The following is reprinted from
Cycle Magazine, October 1977. Nearly all of it is
just as pertinent today as it was twenty years ago.
Thanks to GJ for all the great tech articles over
the years, and for permission to reprint this one. -js
How
you can read spark plugs and select them - by Gordon
Jennings
Stay with motorcycling long
enough to swat a few gnats with your nose and you
will at least begin to realize how much there is to
know about spark plugs. Bikers like to tinker, and
will replace spark plugs even if they don't venture
anything else. And in just replacing plugs the
motorcyclist becomes acquainted with the fact that
there is more than meets the eye.
The
first thing you have to learn is that there are some
important differences in spark plugs' threaded ends,
which are made in four diameters and lengths. Most
plugs' thread diameter is a nominal 14 millimeters,
but Honda -for example- uses 10mm plugs in small
displacement engines and l2mm plugs spark all the
Honda Fours. There also are 18mm plugs, seen only
rarely in motorcycle applications despite the
advantage they bring to two-stroke engines. At one
time you had to cope with slight differences in
thread configuration on spark plugs from different
countries; this worry mercifully has been ended by
an international standardization of thread forms.
Because differences in thread
diameters are so large, few people get into trouble
through trying to stuff a l4mm plug into a 12mm hole
-or vice versa. The same isn't true of plugs'
threaded lengths, or "reach." Setting aside for the
moment the small variations created by the use of an
inch-based standard in a mostly-metric world, there
are just four nominal reach dimensions: 3/8-inch,
1/2-inch, 7/16-inch and 3/4-inch. These dimensions
are followed by engine manufacturers in the depths
they give plug holes, and the idea is that the lower
end of the plug's threaded shank should come up
flush in the combustion chamber.
We know from personal
observation that people do make plug-reach mistakes;
using 3/4-inch plugs in 1/2-inch holes is the most
common error, and one fraught with unpleasant
consequences. One of the disasters you can have from
using a long-reach plug in a short-reach hole is
purely mechanical in nature. In time the plug
threads exposed inside the combustion chamber may
become filled with hard-baked deposits. If that
happens you'll find it almost impossible to remove
the plug without also removing the plug hole
threads. Reversing this kind of mistake, using a
plug reach too short for the hole, lets deposits
fill the plug hole's exposed threads and may cause
difficulties when you try to install a plug having
the correct reach.
The worst and most immediate
problem created by an overly-long plug in an engine
is that the exposed threads absorb a terrific amount
of heat from the combustion process. This raises the
plug-nose temperatures, and may take them up high
enough to make the side electrode function as a glow
plug. And when that happens you have the white-hot
electrode firing the mixture far too early, like an
over-advanced spark timing but worse because the
early ignition causes yet higher combustion chamber
temperatures, which causes even earlier ignition.
This condition is known as "runaway pre-ignition,"
and if it is allowed to proceed it will wreck your
engine.
Even a single plug thread
exposed in an engine's combustion chamber will raise
electrode temperatures quite markedly. That could be
a real problem as engine makers don't hold plug-hole
depths to close tolerances, and the near-universal
adoption of crushable plug washers gives the user a
chance to compound errors by over-tightening when
installing fresh plugs. Spark plug manufacturers
have solved the problem by leaving an unthreaded
relief at plugs' lower ends. The relief also serves
as a pilot, guiding a plug straight into the plug
hole. Finally, the relief accommodates differences
in opinion between plug makers about how nominal
reach dimensions should translate into actual metal
- and there are some small differences.
Matters of thread diameter and
length resolved, you can still get into trouble with
a spark plug property called "heat range." All
conventional plugs, whatever the application, have
to stay hot enough to burn away deposits (oil,
carbon, etc.) that otherwise would short-circuit the
spark, and that places the lower limit for
temperature at about 700 degrees F. There are
multiple upper limits for plug temperature:
sulfurous fuel elements begin chemical erosion of
the electrodes above 1100 F.; oxidation of
nickel-alloy electrodes begins at 1600-1800 F.; and
at some point (which depends upon compression ratio,
mixture, throttle setting, etc.), the electrodes
will be hot enough to cause pre-ignition. So, to be
safe, plug temperatures must be held between 700 F.
and 1000 F. over the whole range of operating
conditions.
