Miata Mailing List: May 1993, Message #189
sponsored by



     


   From: (none)
Subject: Spark Plugs and the Miata Ignition System (was RE:Introduction and questions)
   Date: (none)

On Sun, 23 May 1993, Eric Fancher wrote:
[...]
> Go with the splitfires, I picked up at least a mile per gallon and felt some 
> kick in performance althoughthis is questionable and not documented on my 
> part.  A while back the Miata Magazine had an article on plugs and their 
> concensus was to avoid the platinums because the Miata engine fires each plug 
> twice during the four cycles.  Also something about the grounding into the 
> head and thus affecting the other cylinders.  I'll try to look up the article 

Ah - this triggered a few sleeping/burned out memory cells.  As I recall,
the Miata ignition system does the same thing as my old Honda 600 Sedan:
spark plugs are fired in pairs by sending the ignition voltage from the
ignition coil, through the center electrode of one plug, to the side
electrode (and thus the engine block), then through the side electrode of
the other plug, to the center electrode, and back to the ignition coil. 
Thus in one plug, the spark jumps from the center electrode to the side
electrode, while in the other plug the spark jumps from the side electrode
to the center electrode.

The problem here is that plugs like Bosch's Platinum's assume that the
spark will always jump one way.  When the spark jumps, it has a tendancy
to carry a teeny weeny amount of the electrode metal with it and deposit
it on the electrode where the spark is juumping to.  With most regular
plugs, there is sufficient electrode mass that this can work either way
without harm.  But with the Platinums, the center electrode has very
little metallic mass and so probably cannot stand too much reverse spark
polarity without holing the center electrode.

If the Mazda engineers were clever, they would have arranged to reverse
the polarity of the spark every other time it fired to even out electrode
mass transfer.  I have no idea whether they did or not. 

God, I'm being a geek again aren't I...  :-)

-Andy

PS- Did anybody else see the little feature during the IROC race about Bosch's
    latest plug design which uses a solid ring as the side electrode?  Kinda
    reminds me of the ole foul-proof plugs with multiple side electrodes.

Andy Poling                              Internet: andy@jhunix.hcf.jhu.edu
UNIX Systems Programmer                  Bitnet: ANDY@JHUNIX
Homewood Academic Computing              Voice: (410)516-8096    
Johns Hopkins University                 UUCP: uunet!mimsy!aplcen!jhunix!andy