Zero to One Hundred Thousand Miles!
Hot rodders, a prideful lot, will tell you nothing eats spark plugs like a top fuel dragster. Those things will, they claim, gobble up a set of eight spark plugs in a single quarter mile run, 1320 feet. That's 165 feet per plug, a joy for Champion, NGK, et al, and the distance the straight-line crowd seems to believe marks the lower extreme of spark plug service life, just as the 100,000-mile durability mandated for those in new emission-controlled cars marks a point approaching the upper limit.
As it happens, top fuel dragsters really are not the world's championship spark plug destroyers. The title belongs to the motocross motorcycles operated by young riders typically filled with enthusiasm and ignorance. Such riders can make a bike's engine terminally wet-foul a plug without ever starting or moving the bike anywhere. That's zero distance per plug, an unbeatable if not entirely enviable record, which is equaled by numerous individuals in successive generations of young motocross riders. I have done it myself, more than once.
Because motorcycles of all types, and whatever the abilities of their riders, subject spark plugs to such taxing conditions, I have chosen to focus on them more than automobiles. However, most of my comments also apply to the various old cars that had equally weak and primitive ignition systems, or to new ones modified for racing.
The spark plug hasn't changed much in outward appearance since 1900. However, some of the earlier, mid-18th century examples were a little strange. One designed by Giovanni Babacci in 1854 had a crank-driven, timed rotating center electrode that made a spark when it moved away from contract with its ground electrode.
Another Victorian approach to spark ignition had an electrode extended downward from the cylinder head to a position almost touched by the piston. When the piston neared the electrode a spark would snap across to the piston crown and ignite the charge.
By the time the single-cylinder De Dion Bouton motorcycle engine of 1898 was produced, it had an igniter any of us could identify as a spark plug. All the now-familiar spark plug elements were there: A metal body that threaded into the engine's cylinder head, a ground electrode fixed to that body, and a center electrode surrounded by an insulator. However, those early spark plugs had mica insulators instead of the aluminum oxide ceramics now universally used, and were in other ways inferior to the modern product.
Spark plugs, as good as they have become, cannot expect a long service life in motorcycles, though their life doesn't have to be as brief as it sometimes is. The causes of early plug mortality in motorcycles are rooted in two areas: First, motorcycle engines have a very high power output for their displacement, and this subjects spark plugs to a broader range of conditions than they find comfortable. Second, motorcycles' ignition systems are not nearly as advanced as their automotive counterparts.
Part of the motorcycle plug service life problem is due to their engines' lack of any means to adjust ignition timing to suit load. With the very rare exception (Yamaha's old XS11 being the example that comes to mind), motorcycle engines have essentially fixed ignition timing. Yes, they do have those centrifugal flyweights that advance the ignition timing 30 degrees or so as crank speed rises. But such mechanisms serve only to make starting easier and very low speed running smoother. They cannot match ignition advance to engine operating conditions.
Imagine yourself cruising down the road at a steady 55 mph on a straight and level road holding a constant throttle setting. (Okay, yes, I agree it's an unlikely scenario. If you have trouble with it, imagine also a police car in your mirrors). When you come to a rise, you hold your speed by turning the throttle grip a few degrees. For you, nothing changes but the position of your right hand. But inside your engine, it's a whole new world.
As you open that throttle, a larger mass of air and fuel is admitted into each cylinder, yet it ends up being squeezed into the same combustion chamber space. As a result, air and fuel molecules are crowded closer together and so the fire ignited by the spark plug spreads faster. The more rapid march of the flame away from the ignition point means the engine then needs less ignition advance.
Automotive practice is to provide a "vacuum advance" feature to adjust ignition timing to suit load. When you apply more throttle in a car the engine's ignition advance is reduced; ease off on the throttle and it's increased. Motorcycles, with the noted rare exceptions, light the fire at a fixed point, trading broad-range optimization -- meaning fuel efficiency and extended plug life -- for mechanical simplicity.
Plug life is shortened by fixed ignition timing because the advance chosen by manufacturers usually represents a compromise intended to satisfy all load conditions. In practice, the timing settled upon is one that doesn't produce too much detonation on full throttle at the engine's torque peak, where cylinder pressures are highest. It usually is a few degrees more than will yield best peak power, but a lot less than is needed for light-throttle efficiency.
