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  • Title Page- All About Spark Plugs
  • Original Author- GOrilla

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Posted by GOrilla on Mon Oct 12 08:10:01 1998:

In reply to: Re: Replacing Spark Plugs?? posted by Marty Rood on Mon Oct 12 01:18:00

I'm with Rikki, the tech board has been slow. And I woke up early feeling techy today. I
have been doing more of the artsy-fartsy picture stuff and posting ride info on the
TXVOA. (Ya'll come by for a visit :)

Below is a long winded post, just got to typing and created a post on gathered facts and
in general what I know about spark plugs.

Marty has a good philosophy, if it at broke don't fix it. I generally don't consider changing
my everyday vehicle plugs til about the 60k range. My Hi-pro Mustang plugs are changed
every 5k miles for reasons I will list below. My Valk has almost 9k miles and I feel they have
a ways to go before I will change them, probably around 15k miles or just before I go to
Crawford. Not, that I would need to, it's all a matter of personal preference and how I feel
about engine performance. My selection of the Valks next plug has yet to be determined. I
will probably remain with the stock plugs because they seem to be doing fine. There are a
lot of hype plugs out their that claim to increase horsepower and mileage but I have never
seen one yet with a good write-up from a outside reputable source.

As spark plugs grow older, they lose their sharp edges and material from the center and the
ground electrode is slowly eroded away. As the gap between these two points grows, the
voltage required to bridge this gap increases proportionately. The ignition system must
work harder to compensate for this higher voltage requirement and hence there are a
greater rate of misfires, or incomplete combustion cycles. Each misfire means lost
horsepower, reduced fuel economy and higher emissions. Replacing worn out spark plugs
with new ones (with sharp new edges) effectively restores the ignition system's efficiency
and reduces the percentage of misfires, restoring power, economy, and reducing emissions.
How long spark plugs last will depend on a variety of factors, including engine
compression, fuel used, gap, center/ground electrode material, how the vehicle is used, etc.

Installing spark plugs

The spark plug performs four main functions:

1. It fills a hole in the cylinder head.(a little levity:)
2. It acts as a dielectric insulator for the ignition system.
3. It provides spark for the combustion process to occur.
4. It removes heat from the combustion chamber.

It is important to remember that spark plugs do not create heat, they help remove it.
Anything that prevents a spark plug from removing the proper amount of heat can lead to
pre-ignition, detonation, premature spark plug failure and even internal engine damage,
especially in two stroke engines or modified engines.

In the simplest of terms, the spark plug acts as the thermometer of the engine....much like a
doctor examining a patient, this "thermometer" can be used to effectively diagnose the
amount of heat present in each combustion chamber.

With some experience you can interpet the plugs' visual cues and accurately determine the
engine's overall operating condition, get a feel for air/fuel ratios, and even diagnose
driveability problems. Spark plugs are valuable tuning tools, when interpreted correctly.

They will show symptoms of other problems and can reveal a great deal about the engine's
overall condition.

With common sense and experince you can evaluate the appearance of the spark plug's
firing tip and will begin to make a diagnosis or some basic assumptions based on the
physical appearance of the spark plugs.

Reading spark plugs can be a valuable tuning aid. By examining the Insulator firing nose
color, you can determine much about the engine's overall operating condition. In general,
a light tan/gray color tells you that the spark plug is at the optimum temperature and that
the engine is in good operating condition. Dark coloring, such as heavy black wet or dry
deposits usually indicate a fouling problem. Heavy, dry deposits can indicate an overly
rich condition, too cold a heat range spark plug, possible vacuum leak, low compression,
overly retarded timing, or too large a plug gap. If the deposits are wet, it can be an
indication of a breached head gasket, oil control from rings or valvetrain problems, or an
extremely rich condition, depending on what liquid is present at the firing tip. One must
also look for signs of detonation, such as silver specs, black specs, or melting or breakage
at the firing tip. Signs of fouling or excessive heat must be traced quickly to prevent further
deterioration of performance and to prevent possible engine damage.

Believe it or not torque is one of the most critical aspects of spark plug installation. Torque
directly affects the spark plugs' ability to transfer  heat out of the combustion chamber. A
spark plug that is under-torqued will not be fully seated on the cylinder head, hence heat
transfer will be slowed. This will tend to elevate combustion chamber temperatures to
unsafe levels, and pre-ignition and detonation will usually follow. Serious engine damage
is not far behind. An over-torqued spark plug can suffer from severe stress to the Metal
Shell which in turn can distort the spark plug's inner gas seals or even cause a hairline
fracture to the spark plug's either case, heat transfer can again be slowed and
the above mentioned conditions can occur.

The spark plug holes must always be cleaned prior to installation, otherwise you may be
torquing against dirt or debris and the spark plug may actually end up under-torqued, even
though your torque wrench says otherwise. Of course, you should only install spark plugs
in a cool engine, because metal expands when its hot and installation may prove difficult.
Proper torque specs for both aluminum and cast iron cylinder heads are listed below.