If all engines, and riders,
were identical, the spark plug manufacturers' jobs
would be easy, as a single plug would be suitable
for all applications. Instead, engines vary
enormously, as do specific operating conditions, and
so the plugs themselves have to be given equally
varied thermal characteristics. This is done by
varying the length of the path taken by heat as it
travels from the very hot center electrode and
insulator nose to the relatively cool areas around
the body's threads and the plug washer. Plugs with a
long insulator nose, which leads heat high into the
plug body before it turns back toward the cooler
cylinder head, are "hot." Short-nosed plugs, with a
shorter heat path, are "cold." And these terms are
very misleading, as in all cases the object is to
match the thermal characteristics of plug and engine
so the electrode temperature will stay between 700
F. and 1000 F. We must emphasize that it is the
engine that puts heat into the plug, and not the
reverse. A "hot" plug does not make an engine run
hotter; neither does a "cold" plug make if run
cooler.
The entire question of heat
range is something most people find terribly
perplexing - and deal with simply by following the
recommendations of their bike's manufacturer. But
this does not always yield satisfactory results,
because many motorcycle engines make impossible heat
range demands. Free-air cooling broadens the range
of engine temperatures; so does the typical bike
engine's specific power output, which is a level
encountered only in outright racing engines little
more than a decade ago. Manufacturers tend to
specify plugs with heat ranges chosen with an eye
toward "worst-condition" operation, which means that
bikes' original equipment spark plugs often are a
bit cold for those who ride conservatively.
Unfortunately, the conservative rider is mostly
likely to also be conservative in other ways, and in
most cases will stick with whatever plug his owners
manual suggests; the speed merchants, who are the
people manufacturers have in mind when they make
their heat-range recommendations, usually assume
their own bikes need colder plugs.
Knowing which plugs are hotter
or colder than the ones you presently have in your
bike is easy if you're content to stay with the same
brand. Nearly all of the world's plug makers use a
number-based code to designate heat range: foreign
firms follow a system in which higher numbers mean
colder plugs; American companies do just the
opposite, assigning hotter plugs higher numbers.
Unfortunately, there is no semblance of order beyond
this point. One company, Champion, is in a state of
nomenclature transition that makes its product line
inordinately confusing. The American Rule applies at
Champion, but in an odd way, spread across three
series of heat ranges that encompass touring and
racing spark plugs, old and new, with double-digit
numbers assigned to some and single digits for
others.
Bosch's three-digit numbers
are a holdover from the early days, when plugs were
rated for engines' "indicated mean effective
pressure." But combustion chamber pressures alone
soon proved inadequate, for it was found that the
thermal load on a plug also depended upon spark
timing, cylinder head cooling and even on the flow
of mixture into the cylinder. These factors greatly
complicate the business of assigning plugs thermal
ratings. Each spark plug manufacturing firm has its
own test procedure, and though there are efforts
being made to bring the whole thing under some
international standard no agreement exists today.
On the other hand, there is an
enormous amount of mutual product testing being
done, and this enables plug manufacturers to offer
accurate cross-brand conversion charts. However, it
should be understood that the equivalents are not
exact. When plug maker-A's chart shows "equivalents"
from maker-B and maker-C it only means those are the
nearest equivalents; they aren't necessarily
identical. This creates a little confusion, and an
opportunity: if you think a particular plug is just
a hair too hot or too cold, try its equivalents in
other brands. You might hit upon precisely the
thermal characteristics you want.
The last point of confusion in
the area of heat range is the fact that the
progression of numbers within a manufacturer's line
of plugs may not accurately reflect the extent of
the shift toward hotter or colder thermal grades. It
appears that all the companies began with some neat,
evenly-spaced arrangement of numbers and heat
ranges, and then had to shuffle everything around to
align themselves with reality. Apparently some plugs
are thermally biased, hotter or colder, to make them
better suited to particular applications - as when
an engine manufacturer is willing to order large
volumes of plugs if they're biased to suit his
needs. And if one of a plug maker's best-sellers is
biased colder, while the next-warmer thermal grade
is biased a bit hotter, you get a kind of heat-range
gap, which can be bridged only by switching brands.
