[Jonny H]

The SC rotor is 5k, each lead is about 1k and there is 3k of resistors in the plug cap making a total of about 9k ohms for the secondary coil circuit.

The PK reply is all 'good practice' but is not the primary reason why the Bosch black coils last a long time and the others don't.

The Bosch black coil uses a oil/tar like filler material that is impregnated into the winding to insulate from flash over between coil windings and/or the case which is grounded. The oil/tar melts as it heats so the insulation is kind of self-healing to some extent. E.g. if there is an air pocket, it will eventually be filled with oil/tar. That is also the reason why the coils are mounted upside down, since there is a gap at the base (top) where a void of insulation is of no concern as the windings are closer to the nose end of the coil.

The Bosch silver and other coils are filled with an epoxy compound which sets hard. For this reason, before the epoxy sets, the coil must be held under vacuum so that the epoxy can penetrate the windings and all air bubbles can be removed. This is very difficult in practice due to epoxy setting time, temperature, thickness, pot life and a host of other factors. For example, if the vacuum is too great, the epoxy will boil which introduces bubbles again. I do not believe any vendor (including Bosch) has managed to get repeatability in this process, mainly because epoxy resin is not the correct material for this function. Back in the day both Cosworth and Lucas had similar problems. Cosworth actually used a clear resin as a insulator which could be optically inspected for bubbles!

We have cut open failed Porsche silver and aftermarket CDI coils and air pockets and bubbles are clearly visible.

For our own coil development we are working with a transformer winding specialist to select a material that has the same self-healing effect as the original tar. Even so, the filling/vacuum 'recipe' and process has to be repeatable every time to get a long lasting product. This is what is taking so long to get our coils to market.