Even when the exhaust valve is at relatively low lift, the exhaust gasses can exit the exhaust valve's seat area at super sonic speeds, during this phase the exhaust gasses responds more to the opening area of the exhaust valve rather than the shape of the exhaust port.
As the exhaust valve’s lift increases, the exhaust gas velocity drops to subsonic, and now the shape of the exhaust port becomes the overall factor towards creating high flow. It is also worth noting that for a given size, an exhaust port flows better than an intake port. This is due to the fact that as the exhaust passes from the cylinder into the exhaust port, the flow becomes more organized, which is just the opposite of what takes place at the intake valve.
Another condition that helps the exhaust gasses reach a higher flow efficiency is due to the exhaust valve is typically lifted higher in proportion to its diameter than the intake valve, thus creating the situation that the valve head spends more time out of the influence of the exhaust valve seat.
If an exhaust port has a steep up draft angle and a large short side turn, then the exhaust port begins to resemble a venture like nozzle and a pressure recovery will occur after the gases have passed through the port's minor diameter. If an exhaust port has a reasonable up draft angle and a short side turn, it will work well for evacuating the cylinder's exhaust gasses.
Due to the way the exhaust works, a good exhaust port must have an efficient approach to and from the actual exhaust valve seat, otherwise there will be no effectiveness no matter how good the rest of the exhaust port is.
For a general rule, the minor diameter right under the valve seat needs to be 85% to 88% of the exhaust valve’s diameter. Additionally the outward taper from the minor diameter needs to be about 4 to 6 degrees. When the port area gets to be equal to the exhaust valve, for most purposes, it is as large as it needs to be. Also you must make sure there are no low flow spots in the port, as these areas will amplify the low-speed losses seen with Hi-performance camshafts.