What is the reason for a negative ground system?

Answer 1

Modern vehicle ignition systems use a negative ground system.

The reason some older vehicles used a positive ground system was because their manufacturers did not realize that metal erosion caused by sparking causes the center electrodes of the engine's spark plugs to be eaten away by the sparks if that electrode is connected to the negative side of the ignition coil's high tension output.

Such erosion can quickly make the spark-gap too large, causing late (aka "retarded") ignition problems which can seriously reduce the power output of the engine.

So, when using a positive ground system: a) the spark plugs had to have their spark-gaps adjusted every one or two thousand miles and b) the lifetime of the plugs was only around 5,000 miles or less.

Explanation

It is a fact of physics that, after millions of sparks, due to metal erosion there will be less metal on the electrode that is connected to the negative side of a dc (direct current) ignition coil.

So the center electrode of a spark plug will not erode away due to sparking if it is connected to the positive side of the ignition coil's high tension circuit and will erode away if is connected to the negative side.

As the spark plug's outer casing is much larger in area than its center electrode, if it is connected to the negative side of the ignition coil's high tension output, the erosion of the outer casing caused by sparking is hardly noticeable and this fact has the additional benefit that the spark-gaps of the plugs hardly ever need to be adjusted during their lifetime which, nowadays, can be well over 20,000 miles.