Low compression ratio engines run hot and have cold spark plugs. True or False?

• A. True
• B. False
• C. Not applicable
• D. Sometimes

Answer: (B)

Compression ratio directly influences the peak temperature of the combustion process: higher compression leads to higher cylinder temperatures, while lower compression produces cooler combustion. Since spark plug heat range is chosen to match the engine’s heat load, a hotter engine (more heat to dissipate) typically uses a colder spark plug to prevent the plug from overheating, whereas a cooler-running engine tends toward a hotter spark plug to keep the plug from fouling and to ensure proper combustion. Therefore, a low compression ratio engine would generally run cooler and would not be characterized by cold spark plugs; the statement is false.