Since the gap size has a direct affect on the spark plugs' tip temperature and on the voltage
necessary to ionize(light) the air/fuel mixture, careful attention is required. While it is a
popular misconception that plugs are pre-gapped from the factory, the fact remains that the
gap must be adjusted for the vehicle that the spark plug is intended for. Those with
modified engines must remember that a modified engine with higher compression or forced
induction will typically require a smaller gap settings (to ensure ignitability in these denser
air/fuel mixtures). As a rule, the more power you are making, the smaller the gap you will

A spark plug's voltage requirement is directly proportionate to the gap size.  The larger the
gap, the more voltage is needed to bridge the gap. Most experienced tuners know that
opening gaps up to present a larger spark to the air/fuel mixture, maximizing burn efficiency.
It is for this reason that most racers add high power ignition systems. The added power
allows them to open the gap, yet still provide a strong spark. With this mind, many think
the larger the gap the better. In fact, some aftermarket ignition systems boast that their
systems can tolerate gaps that are extreme, be wary of such claims. In most cases, the
largest gap you can run may still be smaller than you think.


This is for racers only !!
Indexing refers to a process whereby auxiliary washers of varying thickness are placed
under the spark plug's shoulder so that when the spark plug is tightened, the gap will point
in the desired direction. However, without running an engine on a dyno, it is impossible to
gauge which type of indexing works best in your engine...while most engines like the spark
plug's gap open to the intake valve, there are still other combinations that make more power
with the gap pointed toward the exhaust valve.

In any case, engines with indexed spark plugs will typically make only a few more
horsepower, typically less than 1% of total engine output...for a 500hp engine, you'd be
lucky to get 5hp...while there are exceptions, the bottom line is that without a dyno,
gauging success will be difficult.

Heat Range Selection

Let's make this really simple: when you need your engine to run a little cooler, run a colder
plug. When you need your engine to run a little hotter, run a hotter spark plug. However,
going to a hotter spark plug can sometimes mask a serious symptom of another problem
that can lead to engine damage. Be very careful with heat ranges. Seek professional
guidance if you are unsure. With modified engines (those engines that have increased
their compression), more heat is a by-product of the added power that normally comes with
increased compression.

In short, select one heat range colder for every 75-100 hp you add, or when you
significantly raise compression. Also remember to retard the timing a little and to increase
fuel enrichment and octane. These tips are critical when adding forced induction (turbos,
superchargers or Nitrous kits), and failure to address ALL of these areas will virtually
guarantee engine damage.

An engine that has poor oil control can sometimes mask the symptom temporarily by
running a slightly hotter spark plug. While this is a "Band-Aid" approach, it is one of the
only examples of when and why one would select a hotter spark plug.

Using "racing" spark plugs

Be cautious!! In reality, most "racing" spark plugs are just colder heat ranges of the street
versions of the street spark plug...they don't provide any more voltage to the spark plug
tip!! Their internal construction is no different than most standard spark plugs. There are
some exceptions, though. Extremely high compression cars or bikes or those running exotic
fuels will have different spark plug requirements and hence spark plugs that are well-suited
for these requirements...they are classified as "specialized spark plugs for racing
applications". Some are built with precious metal alloy tips for greater durability for their
ability to fire in denser or leaner air/fuel mixtures. If you are looking to "screw in" some
cheap horsepower , in most cases, spark plugs are not the answer.

To be blunt, when experienced tuners build race motors, they select their spark plugs for
different reasons: to remove heat more efficiently, provide sufficient spark to completely
light all the air/fuel mixture, and to survive  the added stresses placed upon a high
performance engine's spark plugs, and to achieve optimum piston-to-plug clearance.

Some of these "specialized racing plugs" are made with precious metal alloy center/ground
electrodes or fine wire tips or retracted-nose insulators...again, these features do not
necessarily mean that the spark plug will allow the engine to make more power, but these
features are what allow the spark plug to survive in these tortuous conditions....most racers
know screwing in a new set of spark plugs will not magically "unlock" hidden horsepower.

Using high power ignition systems (or what I know about car ignitions) Many of the more
popular aftermarket ignition systems are of the capacitive discharge type. They store
voltage, or accumulate it, until a point at which a trigger signal allows release of this more
powerful spark. Companies like Mallory, MSD, Crane and Accel, to name a few, offer such
systems. They affect spark plugs in that they allow the gaps to be opened up to take
advantage of the increased capacity. The theory is that the larger and the more intense the
spark you are able to present to the air/fuel mixture, the more likely you will be to burn more
fuel, and hence the more power you will make. I have only used hi-pro ignitions on
modified or older non-computer controlled vehicles. In reality, computer controlled vehicles
do such a good job of lighting off the air/fuel mixture (as evidence by the ultra-low
emissions), added ignition capacity would do little to burn more fuel since the stock
configuration is doing such a good job. Older non-computer controlled vehicles or those
that have been modified with higher compression or boosted (nitrous, turbo, supercharged)
engines can certainly take advantage of a more powerful ignition system.

Looks like I rambled on for more than my 2 cents worth.


Addendum Notes From GOrilla

Different thread diameter spark plugs require different torque specs.

For most 14 mm spark plugs, about 18-22 lb/ft is accurate.

Aluminum heads will take slightly less torque than cast iron heads.

As a basic rule, for gasket type spark plugs, use 1/2 - 3/4 of a turn after finger tight.

For taper seat spark plugs, use only 1/16 - 1/8 of a turn after finger tight.  F6Rider Webzine    ShopTalk    Just Pics    Valkyrie Hot Links