From Yamaha Wiki
The YZ450F is a Four stroke big bore racing motocross bike built by Yamaha. It is considered by most to be the starter of the four stroke dirt bike revolution. It is a replacement for the YZ250, which is slowly being phased out.
The WR450F is the Enduro version of the YZ450F.
For many years, the motocross world almost exclusively used two stroke engines. American Motorcyclist Association racing classes had two classes: 125 cc and 250 cc two strokes, with no provision for four strokes. Most riders considered four stroke engine technology to be antiquated and uncompetitive.  Additionally, most four strokes produced very little power (the 1996 Honda XR400 made 32 bhp (24 kW), compared to the 40 bhp (30 kW) produced by most 250 two strokes of the time).
In 1996, the AMA changed racing rules to allow 450 cc four strokes to compete in the same racing class as 250 cc two strokes.  Yamaha engineer Yoshiharu Nakayama first came up with the idea of creating the first competitive four stroke race motocross bike.  The Yamaha YZ400F was developed to fit into this category. It solved the power dilemma by borrowing superbike technology and giving the YZ a five valve head, liquid cooling and a 12.5-1 compression ratio.
In 1997, Yamaha rocked the motocross world with the introduction of the YZ400M prototype, a concept motorcycle which borrowed much technology from road racing. The YZM was far ahead of all competition among four stroke motocross bikes. Doug Henry piloted the YZ400M to its first victory in 1997 at the Las Vegas Supercross. This was the first time any four stroke had won an AMA event.  The YZ400M was the predecessor of the production YZ400F, which was released the next year.
|Manufacturer||Yamaha Motor Corp.|
|Engine||400 cc single-cylinder, water-cooled, four-stroke, DOHC five valve|
|Weight||250 lb (110 kg)|
First Generation: YZ400F - 1998-2000
Yamaha Motor Corporation introduced the YZ400F in 1998. It was "the first modern production four-stroke motocrosser that was directly competitive against two-strokes."  Initially, Yamaha targeted a dry weight of 233 pounds (106 kg) (on par with the 250 two strokes of the time), but by production, the bike weighed 250 pounds. The bike had an 11,600 rpm redline  and more power and torque than its 250 cc two stroke rivals.  It benefited from engine compression braking, which allowed the engine to slow the bike down during deceleration, giving the brakes a rest. 
Though the YZ 400F had a wider powerband than its two stroke counterparts, the bike had some problems. It would stall far more easily than two strokes, and had a very difficult time starting. 
In 1998, Doug Henry won the AMA National Motocross Championship aboard the YZF, becoming the first rider to win a championship on a four stroke motorcycle. This victory is considered by some to be the major turing point in the motocross world--for the first time, four strokes were considered a competitive racing machine. 
|Manufacturer||Yamaha motor co.|
|Engine||426 cc single-cylinder, water-cooled, four-stroke, DOHC titanium five valve|
|Fuel capacity||2.1 gallons|
Second Generation: YZ426F - 2000-2002
In 2000, Yamaha updated the YZ400F, increasing the displacement to 426 cc for greater power and throttle response. In addition, the carburetor and jets were updated to ease the YZ400F's starting woes.
The next year, in 2001, Yamaha replaced the previous steel valves with titanium ones. With the valves now more than forty percent lighter than last year's valves, the new valve springs can be designed lighter and softer, allowing a quicker revving engine, improved throttle response, higher rev ceilings, and more power.  The crankshaft has also been reshaped and the whole assembly has been redesigned for quicker throttle response and, Yamaha claims, "less high-end horsepower loss." In addition to motor modifications, a few changes were made to the transmission to help contain the power and ensure longevity. The suspension has also received a bit of an overhaul with the goal being reduced weight and smoother action throughout the stroke.  Yamaha also designed a new exaust pipe design so that the exhaust header does not have to be removed to replace the oil filter like how it had to be done on the previous model. Also the carburetor was tuned differently to fix the difficulty when starting and the off idle take off.
Also in 2001 the subframe was changed from a steel to a blue painted aluminum style. In 2002 the blue painting was stopped and left to a bare aluminum look.  Motorcycle.com says that "The gas tank is reasonably thin and allows good foreward and backward movement while providing something nicely shaped to hold onto with your knees. In fact, the entire ergonomic package on this YZF is well thought out. The handlebars have a nice bend to them and are well-placed for good rider control and legroom. The footpegs are well-made units with a wide platform and sit in a position that keeps them from dragging in ruts without cramping a rider's legs."
In 2002 Yamaha remapped the digital CD ignition system which delivers a more precise spark and optimal timing for faster, stronger response during hard acceleration, and less kickback during starting. Also an all new swingarm which is lighter and stronger for reduced weight, greater rigidity and more compliant rear suspension action. While an anodized finish gives the bike a tricked out look. The 426 also includes a larger pivot shift for increased durability. A larger rear brake disc was also added which means greater stopping power.
|Engine||449 cc single-cylinder, water-cooled, four-stroke, DOHC titanium five valve|
Third Generation: YZ450F - 2003-2005
For 2003, the YZF received the biggest update in its history. First, the engine displacement was increased to 449 cc, in compliance to the new AMA Motocross maximum displacement rule. Next, the frame and plastic were all updated for a new, sleeker look. Additionally, the YZ's weight was engineered from its original 250 pounds down to 233 pounds. The new YZF also had a gearing change, going from a traditional 5-speed to a 4-speed transmission. The new YZ was much easier to start as well.  The bike made tremendous power; however, many thought that the bike had too much power for a motocross track. 
Forth Generation: Yamaha YZ450F 2006-present
The YZ450F received a major update in 2006, with over 300 parts changed and improved. The power was smoothed out, and the bike was made easier to ride, putting to rest the complaint that the YZ had too much power.  The four speed transmission was replaced with a five speed. An all new aluminum single backbone frame shaved another 10 pounds off the weight.
- ↑ 1.0 1.1 2001 Yamaha YZ426F: MD Ride Review
- ↑ MX 2006 Rulebook Final.pdf
- ↑ YZ426F: Miracle Machine? – Motocross News, Race Reports, Videos, Photos, Interviews, How-Tos
- ↑ 4.0 4.1 Motorcycle Hall of Fame: Doug Henry
- ↑ 5.0 5.1 5.2 5.3 Motorcycle News Extreme Motorsports Sportbikes Powersports Girls Babes Racing Apparel - Four Stroke Motocross Shoot out!
- ↑ GT Thunder Test Bench
- ↑ http://www.motorcycle.com/manufacturer/yamaha/year-2001-yamaha-yz426f-14620.html
- ↑ Year 2001 Yamaha YZ426F
- ↑ http://dirtbike.off-road.com/dirtbike/article/articleDetail.jsp?id=266204
- ↑ 2003 Yamaha YZ450F: MD First Ride
- ↑ First Impression: 2004 Yamaha YZ450F – Motocross News, Race Reports, Videos, Photos, Interviews, How-Tos
- ↑ 2006 Yamaha YZ450F - First Ride - MotorcycleUSA.com