In reference to another thread, thought i would start this thread... here's some info that i've obtained over the years:
Distributorless (Simultaneous Ignition) Operation
Distributorless Ignition Systems and Direct Ignition systems that use one coil for two cylinders use a method known as simultaneous ignition. With simultaneous ignition systems, two cylinders are paired according to piston position. This has the effect of simplifying ignition timing and reducing the secondary voltage requirement.
For example, on a V6 engine, on cylinders one and four, the pistons occupy the same cylinder position (both are at TDC and BDC at the same time), and move in unison, but they are on different strokes. When cylinder one is on the compression stroke, cylinder four is on the exhaust stroke, and vice versa on the next revolution. (see Diagram)
The high voltage generated in the secondary winding is applied directly to each spark plug. In one of the spark plugs, the spark passes from the center electrode to the side electrode, and at the other spark plug, the spark is from the side to the center electrode. (See Diagram)
Typically, the spark plugs with this style of ignition system are platinum tipped (double platinum) for stable ignition characteristics.
The voltage necessary for a spark discharge to occur is determined by the spark plug gap and compression pressure. If the spark plug gap between both cylinders is equal, then a voltage proportional to the cylinder pressure is required for a discharge. The high voltage generated is divided according to the relative pressure of the cylinders. The cylinder on compression will require and use more of the voltage discharge than the cylinder on exhaust. This is because the cylinder on the exhaust stroke is nearly at atmospheric pressure, so the voltage requirement is much lower.
When compared to a distributor ignition system, the total voltage requirement for distributorless ignition is practically the same. The voltage loss from the spark gap between the distributor rotor and cap terminal, is replaced by the voltage loss in the cylinder on the exhaust stroke in the Distributorless Ignition System.
now, onto the theory and science behind the corrosion of alternating coil terminals...
Presuming that the contacts in the plug wires and the coil terminals are not of completely the same metals, the possibility exists for galvanic corrosion (or two metal corrosion)
This reaction is typically present in places where there is an electrolyte (i.e. a battery) but there can indeed be enough moisture/other elements in the atmosphere for this to occur in the engine bay. -- here's how it goes --
When two or more different sorts of metal come into contact in the presence of an electrolyte a galvanic couple is set up as different metals have different electrode potentials. The electrolyte provides a means for ion migration whereby metallic ions can move from the anode to the cathode. This leads to the anodic metal corroding more quickly than it otherwise would; the corrosion of the cathodic metal is retarded even to the point of stopping. The presence of electrolyte and a conducting path between the metals may cause corrosion where otherwise neither metal alone would have corroded.
