Anyone into modifying cars will have read or heard the term ‘gas-flowed cylinder head’. This involves modifying the cylinder head to improve flow capabilities, and can be done optimising the shape of the port, removing manufacturing flash, etc.
The details are relevant to individual cylinder heads but the basic principles are the same – to improve flow and the mixing action and movement of the mixture going into the cylinder head.
Typically this was always done by hand, but with modern machining techniques its possible to get this done by computer-controlled machines for optimum flow.
Work is generally carried out mainly on the inlet side, because that is where most benefit can be gained. The finish of the ports is also worth considering. In the old days, it was considered that a mirror finish was desirable but common thinking now suggests that a slightly rough finish helps to create a boundary layer of air, which leads to better efficiency.