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[AzCivic]

and some more, this time from yawpower.com:

Velocity refers to the speed at which the exhaust gasses are travelling. The exact speed is not important to this discussion, but an uderstanding of how velocity affects exhaust flow is. There are two ways that velocity can be increased. One, by decreasing the cross sectional area of the orifice that the gasses are flowing through. (Making the headers or exhaust ports smaller) Two, by increasing the volume of air that is flowing through the orifice. (Increasing engine rpm) Velocity will increase proportionally with an increase in rpm. In other words, if you double the rpm, the velocity will also double. Velocity is inversely proportional to an increase in cross sectional area. Doubling the cross sectional are will halve the velocity, and halving the cross sectional area will double the velocity.

Velocity is important for one simple reason. Inertia. Websters dictionary describes inertia as "The property of matter by which it retains its state of rest or velocity so long as it is not acted upon by an external force." In other words, once it is moving, it will continue to move until some external force stops it. If you apply this theory to the gasses in the exhaust system you can see that once they have been accelerated by the pressure in the combustion chamber, It will take a given amount of energy to stop them, and even more to cause them to reverse direction. Since energy equals mass times velocity squared, you can see that doubling the velocity of the gasses will quadruple the amount of energy required to stop them. This is important because the flow of exhaust gasses is not steady. During each exhaust cycle, the gasses are accelerated, and decellerated rapidly. Often in the forward and reverse direction.