Spark Plug Tech Tips – as taken
from NGK’s technical site
1. Installing spark plugs
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 insulator...in 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. (a good
practice is to blow off the spark plug base/head area of the car with compressed
air WHILE the plugs are still in the heads, to clean away any sand/dirt and
grit that may fall into the cylinder head and cause damage.) 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. And remember,
ALWAYS lightly coat new plugs with some form of Anti-Sieze to prevent plug
galling and allowing proper torque specs on dissimilar metals. Also
this will prevent seizing of plug threads into heads and force expensive
and risky re-threading of spark plug holes in cylinder heads
Spark plug type Thread Diameter
Cast Iron Cylinder Head (lb-ft.)
Aluminum Clyinder Head (lb-ft.)
Flat seat type
(with gasket) 18
(with gasket) 14
(with gasket) 12
(with gasket) 10
(with gasket) 08
Conical seat type
(without gasket) 18 ø mm
Conical seat type
(without gasket) 14 ø mm
Since the gap size has a direct affect on the spark plug's 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 need.
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 maximizes 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.
On modern fuel injected cars, PDM Racing has found indexing of plugs a time
consuming, and often exercise in futility. Only on some of our older
(read: pre 1970’s) vehicles, have we found noticeable differences by indexing
plugs. But this is with factory ignition systems on non-multi valve
4. Heat Range selection
Let's make this really simple: when you need your engine to run a little
cooler, run a colder plug (higher number). When you need your engine to run
a little hotter, run a hotter spark plug (lower number). However, NGK strongly
cautions people that 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
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
What we recommend in our cars is from years of experience, and testing of
plugs back to back in race situations. However, when in doubt, we always
recommend going back to the manufacturer’s suggested heat range.
5. Using "racing" spark plugs
Be cautious! In reality, most "racing" spark plugs are just colder heat ranges
of the street versions of the spark plug. They don't provide any more voltage
to the spark plug tip! Their internal construction is no different (in NGK's
case, as all of our spark plugs must conform to the same level of quality
controls) than most standard spark plugs.
There are some exceptions, though. Extremely high compression cars or those
running exotic fuels will have different spark plug requirements and hence
NGK makes 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 and the ability
to fire in denser or leaner air/fuel mixtures. However, installing the same
spark plugs Kenny Bernstein uses in his 300+ mph Top Fuel car (running Nitromethane
at a 2:1 air/fuel ratio and over 20:1 dynamic compression) in your basically
stock Nissan 240SX (running 15:1 a/f ratios with roughly 9.5:1 compression)
will do nothing for you! In fact, since Kenny's plugs are fully 4 heat ranges
colder, they'd foul out in your Nissan in just a few minutes.
NGK as a company tries to stay clear of saying that a racing spark plug (or
ANY spark plug) will give you large gains in horsepower. While certain spark
plugs are better suited to certain applications (and we're happy to counsel
you in the right direction) we try to tell people that are looking to "screw
in" some cheap horsepower that, 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, 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.
6. Using high power ignition systems
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
We encourage the use of such systems, but only on modified or older non-computer
In reality, computer controlled vehicles do such a good job of lighting off
the air/fuel mixture (as evidenced 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
If your Nissan or Subaru is relatively stock, your OEM ignition system is
more than enough to handle your car’s spark requirement.