Just to add more confusion: I have an old Ford tractor that is 6v. positive ground. When I rebuilt it, I had asked the same question as to why positive ground. The Ford dealership told me that when Ford decided on positive ground, it was because it was felt that the electrodes on the spark plugs would last longer, something to do with electron transfer from the electrode to the head if negative ground were used instead. They also told me that the coil was made for positive ground, and to change to negaive ground would cause some percentage in loss of spark. Is this folklore, don't know.
