I have a sumo 09 yz450f with athena big bore + aftermarket exhaust. This bike works really well on low and mid end but miss a lot of power on top. On quick straight track all the others 450 toast me. The difference is huge. I compared with a friend 06 yz450f also with athena big bore and the same exhaust as me + bored carb, ported head and hotcam intake. This bike works really well. It works as mine on low and mid end but has a lot of power on top end. I was looking for yz camshaft specs and I found this topic http://www.thumpertalk.com/topic/936760-cam-differences-between-wr-and-yz/ Based on these values and on webcam explanations, it seems that yz 08/09 should have more low and mid and compared to 06. But Gray always says that 08/09 yz450f have 2 horsepower more than 06 yz on top. What is the thruth? What could explained this difference between my bike and that of my friend? I think that dored carb, cam spec and ported head all together explained this, but what is the most important factor? Sorry for my English. I hope that you undestood my question.