Ignition Coil Reliability
 

Canister type ignition coils as we use in our Model "A's" are either oil filled, epoxy filled or tar filled. The question is, which of these coils is more reliable? I am judging these coil filler materials by their electrical insulation strength (AKA dielectric strength), their ability to transfer heat, and their ability to fill the coil without voids.

Tar Filled Coil: Tar is a very high viscosity material that has a propensity to have voids at the windings enabling hi-voltage arcing. However, as the coil warms, the viscosity of the tar decreases and voids become less of an issue as the tar flows to fill them;
Epoxy Filled Coil: Like tar, epoxy also has a propensity to have voids that enable hi-voltage arcing. However unlike tar, the epoxy is a solid that does not turn to a liquid and flow to fill voids;
Oil Filled Coil: Oil has a high insulating strength, and being a liquid it fills its container without voids. Oil is the better the heat conductor, so an oil filled coil wil run cooler. Oil has two concerns that affect its performance: (1) Impurities reduce its insulating strength and, (2) Leakage in service.

In my opinion, an oil filled coil is superior, and the epoxy and tar filled coils are similar second place technologies. Now let the debate begin.

Re: Ignition Coil Reliability
 

I've had 3 oil filled "new coils" fail or work intermittently including Lucas, Standard, TSP . Even installed with terminals up. I can shake all the oil filled and hear a lot of sloshing so the windings may not be covered completely. After much research, none appear to be made in the US, Germany (Bosch ) with stringent environ. reg's, etc. and quality control may be not that great. So far, the old paste or tar filled coils seem to function consistently in any orientation. Have a new Peritronix epoxy as back up, but some users suggest the epoxy may not dissipate heat very well.