You may not give a small mouse's haunches about inadequate light throttle spark advance (Mileage? We don' need no stinkin' mileage!), but your bike's plugs have a different view of the matter. A spark plug's insulator needs to be kept at a temperature above about 700 degrees F. to burn off wet fouling deposits, which can be oil, carbon from fuel, or both. Cruising around at low speed and part throttle with an ignition timing effectively retarded will let your engine's plugs cool too much to stay clean.
The second punch of a big double-whammy awaits at the opposite extreme of throttle setting and engine speed. When your fixed-advance engine is pulling wide-open throttle with the revs up, the heat really pours into spark plugs' insulators. When too much pours in, and the center electrodes' temperature rises too high, you have the potential for disaster.
Plugs' center electrodes are heated to dull-red temperatures under full-throttle conditions. Above 1600 degrees F. the center electrode is glowing brightly, hot enough to be an ignition source in its own right. It can fire the mixture before a spark occurs.
When a spark plug becomes a glow plug and begins prematurely firing the mixture you have the condition known as "pre-ignition," a real engine killer. Once pre-ignition starts it makes the plug even more overheated, which become pre-pre-ignition and raises the electrode temperature even higher. This vicious circle spirals ever more viciously until the plug electrode melts or the engine does.
Today's center plug electrodes are made of metals more conductive that plain iron, and a lot of them are multi-part. Most now begin with a steel terminal stud at the top, and end with a nickel-alloy electrode that may have a copper center and/or end in a precious-metal (silver, or even platinum) tip. The copper is there to carry heat away from the insulator nose; the fancier electrode tips provide resistance to spark and chemical erosion, lower spark voltage requirements, or both.
Under conditions anywhere near normal the the spark plug's side electrode, which usually is welded to the plug shell, doesn't get hot enough to become a problem. Severe detonation (or a big load of nitromethanol packed in by a supercharger) can destroy it, but mostly the side electrode is just a destination for electrons flashing away from the hot center electrode. The exception is if you have selected a grossly over-hot plug heat range. In that case the side electrode will show oxide scaling, like it had been heated with a welding torch.
People who have some electronics background will know that electrons travel from negative to positive, and leap from an electrode more readily if it is heated. This is why the cathode in a cathode ray tube has a heating coil wrapped around it.
Ignition system polarity once was vitally important because a spark plug's center electrode is hot and ignition coils were not. The voltage required to force a spark across a plug gap can be 20% higher when polarity is reversed and the center electrode is positive. This was a big deal when spark coils strained to pump up an output of maybe 20 kilovolts, and it remains a consideration though not one of crucial importance.
You can test for ignition polarity, despite the spark jumping across a gap a trifle too fast to see where it started and landed. Let the spark jump across from the lead to ground and stick the tip of a soft pencil in the spark's path. When the spark leaves the graphite it makes a little flare in the positive direction. If the flare goes toward "ground," you have the correct polarity. I must mention here that the ignition coil itself operates at voltages so high that the polarity of a 12V battery hardly matters.
Don't bother checking the polarity of twin-lead spark coils of the sort found igniting most motorcycle four cylinder engines and many of the latest automobile engines. These fire across two plug gaps, only one of which can have the correct negative polarity at its center electrode. Does it matter? Probably not very much. Polarity can make a 20% difference in the voltage required to force a spark across a plug's electrodes, but this is of no practical importance if the ignition coil delivers 200% or more of the actual voltage required. Alas, not all motorcycles nor all of the older European cars have an adequate surplus of spark voltage.
Compared with automotive ignition systems, those on many motorcycles are absolutely anemic. You can judge the power of the ignition system on your bike simply by checking its manufacturer's plug gap recommendations.
The outstandingly feeble old Harley-Davidson magnetos (supplied by Fairbanks-Morse, AKA "Fireblanks-Remorse") strained to push a spark across an .018-inch plug gap. New cars have plug gap specifications in the order of .040- to .050-inch, with some as wide as .080-inch.
Can you guess which of the above specifications go with the higher spark voltages? And where does your vehicle's plug gap specification place it in the rankings? If the gap recommended is less than about .028-inch your engine could use help.
