The amount that an inlet port is opened up is dependent upon the inlet valve size.  The port is made a fraction of the value size based on empirical results.  The reason for this is to compensate for the valve stem blockage, shown left, as well as the fact that the valve is reciprocating.  Also, the shrouding factor is increased the optimum port size is reduced.

Cylinder Head Porting

 

The more efficient an inlet port is, the less the pressure drop is between the carburettor and cylinder, therefore effecting the result in power output.  Port reshaping and resizing improves the air fuel mixture flow, allowing the combustion chamber to fill up quicker.

This increases both power and torque, especially at higher revs. There are also less energy losses throughout the ports due to their improved consistency in size. 

 

 

Combustion Chamber Reshaping

 

The combustion chamber has an effect upon the breathing potential of an engine.  The shape can be modified to increase the flow of the FAM throughout the engine.  This must be done when fitting larger valves to gain full benefit from extra FAM flowing into the engine.

 

When larger valves are fitted to an engine this results in the shrouding factor being increased.  If the chamber remained the standard size then the shrouding factor would increase, reducing the breathing potential of the engine and effectively reducing the power output.  In some cases this is less than with the standard valves.