There is more to spark plugs
than just thread diameter and reach, and heat range.
Cramped installations have created plugs with stubby
insulators and small-hex bodies; aircraft plugs
often require strange provisions for shielding;
aerospace work has brought us spark plugs that look
like a death ray firing-pin. Most of the far-out
variety have no conceivable application in
motorcycling and can be ignored; but there are a few
"special" spark plugs you definitely should know
about.
One
very useful variation of the standard spark plug has
its insulator nose and electrodes extended from its
metal shell. The projected-nose configuration moves
the spark gap a bit farther into the combustion
chamber, which tends to improve efficiency by
shortening the distance traveled by the flame front
and also making the combustion process more regular.
But there is a more important benefit: the
projected-nose plug provides, in many engines, what
effectively is a broader heat range than you get
with the conventional flush-nose type. The projected
nose is more directly exposed to the fire in the
combustion chamber, and quickly comes up to a
temperature high enough to burn away fouling
deposits after ignition occurs. Then during the
subsequent intake phase this plug's exposed tip is
cooled by the swirling air/fuel mixture. In this
fashion the higher temperatures existing at
full-throttle operating conditions are to some
extent compensated by the greater volume of cooling
air, and the net effect is to make the
projected-nose plug better able to cope with the
conflicting demands of traffic and highway travel.
It should be evident that the
projected-nose plug's effectiveness depends on the
pattern of incoming mixture flow. Four-stroke
engines often have intake ports angled to promote
turbulence. If the plug is positioned directly in
the path of the intake flow there will be a large
amount of heat removed from the plug's tip by this
direct air cooling, and that is just what you get in
most four-cylinder motorcycle engines. Indeed, any
hemi-head four-stroke engine gives its plugs' tips
quite a useful blast of cold air during the intake
stroke, and we think projected-nose plugs probably
should be in wider use in bikes than is the case.
Two-stroke engines can benefit from projected-nose
plugs' fouling resistance which they get simply
through the sheer length of their insulator (it's a
long way from the center electrode's tip back up to
the metal shell). However, the two-stroke's incoming
charge doesn't always do a good job of cooling its
plug, and you have to be very cautious in using
projected-nose plugs in the valveless wonders.
Some four-stroke hemi-head
engines' domed pistons extend up into the combustion
chamber too far, at TDC, to leave room for plug tips
that extend inward. This can prevent the use of
projected-nose plugs; it's something you check by
covering the plug nose with modeling clay, shaping
it so you have a 360-degree electrode contour, and
inspecting for signs of contact after you've
installed your "clearance" plug and cranked the
engine over a couple of turns.
Limited
plug/piston clearance in certain racing engines has
prompted plug makers to create the recessed, or
retracted gap, configuration. Champion inadvertently
did everyone a great disservice by labeling its
retracted-gap design as an "R" plug: people thought
the letter meant "racing" and used the R-series in
all kinds of high-performance applications, which
was a terrible mistake. Even if an R-plug's heat
range (all are very cold) is right, its gap
placement lights the fire back in a hole and the
combustion process never is quite as regular as it
should be. The retracted-gap plug exists only
because some engines present a clearance problem; it
never was intended for use where conventional or
projected-nose plugs can be fitted.
At one time there was a lot of
excitement over another unconventional plug-nose
configuration. In the "surface-fire" plug the spark
gap was between the center electrode and the
flanged-inward end of the metal shell, and the
insulator material filled its interior out almost
flush with the electrode's tip. Surface-fire plugs
don't even have a heat range; they run at about the
same temperature as the combustion chamber's walls
and are completely immune to overheating. Neither
can they cause pre-ignition. These features were
stressed at the time of their introduction, and
everyone thought surface-fire plugs were just
wonderful. They aren't, because they make their
spark too close to the chamber wall, and require an
incredibly powerful, CDI ignition system.
