By reversing the cylinder on the 2010 Yamaha YZ450F, inertial mass is centralized and the intake track is straightened. That means more power and better handling. Additionally, the cylinder is offset so the connecting rod is vertical when the force of combustion is at its greatest, minimizing piston/cylinder friction. That means better response. The smaller, 1.6-gallon tank is now located under the seat, for a lower center of gravity. Like we told you in June when the technical diagram leaked, the YZ450F has received its most thorough redesign since Yamaha first went four-stroke in 1998.
Other upgrades include the curlycue "Tornado-style" exhaust header
which is claimed to boost low- and mid-range torque and power; a
4-valve head with titanium valves replaces the 5-valve design; a
battery-less 14-volt/110w generator powers a 12-hole fuel injector and
44mm Keihin throttle body, all of which combines to constantly adjust
the air/fuel ratio to suit temperature and altitude; and stroke is
shortened to 60.8mm while the bore is expanded to 97.0mm.
Additionally, the reversed cylinder means the airbox is moved up front,
which keeps it cleaner and creates more room for the shock, which now
sits along the frame's central axis.
Full details can be found in the following document: