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.

VTEC (pronounced V-tek) is Honda's variable valve timing and valve lift system.

VTEC stands for Variable Valve Timing and Lift Electronic Control.

See variable valve timing

Valve seats are the machined sections of the cylinder head against which the valves seal when closed.

Often the valve seats are formed in steel rings (valve seat inserts) which are inserted into the cylinder head and held by an interference fit.

Variable valve timing (VVT) is a system which optimises an engine's valve timing at all engine speeds, improving power throughout the rev range.

Engines without variable valve timing have a fixed relationship between the opening and closing of the valves and the motion of the piston. This relationship can only be optimal for one engine speed, because of inertia effects in the flow of gas past the valves and into the combustion chamber.

Variable valve timing systems allow the valve timing to be altered so that it is closer to optimal at all engine speed and load conditions. This allows the engine to produce more power at most engine speeds.

Volumetric efficiency is a measure of an engine's ability to breathe.

During induction air is drawn into the cylinder, and if the intake system allows the cylinder to completely fill with air, volumetric efficiency is 100%.

Volumetric efficiency varies with engine speed and throttle opening, and is usually at is highest in the middle of the engine speed range.

Tuned length intake systems and forced induction can lead to volumetric efficiencies greater than 100%.