Standard cylinder heads (shown above) contain a number of 'casting marks', left by the production process, that are very restrictive to air flow and speed.
The first stage in the porting process is the removal of these 'casting marks' and the smoothing of the surfaces over which air will flow. Although this does not greatly increase the volume of air that can be delivered by the ports on the head it does promote better air speed and less turbulence in the induction tract and therefore is a worthwhile process on mildly tuned or forced induction applications.
The picture below shows the inlet ports in the initial stage of "porting". The ports are very slightly larger than standard but essentially they are just smoothed and polished. (note that the guides have been removed at this stage)

The next step in the modification process is to remove a considerable amount of material from the exhaust port castings to increase the volume of the ports and allow for greater volumes of air to be scavenged from to the cylinder.
This in turn requires that we shape the ports correctly to prevent turbulence and 'stalling' of the air in the induction tract.
"Phantom" heads will function well in most engines where the capacity does not exceed 3.9 litres, or where forced induction is being used to push the gas mixture into the cylinder. The Phantom exhaust ports allow for greater horsepower to be produced without reducing the speed of the air in the induction tract, essential for the production of mid range and higher output.
There are options available for the mixed sizing of the ports to accommodate supercharged engines and if Phantom heads are to be properly designed a good understanding of the proposed usage is normally required; consultation with the customer is essential.
The image below shows an exhaust port awaiting final polishing following the Phantom conversion porting.

Things are getting more 'serious' now! At this stage of development the cylinder heads are ported and polished to achieve full air flow on anything up to a mildly tuned 5.2 litre engine. The valves are now bigger and allow a greater charge of gas to be drawn into the chamber. Guides are shortened and 'bulleted' (basically this 'bulleting' reduces the port restriction caused by the guide in the port). Combustion chambers are very precisely matched to give equal volumes of gas for each firing.
Valve springs are changed for much stronger double springs with a progressive pressure to suit the proposed camshaft.
The exhaust ports are opened to match the largest of exhaust gaskets.
The inlet ports are opened to match a 45mm ported inlet manifold.
At this stage the heads are good enough to flow the air and fuel needed for a 4.6 litre engine to produce over 320 BHP as opposed to the standard heads which are capable of little more than 220 BHP.
The picture below shows the cylinder chamber half way through the porting and polishing process.
The inlet port has been shaped and polished whilst the exhaust has just been 'roughed out' and awaits final shaping and polishing.
The head chamber has also been ported and polished and will be matched to the other chambers on the head prior to completion.

At this stage of development there is no material left to remove! We port the heads to the greatest possible volume through from inlet port to exhaust without losing the mixing and speed aspects of the induction tract.
The valve seats are removed and replaced with larger versions to accommodate even larger valves than the Warrior head.
These heads have been used on engines producing in excess of 360BHP and will probably deliver even greater power if combined with an ideal injection system. In terms of a standard Rover casting, they don't get better than this!
The heads have a 'skimmed' cylinder face to ensure correct gasket sealing and optimise compression ratio.
Notice the shortened, bulleted guides and the chamber shaping. The combustion chambers have been ported, polished and precisely matched for size (capacity) and shape.