An earlier paper by Blair and Cahoon (SAE 710082) described the initial design and development of a 60 hp engine of the single-cylinder 2-cycle type for road racing motorcycle applications. This paper continues with the further design of this engine as a 65 hp 5-speed transmission unit and a variation of it as a 40 hp 4-speed transmission engine for moto-cross racing usage. A discussion of the application of the computer to the design of both engines is included. The computer design is used for the prediction of the required torque/speed characteristics for both engines for their particular application in order to eliminate lengthy test bed development. The mechanical design criteria for the engine/transmission units are also discussed and their implementation for the road race or moto-cross application described.