The 2014 Yamaha YZ450F doesn't particularly stand out from the rest of the pack at first glance. However, a closer look reveals a unique engine architecture where the rearward-slanting cylinder is "reversed" compared to traditional designs. The revised throttle body and airbox are front mounted to provide a straight intake path, while the exhaust exits from the rear of the cylinder. The result is optimum mass centralization for excellent handling and a light feeling.

While 2013 was an uneventful year for Yamaha's big dirt bike, plenty of changes are in store for 2014, starting with refreshed styling and improved ergonomics (notice how the new seat stretches all the way to the handlebars for easy rider movements, and features a "cap cover" to access the fuel filler). The engine may be the same, but the header pipe now exits from the rear of the engine and then winds its way around the cylinder body before turning to the rear of the bike (instead of circling around the rear shock and overheating it).


Raging, responsive engine
The various tweaks to the engine were designed to increase throttle response at medium revs. The intake valves are 37mm in diameter (up from 36mm) while the exhaust valves are 30.5mm (up from 30mm), the revised throttle body gets a 12-hole Denso injector, and the higher-capacity airbox is 3,230cc in size (up from 1,210cc) for improved performance and easier maintenance. Moreover, the optional "YZ Power Tuner" allows tailoring of both fuel injection settings and ignition mapping.

The new 5-speed transmission's first and third gear ratios have been modified (the former is longer and the latter is shorter), so it's easier to find the right gear when exiting a corner. Meanwhile, the heavy-duty wet clutch features optimized friction plates and new, stronger clutch springs to match the new engine output.

Consequently, the 2014 Yamaha YZ450F maintains its fury at higher revs and proves a lot more responsive from 4,000-7,000 rpm. While the throttle is still pretty hard to modulate at low speeds, you don't have to shift as much to enjoy spirited acceleration. The 450cc single-cylinder delivers a broader powerband and is a bit easier to start now, although my 70kg were barely enough to kick-start this big machine. Once up to speed, the reduced noise (courtesy of a better insulated airbox) is sweeter for the ears.

The various tweaks to the engine were designed to increase throttle response at medium revs. (Photo: France Ouellet)

Improved handling and damping
The "bilateral beam" aluminum frame benefits from a certain number of improvements that help reduce weight and achieve a more compact design. The Kayaba fork features larger-diameter tubes and a twin-chamber "Speed Sensitive System" design meaning the damper unit is in a separate structure which prevents aerated fork oil from entering the inner damper assembly and reducing its damping abilities. In the back, the new shock's remote gas reservoir has been relocated to accommodate the new fuel tank location (under the seat) and capacity (up from 6.5L to 7.5L).

The 2014 Yamaha YZ450F offers an aggressive riding position with intuitive ergonomics. The new frame makes it more agile while still inspiring confidence on the run-in to a jump. The rear shock works more effectively and shows way more endurance than the old one, which had a tendency to overheat due to the nearby header pipe. The stiff fork behaves commendably when hitting big mudholes and casing a jump, but has a tougher time dealing with smaller obstacles and trail imperfections (the impacts can be felt directly through your arms). Slightly softer springs could make a huge difference, here.

The "bilateral beam" aluminum frame benefits from a certain number of improvements that help reduce weight and achieve a more compact design. (Photo: France Ouellet)

Bottom line
My first experience with the reverse-engine Yamaha YZ450F, back in 2012, wasn't totally positive. However, with the latest upgrades, the machine has significantly matured in terms of responsiveness and versatility. The new YZ450F will happily take on its direct competitors in national motocross championships across the globe, even in stock form.

Pros

• Agile and stable
• New ergonomics and greater mass centralization
• More responsive engine with a broader powerband

Cons

• Front fork is a bit too stiff
• Kick starter demands a lot of strength