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Message: [QUOTE="Rich Rohrich, post: 1308536, member: 16241"] Maybe this will help. As strange as it seems air has mass and it takes some time to accelerate it. Once you accelerate the air to it's maximum speed it will try and stay moving (body in motion and all that foof). So intake valve closing is a matter of figuring out the point when the air will have slowed down enough to reverse. For a given intake valve close (IVC) timing this closing point will only be correct for a band of about 1000-2000 rpm, everywhere else it will be closing to late or too early for the rpm and the port velocity. Higher velocity ports tend to be less sensitive to the IVC being "out of time" for the rpm, but the trade off is they tend to flow less air. When the ports get bigger they get lazier and it takes more time to get the air up to speed so reversals at lower rpm are more common. So big ports will tend to move more total air and show bigger numbers on a flow bench and possibly the dyno, they also tend to be more sensitive to getting the IVC perfect and only get it right in a much more narrow rpm range. They also show dips in the torque curve in places where the cam timing (and by extension the intake and exhaust lengths) are "out of time" In other words they get peaky. MotoGP engineers have been battling this phenomena since they switched to four-strokes. It's become more complicated since they dropped the displacement limit to 800cc. See four-strokes are more interesting than you guys realized. ;) [/QUOTE]

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