Many things go on in the head of an engine. Primarily the air coming into the engine flows through the head and into the cylinder via an open inlet valve. The air moving into an engine is travelling at high speed and any turbulence caused can substantially alter the flow of the air.
Think of air like water. If you dive into a pool of water and land flat, the soft gentle water becomes hard and you really feel it. The difference is the speed you are travelling at. The faster you smash into the surface the harder it seems. Also moving quickly in water takes proportionally more effort the faster you go due to the increased drag.
Air travelling at speed also becomes ’solid’. I know that the physicists out there will pick me up on this but it is really the simplest analogy I can use to describe the way air moves and flows. A small ridge or dimple in the surface that the air is flowing over will create a turbulent stream and effectively slow up the flow of air.
When you gas flow a head the aim is to get the air moving as freely through the head as possible. Bends are either eliminated or made more shallow. Where possible the physical area inside the head is increased to allow for greater flow of air.
Larger valves will also enable the air to flow into the engine more freely. The larger the engine capacity the bigger the benefit of gas flowing the head as more air is moving around. Gas flow is measured on a flow bench and as the head is worked it is remeasured. A professional will get a feel for the type of channels that need to be made inside the head.
Inside the head near the valves there are often steps where a mass produced head is knocked out of the factory for speed. Removing these steps will further enhance the air flow and many car owners report a noticeable power increase solely from this machining away of the steps and grooves around the valve.
Particular attention should be paid to the ports. These are the holes into which the induction manifold connects to the engine and the exhaust headers connect to holes on the other side of the head. Aim to get the holes matched up avoiding steps or ridges for smooth inlet and exhaust from the engine. Porting can be carried out with a drill with a grinding wheel and should be performed with long sweeping strokes to avoid cutting new grooves and steps into the ports.
Areas to be checked are the valve seams. If these do not properly close there will be a loss of air flow and potentially substantial decrease of power. Valves should always be properly seated and a grinding paste can help improve the closure of the valve to the head.
