This is the yellow Acell Super Coil I use on my 63 with the MP orange box and it calls for the .75 ohm ballast which I use. We also used it on my boys Dart before he put the MSD unit on it. It went no faster when we put the MSD on the car and my 63 never misses a beat using the .75 ballast. The ballast controls the current flow thru the coil and control unit or points. At low speeds when the dist is turning slower and there is more dwell time the ballast heats up and drops coil output but at higher speeds when the dist is turning faster and there is less dwell time to heat the ballast the ballast cools off and the resistance goes down some to increase the coil output when needed. I have seen guys burn the chrome MP box up from using the wrong or no ballast. And when I bought that same yellow Acell Super coil back in the 70's for my dual point dist you got and extra .25 ohm ballast to add in series with the stock ballast so it would not burn the points up. All I am saying is be careful if you bypass the ballast on an ign system set up to use it or you may burn the coil or control unit out. Ron