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Re: reversing positive ground

To: "mg-t" <mg-t@Autox.Team.Net>
Subject: Re: reversing positive ground
From: "Bob Donahue" <>
Date: Sat, 8 Jan 2005 14:26:30 -0500
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 -
Cars:       52 MGTD - #17639
               71 MGB - #GHN5UB254361
Member:  NEMGTR #11470
               NAMGBR # 7-3336
               Hoosier MGB Club
               Olde Octagons of Indiana

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