Motorcycle ignition systems
are the weak sisters of the world's spark
generators. Bikes therefore need all the ignition
help you can give them, which brings us to yet
another useful group of special spark plugs: those
with precious-metal electrodes. Conventional plugs
have thick, blunt electrodes made of an alloy that's
mostly iron, with a little nickel added to lend
resistance to erosion. Special-electrode plugs have
a side (ground) post made of ordinary nickel-iron
alloy, but a center electrode of something much more
costly - which may be a silver alloy, or
gold-palladium, or platinum, etc. Bosch still favors
platinum; Champion, ND and NGK offer plugs with
electrodes in materials ranging from silver to
tungsten. Gold-palladium seems to be the alloy that
offers the best price/performance advantage; we
don't entirely trust silver electrodes, which if
overheated will over-expand and crack the insulator
nose.
Platinum
and gold-palladium alloys can survive the combustion
chamber environment as very small wires, and in that
rests their great advantage. Electrons leap away
from the tip of a small-diameter, sharp-edged wire
far more willingly than from one that's fatter and
rounded. So the fine-wire plug requires less voltage
to form a spark than one with conventional
electrodes, and the difference becomes increasingly
biased in the former's favor as hours in service
accumulate and erosion blunts the iron-alloy
electrodes. There are, of course, drawbacks with
precious-metal plugs: they are more expensive, and
they are very sensitive to excessive ignition
advance. The overheating you get with too much spark
lead effects plugs' center electrodes before it can
be detected elsewhere in an engine, and when
subjected to this kind of mistreatment fine-wire
electrodes simply melt. In one sense this is a
disadvantage, as it means the ruination of expensive
spark plugs. Seen in another way it's a bonus
feature: it is better to melt a plug electrode than
an engine.
A final variation on the basic
spark plug theme you should know about is something
NGK calls a "booster gap," and is known at Champion
as an "auxiliary gap." By any name it's an air gap
built into a plug's core, and it improves resistance
to fouling. Conductor deposits on a plug's insulator
nose tend to bleed off the spark coil's electrical
potential as it is trying to build itself up to
spark-level strength. If so much energy is shunted
in this way that firing does not occur we say the
plug is "fouled." It is possible to clear a lightly
fouled plug by holding the spark lead slightly away
from the plug terminal and forcing the spark to jump
across an air gap. The air gap works like a switch,
keeping plug and coil disconnected until the
ignition system's output voltage rises high enough
and is backed by enough energy to fire the plug even
though some of the zap is shunted by the fouling
deposits. Mechanics discovered this trick; plug
makers have incorporated it into some of the plugs
they sell, and booster/auxiliary gap plugs work
really well in bikes with an ignition system strong
enough to cope with the added resistance. Such plugs
more or less mimic the fast-voltage-rise
characteristics of CDI systems - and offer no
advantage used in conjunction with a
capacitor-discharge ignition.
It is necessary to know all
these different plug configurations if you are to be
completely successful in doing your own maintenance
work, and it is absolutely essential that you know
how to "read" plugs if you're dealing with a
high-performance bike (whether factory-built or
do-it-yourself). Sports/touring machines usually are
well sorted out before they're sent to market, but
even the best racing bikes seem to be timed and
jetted a little off-the-mark for our fuels and
riding conditions. We suspect that the
laboratory-quality gasoline that some factories use
in their development work warps manufacturers'
ignition advance recommendations; whatever the
cause, nearly all the factory-built racing engines
with which we have direct experience run better when
their spark timings are slightly retarded.
Typically, too, their spark plugs are one heat range
too cold and they're jetted a bit rich. Also
typically, these same bikes are fitted with even
colder plugs, richer jetting and sometimes are given
more spark advance by those who buy them.
The worst, most destructive,
combination of mistakes we see begin with two
widely-held assumptions: first, that a cold spark
plug will help fend off that old devil detonation;
second, that more spark advance -not less- is the
thing to try when reaching for power. Try to use a
too-cold spark plug and you very likely will have to
jet for a lean mixture to avoid plug fouling - and
as you lean an engine's air/fuel mixture down near
the roughly-14.5:1 chemically-correct level it
becomes extremely detonation-prone.
Excessive spark advance is even worse in its ability
to produce detonation, and when combined with a lean
mixture it's enough to quickly destroy an engine.
