rod angles, head able to flow enough air they are just two of the reasons, i expect daz to Clarify and add some more reasons.. dont let this put you off a zvh because they can be made to rev but it costs, so in the long run you are better off going full zetec my last zvh showed peak power @ 5960 rpm but there were a few problems which i have Rectified for my new zvh, once i get my twin plate paddle fitted, i will get her on the rollers..