Well, the main differences are that VVT engages under certain engine conditions (usually over 4,400 rpm)... so that it's something that switches on and off. When it is engaged, it provides more high-end torque as in high compression engines such as the 4AGE 20-valve. The sacrifice is low-end torque.
VVT-i, however, employs a continuously variable system that maximizes valve timing throughout the revolution range. Hence, it is "available" right throughout, from idle. This is what gives a VVT-i engine that quick burst of power from take-off.
VVT-i has now replaced VVT.
Having driven the new Corolla with VVT-i and a 4AGE with VVT, I find the 4AGE a far more throbbing experience... not only because it is optimized to deliver the goods, but you can actually FEEL when the VVT engages in higher rpms... kinda like the calm before the storm... unlike with VVT-i, you don't actually feel it since it's always there. Both systems achieve the same purpose of delivering more power via an electronic system while maintaining fuel efficiency - unlike that of a forced induction system which induces more air and gas to provide greater combustion and more power. However, the output achieved from forced induction over VVT or VVT-i simply can not be compared. And it's also why you might hear drivers of turbocharged engines say they're trying to stay out of boost to save gas.
