High compression ratio engines run fairly _______ therefore have _________ spark plugs.

• A. Cold; Hot spark plugs.
• B. Hot; Cold spark plugs.
• C. Warm; Standard spark plugs.
• D. Cool; Cool spark plugs.

Answer: (B)

High compression raises in-cylinder temperatures, so engines with high compression tend to run fairly hot. To prevent the spark plug tip from overheating in that hot environment and to reduce the risk of pre-ignition or detonation, a colder spark plug is used. Colder plugs dissipate heat more quickly, keeping the tip temperatures within a safe range while still avoiding fouling. That’s why the correct pairing is that these engines run hot and require cold spark plugs. Using hot plugs would keep the tip too hot for a hot engine, increasing misfire or pre-ignition risk, while other options don’t match the need for extra cooling in a high-temperature condition.