Abstract: The present application discloses an engine configured to selectively operate using a first fuel and a second fuel and adapter devices and methods for converting an engine to operate using a second fuel. In certain embodiments, the adapter device comprises a body member configured to be installed between an upper intake manifold and a lower intake manifold of an engine. A plurality of intake channels and fuel injector ports are formed in the body member. The body member is configured such that the intake channels are substantially aligned with the corresponding intake openings of the upper and lower intake manifolds when the body member is installed between the upper and lower intake manifolds. Each fuel injector port extends from a sidewall of the body member to at least one intake channel and is configured to receive a fuel injector for emitting a second fuel into the intake channel.