Volvo used a 3-prong spark plug in the older NA 5 cylinders on the basis of a "triple spark" providing more complete combustion. Only problem? The electrical energy will choose the path of least resistance, so it will end up sparking across only one of the three prongs. People replace those fancy plugs in their Volvos with the cheaper traditional plug that was factory installed in the turbo cars and have experienced no difference between the two styles.
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