Most Everything You Ever Wanted To Know About Spark Plugs
When it comes to maintaining peak engine performance, nothing is as
important as the spark plugs. The spark that jumps across the gap
between the plugs electrodes ignites the fuel mixture and releases the
explosive power that makes the engine go. So if there's no spark, or the
spark fails to light the fire, the engine experiences a misfire.
Misfires are bad news because they allow unburned fuel to blow right
through the combustion chamber and enter the exhaust. Most of the
unburned hydrocarbons that pass through an engine will be cleaned up by
the catalytic converter. But an overdose of raw fuel due to ignition
misfire can make the converter run dangerously hot. That's why ignition
misfire is a common cause of converter failure.
Misfires also hurt engine performance and fuel economy. Every misfire
equals a missed power stroke. A steady misfire in a four cylinder engine
can rob it of 25% of its potential power output. Misfires are less
noticeable in V6 and V8 engines, but still have the same effect.
Every engine misfires occasionally, but if the spark plugs are worn or
fouled, or the ignition system cant deliver enough voltage for reliable
ignition, misfires may cause hard starting, rough running, hesitation,
increased fuel consumption and elevated hydrocarbon (HC) emissions. It
doesn't take many misfires to make a vehicle fail an emissions test.
A steady misfire is relatively easy to detect because you can hear it
and feel the loss of power. Hook up an ignition scope and the misfire
will be obvious in a secondary parade or raster pattern.
A higher-than-normal firing voltage in one cylinder indicates an open
plug wire or no spark across the plugs electrodes. A lower-than-normal
firing voltage would tell you there's a fouled plug or grounded plug
wire. Intermittent misfires are the ones that can drive you nuts. An
intermittent misfire may only occur when the engine is under load,
accelerating or idling. The problem here may not be the spark plugs but
some other condition that is upsetting the air/fuel mixture causing it
to go lean.
When a V6 engine is cruising down the highway at 3,000 rpm, each spark
plug is firing 25 times per second. At this speed, its virtually
impossible to feel or hear an occasional misfire. That's why all 1996
and newer cars and light trucks are equipped with OBD II to detect
misfires. If the rate of misfire is high enough to cause a 50% or higher
increase in emissions, OBD II will illuminate the Check Engine light and
set a diagnostic fault code that corresponds to the misfire. Using a
scan tool, the OBD II code will then tell you which cylinder is
misfiring, or give you a random misfire code. If the misfire is isolated
to a single cylinder, you can check the spark plug, plug wire, fuel
injector and compression to determine the cause. Or, if you find a
random misfire code, you can look for a vacuum leak or other problem
that is causing the fuel mixture to run lean (leaky EGR valve, weak fuel
pump or fuel pressure regulator, dirty injectors, water in the gas,
To minimize the risk of misfire and maximize ignition performance,
today's spark plugs are designed to resist fouling and wear under a wide
range of operating conditions. Improvements in electrode alloys allow
many plugs to now last up to 100,000 miles.
Platinum is the buzzword today for durability. Platinum is one of the
best conductors of heat and electricity known to man. It also resists
chemical corrosion and electrical erosion much better than steel alloys,
making it an ideal material for the electrode(s) in a spark plug. Some
plugs have a solid platinum center electrode while others have a small
button of platinum welded onto the tip of the center electrode or both
electrodes (single platinum vs. double platinum).
The main reason for using platinum electrodes is to minimize electrode
wear. Every time a plug fires, a tiny amount of metal is vaporized and
lost from the surface of both electrodes. The center electrode typically
suffers the most wear because it runs hotter than the side electrode.
As the electrodes wear, the air gap across which the spark must jump
becomes wider and wider. The gap on a standard spark plug grows about
0.00063" to 0.000126" for every 1,000 miles of normal driving. And the
wider the gap, the greater the voltage needed to jump the gap. On
standard plugs, the firing voltage requirements creep up about 500 volts
for every 10,000 to 15,000 miles of driving. Eventually the plug may
need more volts to fire than the coil can produce, causing the plug to
Using platinum almost eliminates electrode wear. Platinum is expensive,
but it can double or even triple a spark plugs normal service life
from 30,000 to 45,000 miles for a standard plug up to 60,000 to 100,000
miles or more with platinum. Most aftermarket plug suppliers do not make
specific mileage claims for their platinum plugs but say to follow the
OEM replacement intervals - which in most cases is 100,000 miles for
Though long-life platinum plugs cost more than standard spark plugs,
the OEMs are using them for several reasons. One is that they reduce the
risk of misfire, which is essential to meet OBD II requirements. Another
is that they help prolong the life of the catalytic converter (also by
reducing misfires). Third, they almost eliminate the need for periodic
maintenance. On many engines today, replacing the spark plugs can be a
difficult and time-consuming job - particularly the back bank of plugs
on transverse-mounted V6 engines in front-wheel drive cars and mini vans.
