Multi-ground electrodes improve ignition capabilities and are less sensitive to increased voltage requirements due to gap growth. Designed to specific manufacturer ignition requirements in either supplementary gap, intermittent discharge, or semi-surface discharge, all for various firing tip self cleaning abilities. Multi-ground plugs maintain all features of ngk 's traditional plugs including corrugated ribs, pure alumina silicate ceramic insulator for greater strength and better heat transfer, copper core and triple interior seal. Sold each, 12 needed for 911 c2/c4.