| Mainly they are used in race spec engines, as they are a surface discharge type plug they reduce the risk of detonation that can be caused when the tip of a normal plug gets very hot from prolonged high revs, also you do not suffer the same spark errosion of the earth electrode as you would on a normal plug. In my road bikes I run NGK EV type plugs as these give a better spark than normal plugs, but the 955 Corse engine has the QA55Vs |