GP Bike. Flat six Honda Valkyrie flat-6 Main article: Flat-6 The six-cylinder engine is currently used by Honda in the boxer engine of the Rune, Valkyrie and Gold Wing . V-8 Moto Guzzi V8 Main article: V-8 engine Galbusera built a V8 in 1938, and Moto Guzzi experimented over a period