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

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

Answer: (C)

The main idea is how spark plug heat range relates to engine temperature. A lower compression ratio means the cylinder temperatures during combustion are cooler. If you use a plug with a typical or cold heat range, the plug tip may not stay hot enough to burn off carbon deposits, especially during cold starts or light loads, leading to fouling or misfiring. Using a hotter plug keeps the tip hotter, helping ignite the mixture reliably and reducing fouling in a cooler-running engine.

That’s why the statement is true: low compression ratio engines run cool and have hot plugs. In contrast, engines with higher compression run hotter and typically use colder plugs to prevent the plug from overheating.