Overview
You may see labels such as CVVT, DVVT, VVT, VVT-i, or Dual VVT-i on engine covers or in new car specifications. These terms describe different variable valve timing technologies. The following explains each term and the key differences.
VVT (Variable Valve Timing)
VVT stands for Variable Valve Timing. A VVT system adjusts the camshaft phase so that the valve opening and closing times vary with engine speed. This improves volumetric efficiency and increases engine power in different operating ranges.
Advantages include improved fuel economy and better power characteristics. A common limitation is reduced mid-range torque in some implementations.
In practice, VVT systems change cam timing to alter the relationship between intake and exhaust valve events. At low engine speeds, closing the intake valve earlier can increase maximum torque. At high engine speeds, delaying intake valve closing can increase maximum power.
VVT-i (Variable Valve Timing with Intelligence)
Toyota's VVT-i uses a hydraulic mechanism mounted on the camshaft controlled by the engine control unit (ECU). Within a certain angular range, the system advances or retards valve timing, or keeps it unchanged, to optimize engine performance. The camshaft timing gear typically consists of an outer rotor connected to the timing chain and an inner rotor connected to the camshaft.
A solenoid valve controls the oil flow that adjusts the camshaft angle, advancing or retarding it and thus changing valve opening timing.
Dual VVT-i
Dual VVT-i provides independent control of the intake and exhaust valve timing. During acceleration, the intake VVT-i system may advance intake timing and delay exhaust timing to increase valve lift and intake flow, improving combustion and reducing emissions. Many modern Toyota engines use Dual VVT-i.
In such systems, oil pressure controlled electronically can rotate the camshaft within a certain angular range (often around 60 degrees) to continuously adjust valve timing for improved performance across the rev range.
CVVT (Continuously Variable Valve Timing)
CVVT, a term used by some manufacturers, refers to continuously variable valve timing technology derived from VVT concepts. CVVT systems are commonly used in many modern cars. Typical components include an oil pressure control valve, intake cam gear, crankshaft position sensor, camshaft position sensor, oil pump, and the engine ECU.
When the engine starts or stops, the oil pressure control valve changes position so that the intake cam timing is in a retarded state. At idle or low-load conditions, timing is also retarded to stabilize engine operation. Under low-speed, high-load conditions timing is advanced to increase torque output. When engine temperature is low, cam timing may be retarded to stabilize idle and reduce fuel consumption.
DVVT (Dual Variable Valve Timing)
DVVT stands for Dual Variable Valve Timing, meaning continuously variable timing for both intake and exhaust valves. Compared with systems that vary only intake timing, DVVT can provide higher efficiency, lower fuel consumption, and lower emissions while improving low-speed torque and idle stability.
By adjusting both intake and exhaust valve timing continuously, DVVT engines can produce higher torque at low rpm and reduce exhaust emissions thanks to optimized valve events across operating conditions. DVVT is an evolution of earlier VVT systems, addressing limitations of intake-only timing control.
VTEC (Variable Valve Timing and Lift Electronic Control)
VTEC is Honda's variable valve timing and lift electronic control system, introduced in 1989. It was one of the first production systems able to control valve timing and valve lift simultaneously. VTEC consists of a control section, an actuation section, and sensors. The control section includes the ECU and the VTEC solenoid; the actuation section includes cam profiles, rocker arms, and related hardware. Sensors typically include engine speed, vehicle speed, and coolant temperature sensors.
The ECU evaluates sensor signals to determine when to change valve timing and lift. VTEC engagement typically occurs at a predetermined engine speed or operating condition to provide increased intake flow and power at high rpm. When VTEC engages, the exhaust note often becomes noticeably louder and more aggressive.