Conventional wisdom once held that enough spark to light the cylinders' fire was plenty. My experience and a lot of research says otherwise. For example, you'd be amazed how much better your bike's carburetion is after you've fitted a set of hotter coils and opened up those plug gaps to .035-inch or so.
Wider plug gaps provide a broader tolerance for air/fuel mixture strengths because the volume of the spark itself is increased as the gap is widened. Air/fuel mixtures are not uniform at the molecular level. They have rich spots here, and lean ones there, and if there are too few fuel molecules in the tiny volume of mixture the spark passes through you get a misfire, or a fire that fizzles and is tardy in spreading. When the plug gap is widened from, say, .025" to .035" you get a 40% increase in spark volume with a like increase in the chances the mixture will both ignite and burn briskly.
You should keep in mind the fact that engines makes spark plugs hotter or colder, and not the other way around. A "cold" plug will not cool down an over-heating engine; a "hot" plug won't make one overheat. In reality, the only difference between hot and cold plugs, the property that decides a spark plug's "heat range," is the length of the heat path from the insulator's nose to the plug body.
In all modern spark plugs the insulator is inserted from the top of the plug's shell and is forced against a tapered seat when the shell's top is rolled over to hold the insulator in place. A cold plug is cold (in reality, just less hot than a hot plug) because its insulator seats near the bottom of the shell, thus providing a short heat path to the cooler metal and exposing less of the insulator to combustion gases.
Exactly as you would expect, the spark plug insulator's nose is coolest where it seats inside the shell. Remember this, because that relatively cool area is where you look for evidence of excess fuel in the engine's air/fuel mixture. The fuel will manifest itself as a black ring of carbon on the insulator right where it disappears into the shell. The black ring is one of several things you're looking for when you "read" a spark plug.
The above assumes you will have applied full throttle for a long moment, then de-clutched, killed the ignition and stopped the engine while the plug insulator nose is hot. Operating an engine at light throttle allows the insulator to cool enough so it can collect carbon from excess fuel over its whole exposed length. You can use this as evidence of an overly rich mixture at part throttle but it's a tricky read, as it depends so much on plug heat range being right.
Getting the correct heat range is a matter of choosing one that isn't too hot. Plug overheating shows up as three things: Oxide scale on the side electrode, as previously mentioned; obvious heat erosion of the center electrode; and an insulator nose that looks grainy, porous. If you don't see those symptoms you're solid, though the insulator will be white enough to scare the hell out of guys who have been mislead as regards the significance of "color," which is nothing more than baked-on oil.
The danger in using too-cold plugs is not merely that they foul easily, though they certainly do that. The real problem is that an overly cold plug will tend to conceal from you the evidence of excess spark advance and a lean air/fuel mixture, and of the detonation that is their engine-killing consequence.
There is an air/fuel ratio -- with petroleum-based fuels it is around 14.5:1 by weight -- that can be considered "chemically correct." You can confuse your friends and bore strangers absolutely paralytic by refering to this as a "stoichiometric" mixture. This is arcane engineer-speak for a mixture that provides exactly enough fuel molecules for the available oxygen molecules.
You might suppose, if you didn't know better, that a perfectly balanced, stoichiometric mixture would give the best power. It won't, though it will give low exhaust emissions, which is why new cars have oxygen-sensing probes in their exhaust manifolds sending a control signal to their fuel injection systems. And they also commonly have sensors to listen for detonation, the undesireable companion of chemically correct air/fuel mixtures.
Most engines (Harley-Davidson's fabulous racing XR750 being an odd exception) need about 10% excess fuel for best power, and the excess will be evident in the black ring of carbon on the insulator. If the soot on a cold spark plug fools you into thinking the mixture is too rich, the smaller main jet you'll be tempted to try is just the thing to detonate your engine to death.
Spotting the signs of detonation on a plug is easy. Look for what appears to be flecks of pepper on the insulator nose. Those little black specks are bits of the combustion chamber deposits knocked loose by the detonation's shock waves. Slightly more severe detonation produces tiny balls of aluminum that tend to attach themselves to the center electrode. Really damaging detonation will put a dusting of gray, ash-like aluminum oxides all over the plug nose. If you ever see that, it means you have an engine to rebuild.
