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Re: Installed new parts, car runs like shit -- Spark Plugs



First of all Carl, I hope you don't mind but I decided to share our
conversation with the rest of the list as there are many people that don't
understand this and I thought the list could benefit. I sold foreign auto
parts from 1978 until 1988 and glean this knowledge from many product
training meetings that my employer (BAP/GEON) had at the time.

First Carl wrote;

>Cool, that's what I put in last night for the track tonight ;-).
>The ones that I had in it before were an NGK SR-5, I think?  They
>look like a hotter plug (they stuck farther into the chamber).
>Was running them a bad thing?

to which I replied;

> The distance the electrodes extends into the combustion chamber has nothing
> to do with the heat range of a spark plug.

to which Carl replied;

>It wasn't just the electrode, it was the thread area and everything.
>I thought that WAS what determined the heat range, as in: the farther
>the plug sticks into the chamber the less heat it can soak into the
>head, therefore the hotter it runs...

This is a very common misconception. Put a hotter plug next to a colder
plug and unless you know where to look, they will look identical. Where
they differ in construction is inside where the ceramic insulator around
the center electrode contacts the metal body that gets screwed into the
head. On a colder plug, starting from the tip of the center electrode, the
insulator will flare out quicker and contact the metal body sooner. This
shortens the distance the distance the heat has to travel from the tip of
the plug to the head and allows the plug to dissipate heat more
efficiently. On a hotter plug, starting from the tip of the center
electrode, the insulator will flare out less quickly and contact the metal
body deeper. This lengthens the distance the distance the heat has to
travel from the tip of the plug to the head and allows the plug to
dissipate heat less efficiently.

WHAT DOES THIS MEAN TO US?

Well, keep in mind that the temperature of combustion is constant with
respect to spark plugs. Yes, running a motor leaner or richer will change
the combustion temperature, but changing a spark plug's heat range WILL
HAVE NO EFFECT ON COMBUSTION TEMPERATURE. It just affects the way the spark
plug reacts to the combustion temperature.

Assuming all other factors remain the same, if you run a plug that is too
cold for an application, the plug tip is too efficiently removing heat from
the tip. This allows combustion by products to build up on the tip instead
of being burnt off as it should. Eventually, so much crud builds up that
the plug doesn't fire. If you run a plug that is too hot for an
application, the plug tip is not removing enough heat from the tip.
Eventually, the tip of the plug melts away.

NGK NOMENCLATURE;

First letter (in our case either S or T) indicates plug thread (diameter,
pitch, depth) and construction. R indicates it is a resistor plug. 5
indicates the heat range. So in comparing the TR5 to the SR5, we can see
that they are both the same heat range and they are both resistor plugs.
The difference is in either (or both) construction or thread. I would guess
that at least the thread depth is difference.

Disclaimer: I sold almost exclusively NGKs so I cannot decipher the
nomenclature of other manufacturers. This info is 10 years old but the
principles are the same. Kids, these are trained professionals don't try
this at home. Your mileage may vary. Void where prohibited. Prohibited
where void. I'm not a lawyer or a doctor or an indian chief nor do I play
one on TV.

Rick Meredith
Saddleback Mustang Association - Webmaster
topnotch@ix.netcom.com
http://www.compapp.com/mustang/