Valve lead is the ...

• A. The opening of the exhaust valve before BDC.
• B. The opening of the intake valve after TDC.
• C. The closing of the exhaust valve before TDC.
• D. The duration of valve opening.

Answer: (A)

Valve lead is the advance in timing of when the exhaust valve starts to open, relative to the piston reaching bottom dead center. In a typical four-stroke engine, the exhaust valve begins to open before BDC so exhaust gases can start flowing out while the piston is still moving downward, ensuring effective scavenging and a smoother transition into the exhaust stroke. This makes the statement that valve lead is the opening of the exhaust valve before BDC the correct description. The other notions mix up valve timing aspects—intake valves usually open before TDC, not after; the exhaust valve closes after TDC, not before; and valve lead is about when the valve begins to open, not about how long it stays open.