[steve s]

For those who are comfortable talking about electrons but are otherwise mystified by this discussion, there are three things you need to know: (1) when a battery is discharging, electrons are coming out of the - terminal; when it's being charged, electrons are being crammed into the - terminal, (2) the other end of the high voltage wire coming out of the coil is internally grounded to whichever of the coil's terminals is also connected to the negative post of the battery, and (3) it's easier for electrons to jump from hot metal, like a spark plug tip, than from cooler metal, like the part of a spark plug in thermal contact with the water-cooled head.

In other words, you want the electrons to be traveling from the coil to the spark plugs, not the other way. That means the invisible, internal end of the coil wire must be connected to the negative terminal of the battery and, if it's so marked, the negative terminal of the coil. The coil cannot manufacture electrons; they have to come from somewhere, which means, ultimately, the battery's negative post, whether or not it happens to be grounded (for the convenience of the various low-voltage accessories).

With 10,000 volt incentive, electrons will jump the .025" plug gap no matter what, but you'll get a more dependable bodacious spark if conditions are optimized.