Double Overhead Camshaft (DOHC)
An engine configuration where two camshafts are located in each cylinder head, one operating the intake valves and the other the exhaust valves.
Double Overhead Camshaft (DOHC) is a cylinder head design where two camshafts are located above the combustion chambers. In a typical inline DOHC engine, one camshaft operates the intake valves and the other operates the exhaust valves. In V-type engines, DOHC means there are four camshafts in total—two for each cylinder head block.
DOHC designs allow for a direct connection between the camshaft lobes and the valves, eliminating heavy pushrods and rocker arms. This reduction in valvetrain mass allows the engine to rev higher safely. Furthermore, having separate camshafts for intake and exhaust allows engine designers to implement variable valve timing (VVT) independently on both sides, optimizing engine breathing, emissions, and power output across the entire RPM range.
Maintenance for DOHC engines includes regular oil changes to preserve the camshaft journals and VVT solenoids, and replacing the timing belt or chain at recommended intervals. A failed timing belt on a DOHC engine (which is usually an interference design) will cause the pistons to strike the open valves, causing bent valves and ruined cylinder heads. Diagnostics involve checking VVT codes and using borescopes to inspect for valve damage.