New engine designs are placing an increasing demand on spark plug performance. The quest for higher specific power output and improved fuel economy has prompted advances in areas such as lean burn, fuel injection, turbocharging and valve train design. The spark plug must now ignite leaner mixtures and operate under more severe combustion chamber conditions. The copper-cored ground electrode facilitates spark plug designs which meet these increased performance demands. Performance criteria such as service life, ignitability, preignition protection and fouling resistance are greatly improved with copper-cored ground electrode spark plugs. These improvements are realized with leaded and unleaded fuels including those having high additive levels.