Ask any Dirt Rider tester to describe the YZ450F’s engine in one sentence and they’d probably tell you that, “the YZ450F’s engine does everything that a good 450cc powerplant should.” That is to say that it lugs well, it is tuned cleanly, it accelerates strongly, and it has great over-rev. Fortunately, the YZ450FX retained all of these characteristics, but with a slant toward being slightly more manageable and usable for off-road. Twisting the throttle, no matter how fast, fails to reveal a single hesitation or bad patch. Likewise, traction is great even when the dirt is not, thanks to what is a strong but not overly violent delivery profile. One thing to remember when thinking about the Yamaha’s engine is to consider whom this bike was built for. As mentioned, this is a competitive, GNCC-style race bike, and the average trail rider may be surprised by the amount of raw performance contained in the motor. Truth be told, there’s not a lot to be unhappy about motor-wise, but if Yamaha’s engineers put a t-shirt over our heads and threatened to water board us if we didn’t say something negative, we’d probably tell them that they should have made the flywheel even heavier to help combat flame-outs in situations where the bike comes to an abrupt stop (such as lurching onto a rock or slow-crawling over a log). For EnduroCross racing, we’d probably want to run a fully automatic Rekluse clutch to help prevent this low-speed stalling.