Under heavy load, which spark timing adjustment is typically used?

• A. Advance
• B. Retard
• C. Maintain
• D. None

Answer: (B)

When an engine works hard and experiences heavy load, knocking becomes a bigger risk because cylinder pressures and temperatures rise. Retarding the spark timing—firing the spark a bit later in the compression stroke—lowers the peak pressure and the rate of pressure rise during combustion. This reduces the chance of detonation and pre-ignition, helping protect the engine under demanding conditions. The trade-off is a small loss of peak power and efficiency, but safety and reliability come first in these situations. Advancing timing would increase power but raise the knock risk, and maintaining timing wouldn’t adequately address the knock tendency under heavy load.