Assuming that your engine's spark plugs are running hot enough to keep their insulators clean, and you can see the little excess-fuel ring where the insulator's nose joins with the plug's metal body, then any detonation you see will be caused by too much spark advance. Setting the spark timing per manufacturer's specification won't get you in trouble, but it probably also won't be the best you could do.
For reasons I suspect are related to the lab' quality fuels used in factories dynamometer rooms (also to the best-power/best-cruise tradeoff necessitated by fixed-timing ignitions) all the motorcycles I have worked with over the past umpteen years have proven to have just a tad too much spark advance. It's there in the street engines, and I see it in the racing engines as well.
I must emphasize here that I have never encountered an engine that needed more ignition advance -- or colder plugs -- than its manufacturer recommended. The guy who says, "Well, I'm going racing, so I'd better install a set of ultra-cold racing plugs and bump the ignition timing up a few degrees," is going to go slow and buy a lot of engine parts.
You probably will be spared the worst of the past's mistakes, unless you are unlucky enough to find a supply of the old "retracted gap" racing spark plugs of the sort required by Offy and Aermacchi motorcycle engines. Don't even think about using those plugs unless you're running an Offy or Aermacchi in the vintage races. You can spot one of these plugs by way the ground electrode is inserted in a small hole in the side of the shell, which is why they're also called "sidewire" plugs.
Be happy if your car or motorcycle does not require dinky little 10mm plugs, as once used by Honda and BMW/Bristol. Those have so little clearance volume between the shell and the insulator that they crud up (to use a technical term) depressingly quickly. The 12mm plugs developed to replace 10mm plugs are better, and will fit in places almost as tight, but the almost-universal 14mm plugs really are the smallest size that will stay clean.
In a perfect world there would be plenty of the 18mm plugs, as used in ancient side-valve engines and slightly less antique Eastern European two-stroke engines. Those 18mm plugs had so much clearance volume between their insulators and shells they could tolerate an enormous amount of oil and other contaminents flying around in the combustion chamber. We'd still have a lot of them being specified for new engines but for the difficulty of finding the generous room they require in overhead valve engines, especially in this time of four-valve cylinder heads.
One of the biggest improvements in spark plug design in recent times was the projecteded-nose configuration introduced (if memory serves) back in the '60s. These plugs have what their name says: Both the ground wire and the center electrode extend farther into the combustion chamber, and move the spark gap with them to a point about .200-inch beyond the plug shell.
A projected-nose plug looks like its electrodes and insulator are terribly exposed to the fire inside the combustion chamber. But these extended bits also are out there where the incoming air/fuel charge can do a fine job of cooling them. In an engine, the projected-nose spark plug runs hotter has better resistance to fouling at light throttle openings, yet is cooled by the blast of air and fuel it gets when you crank up a bunch of throttle.
The down side (isn't there always one?) of projected nose plugs is that in some engines there just isn't enough clearance for them among the other hardware, valves and piston crown, also occupying the combustion chamber. It's a point to watch, but where there is room, and assuming the appropriate heat range is available, projected-nose type plugs are the best choice.
An engine's ignition advance must be tweaked a little to adjust for the burn rate changes produced by the different nose configurations. The old retracted gap racing plugs needed more ignition advance because they started the fire back in a hole. Projected nose plugs start the blaze out where it can spread easily, so an engine fitted with such plugs need less advance.
Once you get past the basic spark plug information, such as heat range, electrode types and nose configuration, you get into questions of whether a plug will fit your bike's engine. You have to get the "reach" (the length of the threaded portion) right, and you would not want a resister-type plug in any application where ignition system strength is in doubt.
If you try switching to spark plugs from different manufacturers, the problem of coding will drive you crazy. Not only are the heat-range numbers inconsistent (Bosch gives its hotter plugs higher numbers; NGK does the opposite) but the coding to indicate design features like projected nose, special-alloy electrodes, etc., has no uniformity at all. About the only thing you can do is to collect catalogues and read carefully.
Oh, yes, One final bit of advice: When you're installing a new set of plugs, don't tighten them like you think they hold the engine's cylinder head in place. New plugs should be screwed in until their washers are lightly gripped between plug and cylinder head, then tightened just enough to flatten the washers -- usually a final half turn or so. And when reinstalling used plugs just snug them down; don't use a long handle on your plug wrench and pull until the threads creak. Please.