You're experiencing what spark plug junkies have known for years: stick the spark farther into the chamber, get a bigger boom. Instead of the flame path going across the chamber like it's engineered to do, you're creating a fast-burn situation. This can hurt in some engines, but in most cases, like yours, it really helps. This is also why they make projected tip plugs and the like. I'm supposed to be running RV17YC's in my '87 notch, but I like to run RV15YC4's just because they're a little colder, designed for larger gaps, and they stick in a drab more. I'm not brave enough yet to try some RV12YC6's!
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Capri306, Moderator, The Mustang Works Online
1979 Mercury Capri, 5.0L -- C4 -- 2.73
1987 Mustang LX Notch 
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