The Model EL is a Harley-Davidson icon. The design incorporated many classic elements, including the low frame, valenced fenders, and teardrop gas tanks. The heart of the bike was the powerful 61ci "Knucklehead" engine. The overhead-valve, air-cooled v-twin would become a new standard. The Knucklehead was costly to develop, and Harley-Davidson considered scrapping the project. Even when the Model EL was finally released, the company hedged its bets, telling dealers not to over-hype the bike. But customers embraced it, and within two years production numbers almost doubled.