The Offenhauser Racing Engine, frequently referred to as just the "Offy", was a 4.2 liter inline 4-cylinder DOHC engine, which has been used in various types of racing for over 90 years now. It was based on a 3-liter marine engine invented by Henry Miller in the 1920s, whose original design was inspired by Duesenberg and Peugeot engines that Miller serviced in his shop. A racing version of the Miller engine first won at the Indianapolis 500 in 1922, powering Jimmy Murphy's Duesenberg. Miller also built 1.5 and 2 liter versions of the engine.
Miller's company went bankrupt in 1933, and the rights, designs and parts for the engines were bought out by Miller's chief foreman and machinist, Fred Offenhauser. Offenhauser started a new company down the street, and hired Miller as chief designer. The first version of the "classic" 4.2-liter Offenhauser raced at Indianapolis in 1934, and would go on to win the race 27 times, including all 11 races counting for the World Championship.