Even in the 1970's, it was considered normal for some engines to have major work done at 70 or 80 thousand kilometres. Much of the increase in engine efficiency can be attributed to changes in cylinder head design. Many engines are produced today with four valves per cylinder. A four valve engine uses two intake valves and two exhaust valves. The valves must be smaller to fit in the engine but this has some advantages.
Smaller valves are lighter and can change direction much quicker.
This enables the engine to turn faster. Another advantage of using more valves is better engine breathing. Four small valves can permit more intake mixture and exhaust gas flow when compared to a typical 2-valve engine. The biggest disadvantages of four valve engines are the costly and numerous parts required to build them. The heart of any engine is the camshaft.