Variable camshaft timing

Updated 2026-10-07

Variable camshaft timing (VCT) is an automotive engine technology that alters the angular position (phase) of a camshaft relative to the crankshaft during engine operation. It is a form of variable valve timing (VVT). VCT enables the opening and closing of the engine's intake and/or exhaust valves to be optimized for varying operating conditions, rather than being fixed at a single compromise setting. The term "VCT" is most closely associated with Ford Motor Company, which has used it as the name of its cam-phasing technology; it is also used generically within the automotive repair industry to describe cam-phasing systems.

Overview

In a conventional engine, the rotational relationship between the camshaft and the crankshaft is fixed by the timing chain or belt. This fixed timing represents a compromise among conflicting requirements such as idle stability, low-speed torque, high-speed power, fuel economy, and emissions. Variable camshaft timing allows the camshaft to be advanced or retarded while the engine is running, so that valve events can be scheduled according to engine speed, load, and other conditions. Because phasing rotates the entire camshaft, it changes the timing of valve opening and closing but does not change the duration or lift of the valve events.

Reported benefits include improved engine performance and torque, reduced exhaust emissions, and increased fuel efficiency relative to engines with fixed camshafts.

Mechanism

Most production VCT systems are electro-hydraulic. A camshaft phaser (also called a cam phaser or phase actuator) is mounted at the end of the camshaft and coupled to the timing drive. The phaser typically contains a vane rotor attached to the camshaft and a housing attached to the timing chain or belt sprocket, with hydraulic chambers between them. Engine oil is directed into these chambers by an oil control solenoid or valve, and the resulting differential oil pressure rotates the camshaft relative to its drive, advancing or retarding camshaft timing. The powertrain control module (PCM) commands the solenoid based on sensor inputs such as engine speed, load, throttle position, and temperatures, and compares the commanded position with the actual camshaft position measured by a camshaft position sensor.

Types

Cam-phasing systems are commonly categorized into four types, distinguished by which camshaft is actuated:

Exhaust phase shifting (EPS): the exhaust camshaft is the active cam, typically retarded to improve emissions.
Intake phase shifting (IPS): the intake camshaft is the active cam, typically advanced to improve power and torque.
Dual equal phase shifting (DEPS): both intake and exhaust camshafts are phased together by equal amounts.
Dual independent phase shifting (DIPS): the intake and exhaust camshafts are phased independently. Ford markets this configuration as twin independent variable camshaft timing (Ti-VCT).

Relationship to emissions and exhaust gas recirculation

Retarding the exhaust camshaft timing can increase the retention of residual exhaust gas in the cylinder, achieving effects similar to external exhaust gas recirculation (EGR), such as reduced nitrogen oxide (NOx) formation. As a result, some VCT applications can eliminate the need for a separate external EGR system by controlling valve overlap between intake valve opening and exhaust valve closing. Intake camshaft phasing can also be used to regulate the amount of air drawn into the cylinder and to influence internal exhaust gas recirculation.

Applications

VCT has been used in a range of Ford engines, including the 5.4 L Triton 3-valve V8, the 4.6 L 3-valve V8, the 2.0 L Zetec inline-four (exhaust camshaft), the 1.7 L Zetec-SE, and the 6.2 L V8. Ti-VCT has been applied to numerous later engines, such as the 1.6 L Duratec Ti-VCT and various 3.5 L, 3.7 L, and 5.0 L V6 and V8 units. Some Ford Ti-VCT engines use cam torque actuation (CTA), which uses torsional energy in the valvetrain to rotate the camshaft rather than relying solely on oil pressure.

Related concepts

Variable camshaft timing is one approach within the broader field of variable valve timing. Other approaches include cam profile switching, variable valve lift, variable intake and exhaust duration, and camless (electromechanical) valve actuation. VCT is distinct from these in that it changes only the phase (timing) of the valve events, not their duration or lift.