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VTEC stands for Variable valve Timing and lift Electronic Control. There are many variations of VTEC used in Honda vehicles today, but the actual principle of operation is similar in all of them. For this explanation, the Double Over Head Cam version will be used which is designed to increase engine power at higher engine revs. Each valve in the cylinder head has its own rocker arm that opens and closes the valve to let the fresh air/fuel charge in, called the inlet valve, and to let the exhaust gas out, called the exhaust valve. To simplify the explanation, we will concentrate on the operation of just one pair of valves as the operation of the others is exactly the same. In a normal engine, there would only be two rocker arms for these two valves shown, but in this VTEC example, an additional rocker arm has been installed inbetween the original ones. This additional rocker arm does not have a valve associated with it like the other arms and appears to have no useful function. Each rocker arm is moved up and down by a cam lobe that rotates with the rotation of the engine. A cam lobe is shaped similar to the shape of an egg, so that as the high point of the lobe pushes on one end of the arm, it causes the other end to push the valve open. As the cam lobe rotates and the lower shaped end of the lobe comes around, the valve begins to close. The middle rocker arm has a cam lobe which has a slightly different shape to the other two. When the engine revs are low, the valves are being opened and closed by the regular rocker arms and the middle rocker arm is simply going up and down independantly of the others with absolutely no result for its movement. When the engine revs increase to a predetirmined level, and certain conditions have been met, a series of small pistons inside some of the rocker arms are forced to move across and lock into the arms next to them. This action effectively locks all three rocker arms together so they move as one large arm. As the centre cam lobe that causes the middle rocker arm to move up and down is larger than the two normal lobes, it is now forcing the two valves to open sooner, close later and increase valve lift according to the shape of the centre cam lobe. This action will enable the engine to breath more effectively at these higher engine revs and increase the engines performance.