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What controls velocity?

Piston speed and port/runner shape. In essence, the geometric profile (tube) in which the air travels to the base of the valve head. This geometric shape starts at the top of the intake manifold (carb plate) and some may include the shape of the venturi's w/in the carb/throttle body.

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This is the response I see most often on the internet.

How about valve events and ramp rates?

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Valve timing events are critical. We can get really in depth with this if you like......but for now we'll just keep it basic. As the piston lowers in the cylinder, it "pulls" on the intake port as the valve is opening (the scavenging effect). Just before the piston reaches BDC, the intake valve should close (tightly) thus getting the max cylinder volume of charge (air and fuel). It is very possible to actually achieve more than one atmosphere during this operation w/o forced induction. Then the piston goes up and down, and on the following up stroke it "pushes" the fired charge out the exhaust port.....But, this is where it gets good!!!! The velocity in which it travels out the exhaust while the intake begins to open (split overlap) can actually create a negative charge on the intake chamber thus pulling in the new fuel charge at a greater velocity that standard flow (ie, form of forced inducing). This fills the cylinder as the piston drops and......we're at the beginning. But, possibly more than one atmosphere of volume inn the cylinder = VE approaching and exceeding 100%.

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