Higher compression ratios require which change in fuel grade?

• A. Lower grade
• B. No change
• C. Higher grade
• D. The same grade as engine oil

Answer: (C)

When you increase the compression ratio, the air–fuel mixture is squeezed to higher pressure and temperature. That makes it more prone to detonation, or knocking, if the fuel can ignite too easily before the spark plug fires. To prevent knocking, you need fuel with a higher resistance to premature ignition, which is measured by a higher octane rating. So engines with higher compression ratios require higher octane fuel. Using a lower grade can lead to pinging, reduced performance, and potential engine wear, while using a higher grade than necessary generally doesn’t add extra power but provides a safety margin against knock under demanding conditions.