In emissions testing, the load at which large-volume six and eight-cylinder engines operate during warm-up is insufficient for them to be able to attain high exhaust temperatures quickly. Thus there is a delay before the catalytic converter is able to operate at full efficiency. This design-inherent phenomenon can be prevented by further reducing the heat loss which occurs between the combustion chambers and the catalytic converter. One way this can be achieved is by means of three-valve technology, as used by Mercedes-Benz on all S-Class petrol engines. Dispensing with one exhaust port reduces the heat loss from the exhaust stream so that the catalytic converter reaches its operating temperature some twelve seconds sooner following a cold start.