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
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.