The reason that many ignition systems fail in high altitude is a problem that I see often in vacuum systems. There is a point at low pressure (About 100-300 Torr, 760 Torr = 1 bar), that is LOW pressure but NOT vacuum where ionization of the remaining air takes place VERY EASILY. This causes electrical arcs to happen at much lower voltage. hat was described as ignition failure really was arcing to grounds outside the cylinder. Not fun. CD ignition doesn't cure this, in fact it makes it worse. If your insulator is breaking down or in the case of a big supercharged radial the cylinder pressure is up, it becomes much easier to arc through the spark plug wires than through the plug gap! This seems to be unique to this fairly low pressure (in air) situation, which can be duplicated by a vacuum chamber in which you are pumping down while you are running a supply current to something in the low pressure. (don't ask me how much it cost to re-discover this problem)

Bill Jepson