I installed a electronic ignition kit on my sailboat's Atomic 4 inboard engine. I thought that would eliminate my ignition problems that plague a points based system.
For a while things worked great, the engine started quicker and idled much better. Then my ignition coil failed while running at the dock which seemed really odd. The coil was the original unit I presume (same engine paint color) which had the internal resistor to limit current through the coil. This same coil worked on this boat since 1974 and then suddenly it fails only a month after my electronic ignition conversion?? I thought that was odd. After digging into it I found that is no coincidence.
My Atomic 4 is a later model so I purchased the kit that Moyer Marine sells which is the Pertronix 1146A kit. The kit works fine. The problem is that the Pertronix ignition works too well! The dwell with the Pertronix is much longer than the points based ignition. The dwell is the amount of crankshaft movement angle that the coil is energized with current. I'm not sure what the value is with the Pertronix ignition but I believe it is about 60 degrees of crankshaft rotation which is about twice what the points system was. This means that the coil has more time to saturate the windings with current which is great for a good spark, but the coil gets hotter which can cause premature coil failure. Ignition coils hate heat.
I thought about installing a Pertronix coil since I thought it might be a good match for the Pertronix ignition. Pertronix says that a 4 cylinder engine should use a 3 ohm coil which they sell. However Moyer Marine at one time did sell the Pertronix coil and then they stopped selling that coil. If you dig around a lot you will find that the Pertronix coils have or had some problems. So now Moyer now sells their own 3 ohm coil for use with the Pertronix ignition.
I'm not a snob but I want to know what brand of coil I am buying and I would like to be able to replace it with a standard off the shelf coil. For a first pass I purchased a generic Accel Super Stock replacement coil that was stated to be a 1.5 ohm coil. It was a little over $35 at Autozone. Autozone did not think that they had a 3 ohm coil. In order to avoid overheating the 1.5 ohm coil or cooking the Pertronix 1146A by passing too much current through it, I purchased a Mopar/Dodge/Chrysler ignition ballast resistor from Autozone which was a Duralast AL795. This resistor was perfect in that it mounted under the same 5/16 bolt that holds the coil bracket to the back of the Atomic 4 engine block and that made for a quick and easy installation.
I have the Accel coil and the Duralast resistor in place now and I ran it for about an hour and a half at normal cruise speed the other day and then checked the coil temperature with an IR temp gun and it was running right at 175 degrees which is a perfectly acceptable temperature. So the Accel coil and the Duralast resistor is a good fit.
I wanted to carry backup ignition components on the boat for spares so I purchased another Pertronix 1146A kit as a spare (about $80 off Amazon) and I found that Bosch makes a standard 3 ohm coil that was used for the VW aircooled engine. Its called the Bosch Blue 00012 coil and it is literally blue in color and apparently has been so for decades. This coil has a sterling reputation and so I bought that for a boat spare. The Bosch coil was $44 off Amazon. In hindsight I would go with the Pertronix kit along with the Bosch Blue 00012 coil, but for now those will be my boat spares.