VVT-i stands for variable-valve timing with intelligence, which is Toyota's name for the variable-valve control system it uses on its engines. Variable-valve control continuously varies the timing and duration that the intake valves are open to allow air-fuel mixture into the combustion chambers to provide better performance, higher fuel economy and lower emissions across a range of driving situations.
All major manufacturers use some form of variable-valve timing on at least some of their engines. In Toyota's case, all its engines have VVT-i, and many have dual VVT-i that operates on the exhaust valves as well as the intake valves (including the ones used in the Camry and Corolla, among others).
Variable valve control eliminates the performance compromise of having fixed valve timing that has to work at low speeds and at foot-to-the-floor high-speeds. It allows the engine to "breathe" — move air-fuel mixture in and out — at a pace that matches the driving conditions and provide more efficient operation.
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