Since Japan announced a tax based on engine displacement, Japanese auto manufactures have all focused their R&D efforts for improving the performance of their smaller engine designs through ways other than increasing the displacement.
Apart from forced induction like Toyota and Nissan used, Honda engineer Ikuo Kajitani invented a cam timing profile, that provided Honda with the first successful commercial design for changing cam profile in real time. VTEC systems provides engine with valve timing changed for both low and high RPM operations. VTEC stands for Variable Valve Timing and Lift Electronic Control, and is a way Honda used widely across their vehicle range. Despite the many variations of VTEC, their working principle is identical.
Each valve in the cylinder head has its own rocker arm, to open 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 of just one pair of valves., as the operations of the others is exactly the same. In a normal engine, there will be only two rocker arms, controlling two valves. But in a VTEC engine, a additional rocker arm is being installed in between the original ones. This additional rocker arm does not have a valve associated with it, unlike the two other ones, and appears to have no useful function. Each rocker arm is rocked up and down via a com lobe, that rotates with the rotation of the engine. A cam lobe is shaped similar to the shape of an egg, In such so that when the high point of the lobe pushes on 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 cam lobe has a higher profile than the other two. When the engine works at low RPM, the regular cam pushes open and closes the valve, with the middle higher profile one simply rocking up and down, without result for its movement. When the engine load increased to a predetermined level, and certain condition are being met, a series of pistons inside some of the rocker arms are forced to move across, locking them with the arm next to them. This action effectively locks all three rocker arms together, so they move as one large arm. As the center cam lobe that causes the middle rocker arm to move up and down has a higher profile than the other lobes, it is now forcing the valve to open sooner, close later and increase valve lift, according to the shape of the center cam lobe. This action can enable the engine to breath more effectively at higher engine revs, and increase the engine’s performance.
There are a lot of other types of VTEC engine variants, including the HYPER VTEC in some superbikes, most modded and well known for their impressive modding ability, the VTEC-E, i-VTEC on recent cars, VTEC turbo and the 3-stage VTEC that can open one intake valve with another just slightly opened. From 2500-3000 RPM the 12v mode is engaged and results in swirl of the intake charge which increases combustion efficiency. At 3000-5400RPM, one of the VTEC solenoids engages and causes the second valve to lock onto the first valve’s cam lobe, thus called 4v mode. This mode resembles a normal engine operation and improves mid range power. At 5500-7500RPM, another VTEC solenoid engages in such so that both intake valves are using a middle, third cam lobe. This third cam lobe is tuned for high-performance and provides peak power at the top end of the RPM range.