Intake and exhaust ports

Intake and Exhaust Ports :

With four valves a greater proportion of the head area is available for porting than if only two are employed. Moreover , the sparking plug can be more easily positioned very close to the centre of the chamber , so
the flame path is short.

This means that not only is complete combustion
easier to attain but also the Ignition timing can be
retarded so that the dwell of the gases at high
temperature in the cylinder is reduced,Because
of the large diameters of two valves relative to the
bore of the cylinder , the gas flow past the portions
of their edges adjacent to the Cylinder wall tends
to be masked by it.

With four valves,on the other hand,there need be
little or no masking and more over the interaction
of the two incoming streams of gas can greatly
improve mixing, again leading to more complete
combustion.

With two Exhaust valves instead of one , the ratio
of seat length to area exposed to the hot gases
is higher and so also, therefore, is the rate of cooling by Conduction through the seats.

In general , two valves are adequate for engines developing up to about 35 kW/litre but above this level of specific output four are desirable.