2015 YAMAHA YZ 250FX, Naturally, the FX’s 249cc DOHC four-stroke Single engine architecture mirrors that of its sisters. That includes Yamaha’s reversed cylinder head, rearward inclined cylinder and rearward exiting exhaust. Yamaha engineers first introduced this design on its YZ450F back in 2010, and used it to win an AMA 250cc National Motocross Championship with factory rider Jeremy Martin in 2014. Its main advantages include a straighter intake shot to the combustion chamber and symmetrical intake and exhaust ports for more consistent cylinder filling.Mass centralization is also key to the Yamaha’s design, which places the cylinder in a 6.2-degree rearward incline to place the top-end mass and camshaft rotational inertia toward the center of the chassis. Offsetting the cylinder 3.5mm forward of the crankshaft centerline also frees up horsepower by reducing drag; the connecting rod is more vertical at the moment of greatest combustion force, which reduces piston-to-cylinder wall side thrust