Most people who've had some
experience with racing bikes (especially those with
two-stroke engines) know that detonation is a
piston-killer. Few really know the phenomenon for
what it is: a too-sudden ending to the normal
combustion process. You may imagine that the
ignition spark causes an engine's mixture to
explode, but it actually burns. There's a small
bubble of flame formed at the spark gap when
ignition occurs, and this bubble expands - its
surface made a bit ragged by combustion chamber
turbulence - until all the mixture is burning. This
process begins slowly, but quickly gathers speed
because the mixture beyond the flame_ bubble is
being heated by compression and radiation to
temperatures ever nearer the fuel's ignition point.
When the initial spark is correctly timed the
spreading flame bubble will have almost completely
filled the combustion chamber as the piston reaches
top center, and all burning will have been completed
by the time the piston has moved just a millimeter
or two into the power stroke. But the final phase of
this process can be shifted from simple burning into
a violent detonation of the last fraction of the
whole mixture charge.
Starting the fire too early
will produce detonation, as it gives the mixture out
in the chamber's far corners time enough to reach
explosion-level temperature. And a slightly lean
mixture detonates at a lower temperature. It's all a
function of ignition timing and mixture in any given
engine, and spark plug heat range plays absolutely
no part in it.
Your engine's spark plug
doesn't cause detonation but it can tell you when
and why the phenomenon has occurred. Moreover, the
spark plug can tell you with remarkable precision
how much spark advance and what jetting your engine
needs. Those are things you can "read" in a spark
plug, and all that is written there will be revealed
very clearly when the heat range is right.
So how can you tell whether
you've chosen the right heat range? It's easy: a
spark plug should be getting hot enough to keep its
insulator nose completely clean, with all deposits
burned away, but not so hot that its electrodes show
signs of serious overheating. These are things to
look for on a new plug that has been subjected to a
few minutes of hard running. After many miles of
service insulators acquire a coating of fuel
deposits, with some coloration from oil in
two-stroke applications, and there will be some
erosion of the electrodes even when everything is
normal. Don't try to read old spark plugs; even the
experts find that difficult. New plugs present
unmuddled information about what's happening inside
an engine, and can give you a complete picture after
just minutes of hard running. At least they will if
they're running hot enough, and that should be hot
enough to keep the insulator clean.
It's
impossible to separate the question of ignition
advance from the primary evidence of spark plug
overheating, which is most strongly shown on the
plug's center electrode. If you inspect this
electrode's tip with a magnifying glass and see that
its edges are being rounded by erosion, or melting,
then you know there's overheating. You should also
have a close look at the tip of the ground
electrode, checking for the same symptoms. Finally,
inspect the condition of the insulator, which should
be white but with a surface texture about like it
was when new; a porous, grainy appearance is
evidence of overheating. If the signs of overheating
are confined mostly to the center electrode you can
bet you're using too much ignition advance. Retard
the spark timing in small (two or three degrees)
increments and as you get close to the optimum
advance you'll find two things happening: first, the
whole plug will be running colder; second, the
center electrode will begin to acquire a film of
fuel deposits extending out from the insulator nose
toward its tip.
The fuel film mentioned here
is what you watch when making fine adjustments in
ignition advance. In an engine that's been given
just a few degrees excessive advance (as most have)
the fuel film will only extend outward along part of
the center electrode's exposed length, ending
abruptly a couple of millimeters from the tip. The
portion remaining won't be filmed over simply
because it has been hot enough to burn away the fuel
salts dusted on the rest of the electrode, and
you'll see that sort of localized overheating
created by too much spark advance even on a plug
that is two or three heat ranges too cold. And
you'll have the correct spark advance when the
center electrode's fuel film continues right out to
within a hair of its tip. There are a couple of
caveats to be observed in this matter. An
overly-retarded spark timing won't show except as an
absence of any evidence pointing to too much
advance. Also, the spark itself will blast clean
spots in the electrode's fuel film, and when there's
enough combustion chamber turbulence to blow the
spark sideways into a curved path you'll get a
cleared area on one side of the electrode. This
lop-sided spark blush shouldn't be mistaken for the
more sharply defined ring associated with the
electrode tip overheating produced by excessive
spark advance.
