Jack,
I'm nearly ready to remove the extra line just because I've just about convinced myself that there is simply no good reason to include the line. Not that I think it's goin' to hurt the box ("think" mind you as I still don't "know") ... but because I'm about 99% sure I'll get better shielding on the twisted pair supply lines.
The analysis gets a little weird, but for shielding purposes it's better to connect the ground to only one end of the cable (the box end in this case). What really kicked it for me was a recent journal article about shielding for noise (which I could remember where I read it...).
I'll see if I can remember to take some readings with the system cold (unpowered) and then again once I get the engine fired back up. If for no other reason than to move one of the old o'scopes out into the garage
Don ... oh we do NOT "think too much"... it's just that it gives us something to do when is too d@mn hot outside to work in the garage or drive the Cobra (100 degrees, 90% humidity today!)
Ed ... translation: hey, don't pull that spark plug wire off while the engine is running and NO it is NOT a good idea to check for spark voltage using the tried and true 9v battery "tongue test"
One of these days I'm going to have to post instructions on building up a Jacobs ladder using a spark coil. I used to make one for Halloween using an old model T coil until I burned out the primary on the coil.