One important point to keep in mind about platinum plugs is that
they're not all the same. Some are more durable than others, and some
provide better fouling resistance and ignition performance than others. It
all depends on the design of the plug, the type of alloys used for both the
center and side electrode(s), the configuration of the electrodes and
the engine application.
A variety of exotic alloys are used in electrodes to improve durability
and performance. Bosch, for example, uses a nickel-yttrium alloy for the
side electrodes in its "Platinum Plus 4" spark plugs. In Europe, Bosch
recently introduced a new plug that uses yttrium for both the center and
ground electrodes. For years, Champion manufactured a premium plug with
a gold-palladium center electrode and copper-filled side electrode.
Champion has discontinued their gold plug and is now offering a premium
"Platinum Power" plug with a platinum-tipped center electrode. Autolite
uses a chromium-nickel alloy for the ground electrode with its
platinum-tipped center electrode plug, while ACDelco uses a
silver-nickel alloy side electrode with its platinum-tipped plugs.
NGK and Denso have both introduced new premium plugs with iridium
alloy electrodes. NGK says iridium is even better than platinum in terms
of corrosion and wear resistance, and ignition performance.
Regardless of the type of alloy used in the electrodes, all spark
plugs have to resist fouling. The trick here is to design the plug so the
electrodes run hot enough to burn off any deposits but not so hot that
they cause preignition or detonation. To burn off carbon deposits, the
electrode needs to reach about 700 F quickly. But if the electrodes get
too hot (above 1,500 F), it can ignite the fuel before the spark
occurs, causing preignition and detonation. For most plugs, the ideal
operating temperature is around 1,200 F.
The temperature of the electrodes is controlled by the length of the
ceramic insulator that surrounds the center electrode and the design of
the electrode itself. Ceramics do not conduct heat very well, so an
insulator with a relatively long nose will conduct heat away from the
electrode more slowly than one with a relatively short nose. The longer
the path between the electrode and the surrounding plug shell, the
slower the rate of cooling and the hotter the plug.
Many plugs have a copper core center electrode. Copper is an excellent
conductor of heat, and allows the plug to dissipate heat quickly under
load yet remain hot enough at low speed and idle to burn off fouling
A spark plugs "heat range" (heat rating), therefore depends on the
length of the ceramic insulator and the design of the center electrode.
The heat range must be carefully matched to the engine application
otherwise the plugs may experience fouling problems or run too hot and
cause preignition/detonation problems. Most plugs today have a
relatively broad heat range, which means they reach a self-cleaning
temperature quickly but don't get too hot under load. This allows plug
manufacturers to consolidate applications and use fewer plugs to cover a
wider range of engines.
When replacing spark plugs, the heat range must be correct for the
engine application. Always follow the vehicle or spark plug
manufacturers recommendations. If the plugs are too cold, fouling may
occur if the vehicle spends a lot of time idling or is only used for
short trips (especially during cold weather). If the plugs are too hot,
the engine may experience preignition and detonation problems under load
or during hot weather.
In some situations you may want to install a slightly hotter or colder
plug than the one normally recommended. Switching to a slightly hotter
plug can help reduce fouling in an older engine that uses oil. A hotter
plug can also reduce fouling in vehicles that spend a lot of time idling
or are only used for short-trip, stop-and-go city driving. But a hotter
plug should not be used unless an engine is experiencing a fouling
problem because of the increased risk of preignition and detonation.
Switching to a slightly colder plug can reduce the risk of preignition
and detonation in performance applications (especially turbocharged and
supercharged engines), vehicles used for towing, or those that are
driven primarily on the highway.
