VARIABLE VALVE TIMING, or VVT, is the act of changing the valve timing, or the times the valves in an engine open and close; as well as the duration, the amount of time a valve is open. Various systems perform some combination of changing timing, duration, or both. VVT systems may also control lift, or the distance the valve travels. Optimally, you would be able to change all of these values arbitrarily over time, as an engine behaves very differently at high RPMs than it does at idle. Variable Valve Timing allows you to vary your valve timing to a certain degree, achieving significantly better (or more tunable) performance at all speeds.

The best known VVT system is Honda's VTEC, which stands for Valve Timing Electronic Control. This is a Cam-Changing VVT system. It appeared first on the Civic, CRX, and NSX, but is now available on almost any Honda automobile. VTEC comes in two and three-stage designs. Either way, as you go from the lower-speed to the higher-speed stage or stages, the timing is changed and fuel delivery is increased, and the engine runs at higher RPMs, developing more power. VTEC only adjusts intake timing. Timing is changed by activating additional lobes on the camshaft.

Nissan's Neo VVL VVT system is similar to Honda's VTEC, but it is always a three-stage system, which works on both intake and exhaust camshafts. In the first stage, both intake and exhaust cams are set for economy; While this produces less horsepower, torque remains high. In stage 2, the intake cams are set for performance, but the exhaust cams are set low, to maintain torque during acceleration. In the final stage, both intake and exhaust cams are in the activated configuration. This provides the freest air flow, for the greatest horsepower.

Cam-Phasing VVT works by adding an offset to the rotation of the cams. Advancing or delaying the cams in this fashion alters the timing, but not the duration. It is, however, the cheapest and simplest VVT system, and so is used by nearly everyone. Again, VVT is usually done only on the intake camshaft, but sometimes also on the exhaust. Many engines use this method, including the Ford Zetec engine in the Focus, or the Volkswagen VR6 powerplant.

You can combine cam-changing and cam-phasing VVT. This is done by a couple of manufacturers; Toyota, with VVTL-i, Porsche, with the Variocam Plus, and Honda, with their i-VTEC. All of these systems are superior to the simpler systems, as they can control both lift and duration. The Porsche Variocam Plus system is superior in that it is continuously variable, without staging. Unfortunately, at this time it is only being used on the intake valves. Toyota's system provides staged lift and duration on both intake and exhaust cams, and Honda's i-VTEC gives phase-based staged lift and duration. Unfortunately, it too is only being used on the intake camshaft, and is only paired with a normal VTEC system on the exhaust side on the Integra Type R.

Rover also has its own system using an eccentric disc to actuate valves, called VVC, or Variable Valve Control. It allows continuously variable adjustment of both lift and duration, but it is expensive and unwieldy, as you need one sizable mechanism for every two cylinders.

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