Miata Mailing List: March 1994, Message #195
sponsored by



     


   From: Andy Poling 
Subject: Re: Something's missing...
   Date: Mon, 14 Mar 1994 11:24:31 -0500

On Sun, 13 Mar 1994, John W. Keating III wrote:
> The local parts
> store only had Bosch Platinums in stock for the Miata.  Are these
> recommended?  Or ran-away-from?

I continue to be leary of putting the Bosch Platinums in a Miata because 
of the design of the high-voltage part of the ignition system.  

Because the spark plugs are fired in pairs, one is fired backwards.  This
is fine for ordinary spark plugs with big electrode pairs, but the Bosch
Platinum design uses a VERY small center electrode (compare one to an
ordinary plug sometime and you'll see what I mean).  If the ignition
system doesn't switch polarity every firing cycle (and I doubt it does),
it the spark will carry the tiny center electrode with it until it is
insufficient (or gone - it IS small).  For those who are unaware, a high
voltage spark tends to carry a tiny bit of the originating electrode's
material with it.

There are companies who make knock-offs of the Bosh Platinum's principal
of a small electrode by using a pointed center electrode.  I have used
these successfully in my two-stroke two-wheeled buzz-bomb which has
similar ignition system high-voltage wiring.  They fight plug fouling (the
primary realistic advantage of the Bosch Platinum design) without
endangering the center electrode.  I believe I am currently using plugs
from NGK in the RZ... 

I would consider a modern ignition system like that in the Miata capable
of performing properly with darn near any spark plug.  The only thing I
would look for in a spark plug is a copper core (for temperature control)
and reasonable quality control.  Your spark plug wires are more likely to 
fail you than a modern spark plug...

If you do use the Bosch Platinums, be gentle when gapping them - the 
center insulator is somewhat more fragile than the typical center electrode.

-Andy

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