2004 Yamaha YFZ450

December 12, 2003
Story and Photos By Eric Putter

2004 Yamaha YFZ450

The 4-stroke revolution has done more than give motocross bikes a new sound; it has spawned a radical species of ATV. Just as the company did with the YZ400F motorcycle in 1998, Yamaha has taken the lead in 4-stroke quad technology by unleashing the YFZ450, the first truly competition-oriented 4-stroke ATV from a major manufacturer.

Before this ground-breaking machine “sport” ATVs (Click Here for an ATV History Lesson), were more akin to light-duty trailbikes such as Honda’s XR series and Suzuki’s DR-Zs than competition-only motocrossers. Until the YFZ arrived this summer, Yamaha’s hottest “sport” ATVs consisted of old-tech, bike-engined quads such as the Banshee, introduced in 1987 and stuffed full of an RZ350’s howling 2-stroke Twin, and the Raptor, powered by a 660cc Single first seen on these shores in 1996 powering the MZ streetbike line. The Tuning Fork factory wasn’t alone in the tradition of equipping ATVs with detuned motorcycle engines long past their prime. Take, for example, the Sportrax 400EX, Honda’s best-selling sport ATV of the last few years, was based on an XR400 motor. How’s that for red-hot mid-’80s technology?

One of the powersport industry’s dirtiest little secrets is that quads, not motorcycles, have led the Big Four’s sales ledgers for years, at times propping up the sagging bike economy. Of the 1.7 million powersports vehicles, ATVs, streetbikes, dirtbikes, dual-sporters and scooters, sold this year, it is forecast that four-wheelers will make up 46.5% of the major manufacturers’ combined sales total for 2003.

2004 Yamaha YFZ450

Instead of using quad-generated windfalls to produce state-of-the-art four-wheelers and market them with ATV race teams, the OEMs have been selling millions of these not-so-serious, old-tech machines to quad enthusiasts and funneling the funds to higher-profile motocross and roadrace teams, much to the chagrin of hard-core ATV racers. 

From the bottom up, the YFZ’s dry-sump motor is a WR/YZ hybrid re-engineered specifically for the demands of quad life. To make the YFZ legal for the ATVA’s Pro-class series that has a displacement limit of 440cc, the Yamaquad’s stroke was decreased by 1.4mm, making it a “439” rather than a true 450. The YFZ adopts the WR’s five-speed gearbox, instead of the YZ’s four-speed, albeit with stronger gears to handle the additional loads two fat rear tires put on it. Power is transferred to those wheels with a beefed-up clutch pack. Thankfully, the four-wheeler also features the WR’s electric starting.

Up top, the water-cooled engine features the same five-valve design as the bikes, but with a bit less compression (11.9:1 versus 12.5:1) and a lower rev limit (10,750 rpm versus 11,250 rpm). To generate a combination of screaming top-end and prodigious torque, the titanium valves (the first-ever quad so equipped) are activated by a YZ intake cam and a WR exhaust cam. Fresh air is ingested by a huge foam-type filter in a sealed airbox through to the bikes’ 39mm, throttle-position-sensor-equipped Keihin flat-slide carb, the very one used by most 4-stroke ATV motor builders for years.

2004 Yamaha YFZ450

Other cool, weight-saving features carried over from the bikes include: magnesium cylinder head cover and right-side crankcase cover; compact cam chain tensioner and oil pump; aluminum oil hose; and a super-light CDI unit with the ignition coil integrated into the spark plug cap; again, seen on bikes for years, but the first-ever on an ATV.

This all adds up to one rip-roarin’ motor. In stock form, the YFZ’s 439cc engine churns out 37 dyno-generated horsepower, positively crushing the mid-size competition and making more than Yamaha’s own 660cc Raptor. The YFZ starts with the push of a button in any gear, has a light clutch pull, carburets perfectly and revs quickly. Like the YZ and WR powerplants, the quad’s motor is a little light in the low-end and early midrange, but positively snaps to life and hauls when the revs rise, just what the racers ordered. 

This would be faint praise if the 450 didn’t have a comparable chassis to exploit its hellacious motor. Dimensionally similar to aftermarket quad frames that cost nearly $5000, the YFZ’s mild-steel backbone carries all the right stuff: removable aluminum subframe; A-arm front suspension with aluminum uppers; fully adjustable piggyback shocks all around and a rigid, cast aluminum swingarm with YZ-derived linkage. Along with these exotic materials, a great deal of thought went into engineering a light, right and compact package, which, at a paltry 350 pounds (claimed), make this the lightest 4-stroke sport ATV.

2004 Yamaha YFZ450

From the saddle, this mass figure seems right on, although we didn’t have a chance to put it on our scales. Contributing to its light, compact feel is the YFZ’s tight control layout that feels best for sub-six-footers. With 9 inches of suspension travel up front and 10 inches out back, bump absorption is somewhat stiff initially, yet the YFZ’s legs offer serious bottoming resistance. Great handling from this well-suspended quad can be traced to its short, 50.4-inch wheelbase and low center of gravity that provide excellent steering manners and good traction to the semi-low-profile Dunlop tires engineered specifically for this application.

All of this adds up to a fast, high-tech quad that even jaded motorcycle riders could come to love. Sport-quad riders will dig its class-leading performance and swoon over its spec sheet. Racers will be happy knowing that the YFZ comes closer to a full-on competition mount than anything previously available. Now, instead of investing $10,000 in mods, they can get by with half that amount to build a national-level machine.

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