SMOOTHER, SLIMMER, BETTER
The YZ450F was our 2017 Motocross Shootout winner (February/ March issue), so when Yamaha came out with an all-new bike for 2018 we were excited to see if it raised the bar for the class even higher. A powerhouse since 2010 when the Yamaha engineers reversed the engine and created a straighter, downdraft intake, the new YZ450F gets a slightly straighter (more vertical) intake tract and higher-lift cams. The intake valves now open earlier, open farther, and stay open longer, with the aim of increasing midrange and giving a torquier character. Overlap, when both valves are open, has been increased in an effort to improve top-end and over-rev. The throttle body is now a Mikuni unit (a switch from Keihin), with a push-on cold-start knob that releases with a twist of the throttle. The piston is now a bridge box style, lighter by 2.3 percent and with a compression bump (12.8:1 versus 12.5:1), and the new exhaust header is 12mm longer and narrower at the junction (38mm diameter versus 41mm). There’s even a new spark plug with a longer electrode for improved burn efficiency and a cap holder on the valve cover. According to Yamaha, the changes mean an overall horsepower boost and claimed 3 more ponies at both 6,000 and 9,000 rpm.
The clutch pressure plate is redesigned with more rigidity in its outer area and less at its center, the clutch’s steel plates are precision ground on both sides, and the springs are designed to stay straighter during compression for smoother engagement at high rpm (such as launching off the starting gate. Yes, the bike still has the Launch Control System). Also second, third, and fourth gear cogs are 1mm wider (the ratios are unchanged) for durability with the stronger engine output.
Oh, and there’s that magic button on the handlebar. The engine is electric-start only (it will start in gear with the clutch out), with a starter that is more compact than the one on the FX line, with a claimed 70 percent more cranking power and drawing 20 percent less electricity. As a bonus you might not normally think about, because the bike now has a battery, Yamaha was able to decrease the ignition’s electrical output and therefore decrease the drag the system puts on the engine, which Yamaha reps claim improves low-rpm power and response.
An all-new frame has main spars that are now straight, and the frame is narrower at the rider’s ankles. The engine is held 2 degrees more vertical, and the engine hangers now attach at the rear of the head and are aluminum rather than steel. The steering stem is 6mm forward, the triple clamp offset changes from 25mm to 22mm, and the handlebar mounts put the handlebar 5mm higher.