In 1967 the seeds were sown for what would become one of Porsche's rarest and most desirable models, the 911SR. At this time Porsche started development of the first 911 designed purely for motorsport, the 911R, which featured lightweight body panels and an uprated 210 bhp engine. To meet homologation requirements, the 911R spawned racing versions of the 911S and 911T, which competed in Group Three GT racing in the 1968 and 1969 seasons. These cars were similar to the 911R but had somewhat flared wheel arches to house the wider 7-inch rear wheels. By 1970 changes to the regulations allowed even wider wheels to be used and Porsche further redesigned the 911S to accommodate them. It was this model that Porsche used as the basis for a very limited run of cars that were designed specifically for GT racing and rallying and were designated the 911SR.