DOHCsystem

The DOHC system, or dual overhead camshaft system, is a type of internal combustion engine valvetrain arrangement in which each cylinder bank has two camshafts located in the cylinder head. One camshaft operates the intake valves and the other the exhaust valves. Compared with pushrod designs or single-overhead camshaft systems, DOHC enables more precise valve timing and is commonly used with multiple valves per cylinder, such as four valves per cylinder (two intake, two exhaust) to improve high-speed breathing and efficiency. In inline engines, both camshafts are in a single head; in V engines, there are two camshafts per bank, resulting in four camshafts for a V8, for example. Many DOHC engines also pair with variable valve timing to adjust timing on the fly.

Key components include the camshafts themselves, the timing drive (gear train, belt, or chain), lifters or tappets, rocker arms or direct acting followers, and valve springs. The system is designed to ensure synchronized opening and closing of intake and exhaust valves with piston position.

Advantages include better high-RPM performance, higher volumetric efficiency, smoother valve operation, and compatibility with multiple valves per cylinder. Disadvantages include higher cost and complexity, increased machining precision, and more intricate maintenance. Maintenance considerations include timing drive replacement intervals (for belts), tensioner wear, and potential phasing issues in engines with variable valve timing.

DOHC is common in modern passenger cars, particularly four-, six-, and eight-cylinder engines, including performance-oriented models. It became widespread in the late 20th century as part of efforts to improve efficiency and power without increasing engine size.