VTEC works by utilizing two different optimized cam profiles in order to improve lower RPM performance as well as upper RPM range performance. One cam is used to be fuel efficient and provide stable low-RPM performance, while the other cam is designed to provide additional power at high-RPM output.
Depending on the RPM at which the engine is operating, the engine's computer activates the corresponding cam lobe profile and makes adjustments to the engine's timing. The engine is able to provide both fuel efficiency as well as performance by utilizing this technology.
When the engine is operating at a low RPM, smaller cam lobe profiles push the intake and exhaust valves open and closed and limits how far they open, which results in superb fuel efficiency. The higher lobe profile is not utilized while operating at low RPM. Once the engine reaches the specified higher RPM, a solenoid pushes more oil into the engine's rocker arms, making the larger cam lobe profile active, which allows more air into the valves, translating into increased performance.
By utilizing VTEC in engines, Honda is able to offer engines that are reliable, fuel efficient, as well as powerful in the upper-RPM range. A drawback in utilizing VTEC is that most drivers rarely push their engines to the RPM range where it is usually activated, which is between 5,000 and 6,000 RPM.