GPSpacewalker, CR500 Honda 2 stroke rated horsepower = 55. CR450F 4 stroke rated horsepower = 53. Both motorcycles are from same manufacturer and are intended for Motocross racing. They would compete in different classes though. The 450 in the 250 class and the 500 in the open class. So in many ways we are already at the break even point in displacement. Also the YS 160FZ claims to have the most horsepower of any engine in it's class, 2 stroke or 4 stroke. I'm assumeing that would mean the OS160FX which seems to have been the class leader. I think we are really starting to see a trend to 4 strokes.
Thanks,
Barry