First off, the set up as it is. I have over 100,000 miles experience running the otherwise stock set up with a 2.2 ohm coil. No problems. I have read that a lot of people run the 3 ohm HD coil with no problems. Now lets dual plug it. Two red 1.5 ohm 6 volt coils adds up to 3 ohms. Some say I need to run a booster and some don't. Some say they run two brown 1.5 ohm 12 volt coils without a booster with no problems. I am thinking I would be better off running two .7 ohm 12 volt coils without a booster than two 6 volt 1.5 ohm coils with a booster or without. Any opinions? But then there are those that claim running three ohms with a single HD coil is trouble free so why wouldn't running two coils at three ohms? And my still yet unanswered question: What is the difference between a 1.5 ohm 12volt coil and a 1.5 ohm 6 volt coil? Dyna won't answer that question. Any ideas. A friend thinks it is probably the winding's insulation value. Maybe it has something to do with the secondary windings? All my sources don't know. Even my dad! That's rare! I'll ask Dyna one more time.