Stock for stock there is'nt too much diff on a race, but yes the z is a little faster. But it all depends on where the race takes place and who is riding it. Example- on the street my 350x "250x quad" with turf tamers smokes my buddies 400ex "stock" but on the dirt he smokes me. He weighs 240 I weigh 140 See what I mean! There are too many factors that play in hand. RUSS