Many spark plugs today have unique electrode designs such as V-split,
grooved or clipped ground electrodes, multiple ground electrodes, fluted
center electrodes, V-notched center electrodes, etc. Though each plug
manufacturer takes a slightly different approach and claims various
benefits for their design, the basic idea is to make it as easy as
possible for the spark to jump the gap and ignite the fuel mixture. A
spark jumps more easily between small, sharp surfaces than large dull
ones - which is another reason why new plugs require less firing voltage
than old ones with worn, dull electrodes.
Another issue that plug manufacturers talk about is "unshrouding" the
spark so it has a better opportunity to ignite the fuel mixture. Opening
up the spark also means the flame kernel it creates can expand more
rapidly and evenly inside the combustion chamber, reducing the chance of
the flame kernel being quenched and a misfire occurring. Split-Fires
V-shaped ground electrode as well as Boschs surface gap four electrode
Platinum Plus 4 are both designs that claim to expose more of the spark
to the fuel mixture.
One thing to keep in mind about all "performance" spark plug designs
is that no plug can magically create horsepower out of thin air or add
horsepower that wasn't there in the first place. But improved ignition
reliability can minimize horsepower losses caused by misfires. That's
why some plug manufacturers claim their spark plugs improve power. The
gains come from power that was being lost to misfires.
Plug Replacement Tips
Regardless of what type of plugs you choose to install, use care on
engines with aluminum cylinder heads. Wait until the engine has cooled
to change the plugs. This will minimize the risk of damaging the threads
in the plug hole.
Most threads on spark plugs designed for aluminum head applications are
precoated to reduce the risk of thread damage. If you're in the habit of
applying a drop of antiseize compound to the plug threads before they go
in for added insurance, you might want to reconsider this practice. One
vehicle manufacturer warns against this practice because antiseize acts
like a lubricant and may allow the plugs to be overtightened, which can
damage the threads. If you do use antiseize on the threads, their advice
is to reduce the tightening torque on the plugs 40%.
How much the plugs should be tightened depends on the size of the plugs
and the type of plug seat. Spark plugs with gasket-style seats require
more torque than those with taper seats.
Always follow the vehicle manufacturers torque recommendations, but as
a general rule: 14 mm plugs with a gasket-style seat should be tightened
to 26 to 30 ft. lbs. in cast iron heads, but only 18 to 22 ft. lbs. in
aluminum heads. Likewise, 18 mm plugs with gasket-style seats should be
tightened to 32 to 38 ft. lbs. in cast iron heads but only 28 to 34 ft.
lbs. in aluminum heads. For taper seat spark plugs, 14 mm plugs should
be tightened to 7 to 15 ft. lbs. in both cast iron and aluminum, while
18 mm taper seat plugs should be tightened to 15 to 20 ft. lbs. in both
types of heads.
As for setting the plug gap, always follow the vehicle manufacturer
recommendations - unless you are installing a set of Bosch
spark plugs. These plugs are pregapped at the factory to a standard 1.6
mm gap. This is necessary to achieve maximum plug performance and
longevity so don't change the gap.
Finally, play close attention to the condition of the spark plug wires
and boots. Loose-fitting boots or damaged wires can cause ignition
misfire. Also, make sure the wires are properly routed and supported in
their looms to avoid crossfire problems and contact with the hot exhaust
And also found.....
Courtesy of ACDelco
Proper Installation of Spark Plugs:
STEP 1: Make sure that cylinder head threads and spark plug threads are
clean. If necessary, use a thread chaser and seat cleaning tool.
STEP 2: Make sure that the spark plug gasket seat is clean, then thread
the gasket to fit flush against the gasket seat. Tapered seat spark
plugs do not require gaskets.
STEP 3: Use a gap guide to make sure new spark plugs have the correct
STEP 4: Screw the spark plugs finger-tight into the cylinder head. Use a
torque wrench to tighten spark plugs following manufacturer's
Note: Do not use any type of anti-seize compound on spark plug threads.
Doing this will decrease the amount of friction between the threads. The
result of the lowered friction is that when the spark plug is torqued to
the proper specification, the spark plug is turned too far into the
cylinder head. This increases the likelihood of pulling or stripping the
threads in the cylinder head. Over-tightening of a spark plug can cause
stretching of the spark plug shell and could allow blowby to pass
through the gasket seal between the shell and insulator. Over-tightening
also results in extremely difficult removal.