History
The International Race of Champions was conceived in 1973 as a way of determining who is the world's best race car driver. The main movers were Les Richter, former NFL lineman who had become chief executive of Riverside International Raceway in California, and car builder Roger Penske.
The idea was, and is, rather simple: Pick a dozen top-ranked drivers from various kinds of racing, give them the same equipment to drive, and have them run a series of four races.
For the first IROC series, in 1973-74, three drivers were selected from each of four venues: USAC, NASCAR, SCCA, and Formula 1. Driving identical Porsche Carrera RSR cars, they competed in three races on Riverside's road course in October of 1973. The top six then moved to the final, at the Daytona road course in February of 1974. Mark Donahue, representing SCCA, won three of the races and the series.
The series was temporarily discontinued after 1980, but it was revived in 1984 with several new sponsors. Although the courses and cars used and the exact format of the races has changed somewhat through the years, the idea is still basically the same: Twelve drivers using the same equipment in a series of four races.
NASCAR drivers have dominated IROC pretty thoroughly, winning 18 of the 24 series.