The problems with splitfires are numerous.
1. If you have two possible places for the spark to go, it uses voltage that would've been used for firing to ionize two gaps instead of a single gap. It's a waste of spark energy.
2. Champion tried to split the electrode back sometime in the 1930's. They didn't notice a difference except more carbon buildup, so....
3. The insulator on splitfire plugs is of horrible quality, and so is the rest of the plug. I've had people bring me splitfires with the ground electrode missing. Wouldn't want that chunk of metal flying around in the cylinder, would you? Bad heat control due to the poor ceramics are the reason they tend to foul easy, in addition to a weaker spark.
The only way you're going to get two sparks from one plug is if by some coincidence the gaps are ionized exactly the same. Save yourselves money and get regular copper plugs, cold as you can go without fouling for maximum performance.
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Capri306, Moderator, The Mustang Works Online
1979 Mercury Capri, 5.0L -- C4 -- 2.73
1987 Mustang LX Notch 