Wow, my "old wives tale" comment started quite a thread. I'm an EE working
for a major automotive parts supplier, but not directly involved with
ignition systems. I'll ask around work and see if I can garner any hard
facts about spark plug polarity.
What I do know about sparks:
Arcs do cause material to be transferred from one contact to
another, and the transfer direction depends on the direction of current
flow. This can be a design factor for switches and relays that are used with
direct current. Contacts used with DC must be made of a harder metal, other
things being equal.
Modern automotive ignitions fire pairs of spark plugs in series. Neither
end of the HT secondary goes to ground, instead each end goes to a
complementary spark plug. (So each plug fires both on the compression
stroke, and the exhaust stroke.) There is a separate ignition coil for each
pair of plugs. The spark current goes into the center conductor of one plug,
through it's gap, through "ground", then up though the other spark plug back
to the coil. So the polarity is "plus" on one plug, and "minus" on the
other. If indeed one of the plugs fires "harder" because it is the wrong
polarity, the modern high energy ignition more than makes up for it.
For the record I did reverse the coil leads when I converted my TD to
negative ground (just in case).
Bob Donahue (Still Stuck in the '50s)
Email - email@example.com
Cars: 52 MGTD - #17639
71 MGB - #GHN5UB254361
Member: NEMGTR #11470
NAMGBR # 7-3336
Hoosier MGB Club
Olde Octagons of Indiana
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