Once you have brought your
engine's ignition timing close to optimum you'll
almost certainly have to make a further change in
spark plug heat range. Manufacturers' specifications
for racing models very often advise you to use too
much advance and a too-cold plug, and when you
shorten the spark lead to suit commonly-available
fuels it almost certainly will be necessary to use a
warmer plug. Then, when you have found plugs of a
heat range that will keep that insulator nice and
clean you can start adjusting your engine's air/fuel
mixture - a task that will be easy if you can forget
everything you thought you knew about this aspect of
plug reading.
A lot of amateur tuners, some
of whom are fairly successful, will look at some
plug freshly removed from a two-stroke engine and
offer advice based on the color of the oil deposited
on the insulator nose. In fact, if the plug is hot
enough there won't be any color, and if there is
that still has nothing much to do with air/fuel
mixture. If you think about it you'll realize that
the only color you can get from an air/fuel mixture
is the color of soot. When the mixture trapped in an
engine's combustion chamber has more fuel than can
be burned with the available air, then combustion
will be incomplete and the excess fuel will remain
as soot, which is not brown or tan or magenta or any
color other than black. And if your engine's mixture
is too rich, the sooty evidence will be present on
the spark plug's insulator, in a very particular
area.
You
won't find any soot out near the insulator nose, on
a plug that's running hot enough to keep itself from
fouling, because temperatures there are too high to
let soot collect. But the insulator is much cooler
deep inside the plug body, and coolest where it
contacts the metal shell, which is precisely where
you "read" mixture strength. Look far inside a plug,
where its insulator joins its shell, and what you'll
see there if your engine's mixture is too rich is a
ring of soot. If this ring continues outward along
the insulator to a width of even a millimeter you
can be sure the mixture is rich enough to be safe,
and too rich for maximum output. In most engines
best performance is achieved when the mixture
contains only enough excess fuel to make just a wisp
of a "mixture ring" on the plug insulator. Air
cooled two-stroke engines often will respond
favorably to a slightly richer mixture, which
provides a measure of internal cooling; some
four-stroke engines give their best power when the
mixture is leaned down to such extent that the last
trace of soot deep inside the plug completely
disappears.
Never try to jet too close to
a best-power mixture until after you've taken care
of spark advance. As previously noted, the air/fuel
ratio that yields maximum power is only a shade
richer than the one that is most detonation-prone;
fortunately, the plug will tell you when there has
been even slight detonation inside your engine. The
signs to look for are pepper-like black specks on
the insulator nose, and tiny balls of aluminum
concentrated mostly around the center electrode's
tip. Severe detonation will blast a lot of aluminum
off the piston crown, and give the plug a gray
coating-which is a portent of death for the engine.
A few engines will show just a trace of detonation
when jetted and sparked for maximum power, but that
never produces anything more than a few miniscule
spots of aluminum gathered on the center electrode's
sharp edges. If you see more aluminum and an
extensive peppering evident on your plug, you're in
trouble.
We cannot stress too strongly
the need to give spark advance your closest
attention, because excessive spark lead is the most
frequent cause of detonation, which is a real engine
killer. You can't stop advance-produced detonation
with a cold spark plug, nor with anything but a
wildly over-rich mixture. Also, excessive ignition
advance has a bad effect on performance. We ran a
250cc road racer at the drags a few months ago, and
found that retarding the spark about five degrees
from the manufacturer's setting raised the trap
speed from 106 to 110 mph. Similarly, there's a
125cc motocross machine residing in our shop which
runs a lot stronger and cleaner since it has been
retimed for less advance, jetted leaner, and been
given a hotter spark plug.
Even touring bikes sometimes
benefit from revised spark timings. Only rarely will
their carburetion be off enough to need attention,
but the ignition advance they get represents a
compromise between the optima for power and economy.
For some riders, especially those who use a lot of
throttle much of the time, stock ignition advance is
too much advance. And of course many riders find
that their specific requirements are better met with
non-standard plug configurations.
The trick in all this is to
know enough about spark plugs to be able to choose
the right basic type, and to understand what the
plug has to say about conditions inside your bike's
engine. It's not an altogether easy trick to
perform, with so many things to be remembered all at
once; it's a terrifically effective trick when you
get it right.
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