Although it sounds quite complicated, gas flowing and cylinder head porting are actually quite simple. The main aim of both gas flowing and cylinder head porting is to improve the air-flow through the cylinder head. Understanding what is good for improving air-flow, and what is bad for air low, i.e., what restricts air-flow, will go a long way to making good power gains from your cylinder head porting and gas flow work, as will an understanding of mean gas velocity, its affect on the air/fuel mixture, and how port size affects mean gas velocity.
As "Bad Ass" Bre and "Langer" have mentioned else on this site, the key to engine power and car performance is good air-flow in and out of the engine. Getting more air/fuel mixture into the engine and getting the exhaust gas out efficiently after combustion will get you more power. This is what is called Volumetric Efficiency (VE).
In technical terms, Volumetric Efficiency is the ratio of the volume of fresh air/fuel mixture that is drawn into the cylinder on the intake stroke, relative to the swept volume of the cylinder. Obviously, any exhaust gas that remains in the cylinder after the exhaust stroke will occupy some of the volume that fresh air/fuel mixture should occupy, and would reduce the Volumetric Efficiency of the engine. Thus, how well the exhaust gasses flow out the exhaust system is also important. Generally speaking, a multi-valve cylinder head will have a better Volumetric Efficiency, and hence will create better power, than a two-valve cylinder head. So if you have the option of fitting a two-valve cylinder head, or a multi-valve cylinder head, I’d go with the multi-valve cylinder head.
Improving the Volumetric Efficiency of your engine requires that you improve the air-flow in and out of the engine. Fortunately, there are a number of things that you can do to improve air-flow, particularly through the cylinder head. The first is to ensure that nothing obstructs or restricts the air flow to and through the cylinder head, from the moment air enters the intake system until the moment it exits out of the tail pipe. However, certain obstructions in the cylinder head ports, such as the valve stem and the valve guide boss, cannot be eliminated completely but can be minimized by narrowing the valve stem without weakening it too much and shaping the valve guide boss into a ramp.
Another way of improving air flow through the cylinder head is to form the cylinder head ports and the combustion chamber into an even, smooth and consistent shape. The key word here is consistency; constituency not only in shape and size, but also consistency from one cylinder to the other. You can achieve consistency in shape by ensuring that there are no intrusions or cavities in the port, and that the port does not widen or narrow. Air-flow can be further optimized by eliminating sharp turns and bends in the path of the air-flow.
Enlarging the Port
Enlarging the cylinder head port might appear to be a good way of improving air-flow but it has a major effect on mean gas velocity, which we actually need for better power and performance. A small port, relative to the cylinder, will have a high mean gas velocity at low RPM but it will struggle to fill the cylinder at high RPM. Thus Volumetric Efficiency will tail off at high RPM and power will fall off quickly. Conversely, a relatively large port will have a low mean gas velocity at low RPM, and when we have a low mean gas velocity the fuel in the air/fuel mixture could drop out of suspension and dribble into the combustion chamber as droplets. This leads to poor fuel economy and poor performance as fuel droplets do not burn efficiently and as quickly enough as it would in suspension. It also removes lubricant from the cylinder walls, which increases emissions and reduces engine life. To maintain fuel atomization, i.e., to keep the fuel droplets suspended in the air flow, a high mean gas velocity is required. Therefore, enlarging the cylinder head ports might not be as beneficial as it might appear.
Modern Cylinder Heads
The cylinder heads on modern engines have benefitted quite a lot from emissions control, which has resulted in extensive research into porting and combustion chamber design for better pollution control. These cylinder heads usually have many features that were developed and applied in motor sport racing not too long ago. Consequently, these cylinder heads may not require the removal of much metal from the ports or combustion chambers, and may not benefit for enlarging the ports. This, however, does not mean we cannot improve upon them; it just means the gains will be far less. On these engines, better performance is usually achieved by higher valve lift. The valve head itself, and the valve pocket, i.e., the area from the valve guide to the valve seat is where most restrictions to air-flow at high valve lift is to be found, and thus, the best performance gains are to be made by concentrating our modification efforts on this area.
Now that we've got a good understanding of air-flow, we can move on to the actual cylinder head porting.