As far as the velocity, the 640 fps (or 690) is the actual velocity of the air or air/fuel mixture in the actual running engine when 'choke' occurs. This is the point where further increases in velocity do not bring a corresponding increase in power. In other words, at speeds above 690 fps the engine looses more power from pumping losses than is gains from extra intake flow. This 690 fps would require a much higher differential pressure through the intake tract than the 28" that the flow bench provides. So, at 28" of differential (on the flow bench) 350 fps average velocity is the maximum possible. In a running engine much higher velocities will occur because the pressure differential can be much higher.