The cam moves the power up in the powerband.
Yamaha's then-new 2006 aluminum-framed YZ450F was a fine machine, and a worthy heir to the historic YZ-F line. The return to a five-speed and power that was domesticated but not de-clawed were highlights of the machine as were the solid feel of the light alloy chassis. It missed the target in turning and steering accuracy, though, and the riding position made tall guys look like a pocket knife with the blades half folded. Yamaha calls '07 a refinement year, but we never underestimate the power of small changes. Forged triple clamps (instead of cast) are more than strong enough for the job, but with fewer ribs hidden underneath, they modify the feel of the bike in a very good way, and even help the steering feel. A shock a mere 1.5mm longer is another shot at the turning gremlins, and another hit. The bike rides higher in the rear and puts more weight on the front wheel.
We weren't as sure on the engine. Yamaha made laudable efforts to quiet the exhaust note, and it worked. The bike is pleasantly subdued sound-wise. A more aggressive cam is in place as well, and it makes the bike very lively from 1/3rd throttle on up, but the engine is much smoother below a third. It is also reluctant to pull a tall gear when there is traction. If you actually use the five-speed you will like this motor. If you want to do the whole track in one or two gears, you may lament the passing of the '06-spec engine.
Look for a more in-depth pounding of the bike in Dirt Rider, and a shootout to come soon!
You can feel what the front end is doing better than last year. | 
Bottoming isn't an issue. | 
The bike feels more nimble this year. |