Miata Mailing List: September 1993, Message #64
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   From: Herb Samuels 
Subject: Re: Performance Mods
   Date: Wed, 8 Sep 1993 00:29:03 -0400

The K & N Filter may be good for 1 or 2 horsepower at higher RPMs. 
Regarding Splitfire plugs, I installed them at the suggestion of Miata
Magazine about 2 1/2 years ago. The gave better driability than the
plugs which came with the car.  They were Japanese but not NGK. 
Recently I changed them for NGK plugs which I found as good or perhaps
better then the Splitfire plugs at 1/3 of the cost.  The NGKs gave
excellent drivability.  However, they were newer than the old
Splitfire plugs that I had in the car so I bought a new set (you can
return them for a full refund for up to thirty days) and I compared
then to the new NGK plugs.  Both gave good power and response but the
NGK plugs gave a somewhat smoother response than the Splitfire's.  The
NGK plugs also have a split electrode design with the bottom electrode
split rather than the top electrode as with the Splitfires.  All in
all, from my recent experience the NGKs are comparable or better than
the Splitfires and much less expensive.  I've also heard some horror
story rumors about Splitfire plugs having premature failure with
breakage of the top electrode.  I don't know whether they are true or
not.  

One thing to be careful about with the Splitfire plugs is that
the sealing washer is not permenantly fixed to the plug so that when
you remove the plug the washer can remain behind recessed in the engine. 
I had heard about this and when I recently removed the Splitfire plugs
when testing them I found that the washers of two of the four plugs 
were left behind and they were a pain to get out. Make sure that they
are out before you install anything else.

Herb Samuels