Cylinder Heads (3 or 5 Angle Valve Seats)
The greatest flow restriction in any engine is the cylinder head. Having the air / fuel mixture to efficiently pass through this restriction will increase an engines HP and Torque.
The Intake and Exhaust Valves are part of the cylinder ports, and when they are closed their ability to flow is zero. This means until they have opened to a very large opening, the valves are the main restriction to the engine's cylinder head's airflow.
Even when the valve is at a large lift, it still presents a difficult path for the air to travel on its way into or out of the cylinder. The priority is to make the valve capable of passing as much air as possible, whatever the lift is. To do this both the valve size and the valve's seat must be considered.
Although it may be the last operation during a porting job, the valve’s seat design is the most important priority toward effectively filling the cylinder. It would seem that the hole under the valve head needs to be as large as possible so as to flow the most air. Before flow benches were developed, it was a common practice to make a valve seat as thin as possible in order to achieve the maximum throat diameter. Objective flow bench testing found this to be untrue. In the real world the maximum flow is always a combination of size and form around the valve before and after the seat.
Air has mass and does not like to hug a port wall around a short-side turn. With low-angle ports, the air at mid and high valve lifts do not make the transition around the short-side turn very well. As a result, most of the air goes out of the long-side turn.
This situation is even greater as the higher the valve lift becomes. As a result, the streamlining of the port on the long side needs to be addressed for low, medium and high lifts, while the valve seat approach on the short side needs only to deal with the requirements of low-lift flow.
It does not matter if it is the intake or exhaust port, the worst part of the port for air to travel is the short-side turn. If the air fails to make it around the short-side turn, there obviously won't be much air exiting the valve in that area.
Changes in the valve's seat angles can make the valve appear bigger than it really is and flow more air during the beginning of its opening phase.