to as such by Porsche. The Carrera engine is an all Porsche design with a Hirth crankshaft. Hirth Motoren GmbH was formed in 1927 in a brake up from what became the Mahle company. (Mahle made pistons and cylinders for Porsche.) Hirth aircraft engines was successful with the HM 60 four cylinder engine in the Klemm aircraft, used as a trainer during the thirties by Luftwaffe and a few other air forces in Europe. After the war aviation was banned in Germany and Hirth turned to building smaller industrial and snowmobile engines. The Carrera crank was basically a copy of the design from the 1930 four cylinder aircraft engine, with dramatically shortened stroke. The reason Porsche wanted the rollerbearing Hirth crank was its very low friction. The fourcam engines would in racing rotate at 8-9000 RPM and at that high engine speed the friction of a plain bearing crank could result in a power loss of 5 to 10 % and the oil reaching critical temperature. The Carrera engines were very successful in racing, but poor maintenance destroyed the sensitive rollerbearings. Porsche did not have the tools to rebuild these cranks themselves and sent them back to Hirth. According to rumor the Hirt people got tired of crazy Porsche race drivers hounding them and by 1959 they stopped making the crank. The truth is probably that they didn't make enough money on the small numbers of cranks needed for the carrera production. The 356B and C model Carreras had all plain bearing cranks of Porsches own design and needed separate oil coolers to keep the oil temp at bay.
