Definition
In a reciprocating aircraft engine, the number of degrees of crankshaft rotation by which the actual opening or closing of an intake or exhaust valve is delayed past the theoretical position (top or bottom dead center). Valve lag is built into the valve timing to take advantage of the inertia of the moving gases and to improve cylinder filling and scavenging at operating speed.
Plain English
Valve lag is the small amount of extra crankshaft rotation that happens before a valve actually opens or closes, compared to where you'd expect it to open or close in theory. Engine designers build this delay in on purpose so the engine breathes better when it's running.
Context Anchor
Seen in reciprocating-engine valve timing discussions, engine theory, and maintenance checks.
Derivation
Lag' comes from an old word meaning to fall behind or be slow. Here it describes the valve's action falling behind the piston's position by a measured number of crankshaft degrees.
Why Pilots Care
Correct valve lag improves volumetric efficiency, power output, and fuel economy while preventing valve damage at high RPM.
Intuition Check
Valve lag does not mean a sticky or slow valve. Here, it means a planned part of valve timing: the valve is designed to stay open slightly past a piston end point.
Example Sentence 1
The mechanic checked the valve lag during the engine overhaul to confirm the timing matched the manufacturer's specifications.
Example Sentence 2
During overhaul the mechanic verifies valve lag against the manufacturer specifications before reassembling the cylinder head.