Book excerpt: The piston engines that powered Second World War fighters, the men who designed them, and the secret intelligence work carried out by both Britain and Germany would determine the outcome of the first global air war. Advanced jet engines may have been in development but every militarily significant air battle was fought by piston-engined fighters. Whoever designed the most powerful piston engines would win air superiority and with it the ability to dictate the course of the war as a whole. This is the never before told story of a high-tech race, hidden behind the closed doors of design offices and intelligence agencies, to create the war's best fighter engine. Using the fruits of extensive research in archives around the world together with the previously unpublished memoirs of fighter engine designers, author Calum E. Douglas tells the story of a desperate contest between the world's best engineers - the Secret Horsepower Race.