Hi there Dick. A standard ignition delivers a single spark to each cylinder when the cylinder is at or near TDC on it's power stroke. A spark box connects between the distributor and
coil and causes the
coil to fire three times instead of once. If you add in a higher energy
coil, like the
Pertronix flamethrower
coil, then you're getting three hotter sparks, hence better ignition/more complete combustion of gas. In a 390, with pistons the size of soup cans, that can make a big difference. I put a standard
Pertronix and flamethrower
coil plus spark box on a Galaxie 500 I have and the difference in power was quite noticeable, in fact I had to crank down the idle screw to reduce the idle to where it is supposed to be, that's how much better the gas was being combusted. The engine runs and idles smoother as well.
An MSD spark box will provide the multiple sparks up to 3000 rpm (or so) then a single spark at higher rpms. Spark boxes typically have rev limiters too for the drag racers among us to keep from blowing their engines with over revving. My understanding is the
Pertronix III provides the multiple sparks regardless of the revs (which is a good thing) and also has a rev limiter. So, no spark box needed. By the way, no danger of burning holes in the tops of pistons, that's an urban legend.
