Difference between revisions of "Honda Unicam"

[[Honda]]'s Unicam [[single-overhead-camshaft]] (SOHC) four-valve cylinder head first appeared on the [[Honda CRF450R|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 [[Honda CRF250R|CRF250R]]/[[Honda CRF450X|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 [[Honda CRF450R|CRF450R]]/[[Honda CRF150R|150R]] is slightly different, with the two exhaust valves operating through two separate low-[[friction]] roller [[rocker arm]]s. 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 arm]]s 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 arm]]s were designed to be both lightweight and rugged.