When running on low-octane fuel, ignition timing should be.

When running on low-octane fuel, ignition timing should be.

• A. Advanced
• B. Retarded
• C. Idle
• D. None

Answer: (B)

When fuel is lower in octane, it’s more prone to knocking (pre-ignition) under the same spark timing. To prevent that knock, you retard the ignition timing, meaning the spark plug fires later in the compression stroke. This lowers the peak cylinder pressure and temperature at combustion, making detonation less likely. Advancing the timing would push toward higher pressure and more heat, increasing the chance of knock with low-octane fuel, which is why that option isn’t appropriate. Idle timing is a separate adjustment and not a universal fix for octane issues, and saying to do nothing wouldn’t address the knock risk. So, retarding the timing is the safer, correct approach when running on low-octane fuel.

When fuel is lower in octane, it’s more prone to knocking (pre-ignition) under the same spark timing. To prevent that knock, you retard the ignition timing, meaning the spark plug fires later in the compression stroke. This lowers the peak cylinder pressure and temperature at combustion, making detonation less likely. Advancing the timing would push toward higher pressure and more heat, increasing the chance of knock with low-octane fuel, which is why that option isn’t appropriate. Idle timing is a separate adjustment and not a universal fix for octane issues, and saying to do nothing wouldn’t address the knock risk. So, retarding the timing is the safer, correct approach when running on low-octane fuel.