The 2002 debut of the Honda CRF450R heralded a sea change in the motocross world, as top pro competitors and then amateur racers transitioned from two-stroke single-cylinder race bikes to four-stokes. In terms of engine design, in order to extract maximum, race-ready performance from a four-stoke single, a liquid-cooled powerplant with a four-valve head was a foregone conclusion. Why so? With four valves instead of only two, engine breathing increases significantly, opening the way to more power and higher rev limits.
But how to actuate those valves? A dual-overhead-camshaft (DOHC) head design is typically chosen when an engine features four valves per cylinder, regardless of the number of cylinders. In such a design, two separate camshafts directly actuate the valves; one camshaft for the intake side and the other camshaft for the exhaust side. It’s a good design, one that can be found on the high-performance Honda CBR1000RR and CBR600RR four-cylinder sport bikes, as well as many others.
However, in a four-stroke single-cylinder machine designed for motocross or off-road racing, using a DOHC configuration to actuate the four valves adds bulk and weight—both of which hamper overall performance in a dirt bike. Could there be a better way to accomplish this task, one that would save weight and also create a more compact engine? And so it was that in 2002 the world was introduced to Honda’s unique single-camshaft Unicam® head design on the CRF450R.
A decade later, we can see that all subsequent competition-oriented four-stroke Honda CRF-R models of varying engine displacements and racing applications have likewise incorporated this Unicam single-overhead-camshaft (SOHC) four-valve cylinder head configuration. And while some of the specific details between the CRF450R, CRF250R and CRF150R motocrossers and the CRF450X and CRF250X off-road machines vary slightly, all of these machines ended up employing Unicam engines.
With Honda’s Unicam design, a single camshaft directly actuates the two intake valves, while the two exhaust valves operate through low-friction roller rocker arms. A roller bearing on the rocker arms reduces friction and therefore wear, which allows the cam lobes to be narrower, smaller and lighter than conventional rocker arms. The CRF250R, CRF250X and CRF450X use one forked roller rocker arm to actuate the two exhaust valves, while the CRF450R and CRF150R use two separate roller rocker arms, but operation is otherwise virtually identical. The key benefits gained from the Unicam design are excellent flow and engine breathing for increased power, but with a shorter, lighter and more compact engine over a comparable DOHC engine—attributes that aid handling.
All of these Unicam heads permit a narrow included valve angle, a design that flattens the combustion chamber to facilitate flame propagation for a good, efficient burn for more power, while also allowing a high compression ratio. Since the SOHC configuration takes ups less space in the cylinder head, the camshaft can be situated lower in the head; this produces a more compact engine design and lowers the center of gravity. Obviously, the SOHC design also obviates the weight and bulk required by a second camshaft.
A few other detail differences include lightweight and tough titanium intake valves for the higher-revving CRF250R and CRF450R; the other models feature steel intake valves. Also, the CRF250R and CRF450R feature Honda’s PGM-FI Programmed Fuel Injection systems, whereas the CRF250X, CRF450X and CRF150R use carburetors to mix air and fuel.
But regardless of these various detail differences, all of these machines have one overriding advantage in common: thanks to Honda’s Unicam design, all of these bike feature remarkably compact, light and powerful engines—and that makes every one a winner, even if you never take yours racing.