Honda's Unicam single-overhead-camshaft (SOHC) four-valve cylinder head first appeared on the 2002 CRF450R. The benefits of the Unicam design are a powerful, yet shorter engine. The liquid-cooled head incorporates a carburetor fed single camshaft that directly actuates two intake valves (the CRF250/450X series utilize titanium intake valves). In the CRF250R/CRF450X models, the camshaft's single exhaust lobe actuates two steel exhaust valves via a forked, low-friction roller rocker arm. The Unicam system in the CRF450R/150R is slightly different, with the two exhaust valves operating through two separate low-friction roller rocker arms. The SOHC configuration contributes to a compact design that saves weight over a comparable dual-overhead-camshaft (DOHC) motor and also permits a narrow included valve angle. This, in turn, flattens the combustion chamber to facilitate free flame propagation, allowing a high compression ratio. A roller bearing on the rocker arms reduces friction and therefore wear, allowing the cam lobes to be narrower-and lighter-than conventional designs. Since less space is taken up in the cylinder head, the camshaft sits lower in the head for a more compact engine and a lower center of gravity. This design also offers the advantage of a narrower included valve angle than is possible in a DOHC configuration. Up to the development of the Unicam, conventional wisdom held that rocker-arm systems were unsuited for engine speeds above 10,000 rpm. But the CRF250R achieved engine speeds greater than 13,000 rpm because its rocker arms were designed to be both lightweight and rugged.