Valve timing is the phasing of the opening and closing of the intake and exhaust valves relative to the position of the piston.
When a valve opens the gases passing to or from the combustion chamber take time to start moving. Because of this inertia effect the optimum valve timing varies with engine speed, but until the advent of variable valve timing systems in the 1980s the valve timing in an engine was fixed, and was controlled by the cam profile and the geometry of the valve gear.
