Porting - 4-Stroke
There are many similarities between porting 4-stroke heads and 2-stroke cylinders so much of the information in the previous section will apply in some general way. For this reason we will direct you to the 2-Stroke section if you haven't already read through it, and mention some similarities as well as the main differences below.
The first and most obvious difference is the fact that on 4-Strokes we port the head, rather than the cylinder as is the case with 2-Strokes. With that said, we can still benefit just as much from getting a high quality port and polish.
The clean-up process is basically the the same as it is for the 2-strokes, smooth out the casting seams and resurface and clean up the walls of the ports to promote the best flow possible.
In 4-strokes there are no transfer ports, only the intake and exhaust ports that are opened and closed by the valves (and the timing is controlled by the cam rather than the placement of the ports in the cylinder).
One major area where 4-strokes benefit is the bridge mentioned in the last section, knife-edging these is an extreme improvement in air flow, just as it is in 2-strokes.
The more detailed 4-stroke modifications involve modifying the "swirl" of air/fuel as it flows through the ports. This swirl is largely responsible for mixing the fuel with the air before it gets into the cylinder. It is designed to work in a certain way, and generally only changed in the most specific high performance applications.