There are four spark plugs used on this engine, two for each head. The reason for two is one is for firing the charge for the power stroke and the other is used to lower emissions by firing on the exhaust stroke, but by having two per head the spark timing can be altered at times for more power using computer algorithms. Diesel trucks today use similar algorithms to quiet them, to produce more power, and reduce emissions. In other words, there are four, two use a plug wire similar to those used on the older cars, and two have each a coil on top of them with two wires attached.