Most blokes think they know a thing or two about sparkplugs... hell we've all changed enough of the damn things over the years right... You just gap them and whack 'em in .... What else is there to know ?

Heaps apparently, read on.

The spark plug has two primary functions:

* Ignite air/fuel mixture
* Transfer heat away from the combustion chamber and into the head

A sufficient amount of voltage must be supplied by the ignition system to jump the spark across the spark plug's air gap (between the ground wire and electrode nose). The temperature of the spark plug's firing end must be kept low enough to prevent pre-ignition but high enough to prevent fouling. This is called "Thermal Performance" and is determined by the heat range of the plug.

It's important to remember that the spark plug works as a heat exchanger by pulling unwanted thermal energy away from the combustion chamber (lowering combustion temperature) and transferring the heat to the engine's cooling system. The heat range is defined as a plug's ability to dissipate heat.

The plug's ability to transfer heat effectively is determined by:

* The insulator nose length
* Gas volume around the insulator nose
* The materials/construction of the center electrode and porcelain insulator (thermal conductivity)

The insulator nose length is the distance from the electrode tip of the insulator to the point where insulator meets the metal shell. Since the electrode tip is the hottest part of the spark plug, the tip temperature is a primary factor in pre-ignition and plug fouling. Ideally, the spark plug tip temperature must remain between 500-850C.

If the tip temperature is lower than 500C, the insulator area surrounding the center electrode will not be hot enough to burn off carbon and combustion chamber deposits and they will accumulate over time, resulting in spark plug fouling and misfiring. If the tip temperature is higher than 850C the spark plug will overheat which will cause the ceramic around the center electrode to blister and the electrodes to melt.

This is extreme and often leads to pre-ignition, detonation and expensive engine damage. In identical spark plug types, the difference from one heat range to the next is the ability to remove approximately 70C to 100C from the combustion chamber. In a projected style spark plug firing tip (long nose plug) temperature is increased by 10C to 20C.

The borderline between the fouling and optimum operating regions is called the spark plug self-cleaning temperature, at which point is where the accumulated carbon and combustion deposits are burned off. A long nose plug maintains a higher internal operating temperature to burn off oil and carbon deposits, and has no relationship to spark quality or intensity.

Conversely, a cold spark plug has a shorter insulator nose and absorbs more combustion chamber heat. This heat travels a shorter distance, allowing the plug to operate at a lower internal temperature. A colder heat range is necessary when the engine is modified for performance, subjected to heavy loads or is run at a high rpm for a significant period of time. Colder plugs are utilised to reduce the chance of pre-ignition.

Factors which modify combustion temperature are:

* Rich air/fuel mixtures cause tip temperature to drop, causing fouling and weaker acceleration.
* Lean air/fuel mixtures cause plug tip and cylinder temperature to increase, resulting in pre-ignition, detonation and engine damage.
* Higher Compression Ratios and Forced Induction will elevate spark plug tip and in-cylinder temperatures.
* Camshaft changes often result in a dynamic cylinder pressure change and extended rev ceiling (use a colder plug).
* Advancing ignition timing by 10 degrees causes tip temperature to increase by approx 70-100 C.

Common ignition related Engine problems:


* Defined as: ignition of the air/fuel mixture before the pre-set ignition timing mark
* Caused by hot spots in the combustion chamber...can be caused (or amplified) by over advanced timing, too hot a spark plug, low octane fuel, lean air/fuel mixture, too high compression, or insufficient engine cooling.

To remedy pre-ignition:
* Change to a higher octane fuel, a colder plug, richer fuel mixture or lower compression ratio.
* You may also need to retard ignition timing and check vehicle's cooling system for efficiency.


* The spark plug's worst enemy! - Can break insulators or break off ground electrodes!
* Pre-ignition most often leads to detonation, however they are two separate events.
* Plug tip temperatures can spike to over 3000F during the combustion process (in a racing engine)
* Most frequently caused by hot spots in the combustion chamber, often from excessive carbon buildup, low octane fuel or wrong sparkplug choice.
* Hot spots will allow the air/fuel mixture to pre-ignite. As the piston is being forced upward by mechanical action of the connecting rod, the pre-ignited explosion will try to force the piston downward. If the piston can't go up (because of the force of the premature explosion) and it can't go down (because of the upward mo-tion of the connecting rod), the piston will rattle from side to side. The resulting shock wave causes an audible pinging sound. This is detonation.
* The spark plug is damaged by both the elevated temperatures and the accompanying shock wave.


* A spark plug is said to have misfired when enough voltage has not been delivered to light off all fuel present in the combustion chamber at the proper moment of the power stroke (a few degrees before top dead center)
* A spark plug can deliver a weak spark (or no spark at all) for a variety of reasons...defective coil, too much compression with incorrect plug gap, fouled plugs or out of spec ignition timing.
* Slight misfires cause a loss of performance from a drop in combustion efficiency.
* Severe misfires will cause poor fuel economy, piss poor driveability, and lead to engine damage.


* Occurs when spark plug tip temperature is insufficient to burn off carbon, fuel, oil or other deposits.
* Will cause spark to leach to the metal plug shell, causing a misfire.

Proper Plug Gapping:

* Proper gapping of the spark plug is necessary to get maximum spark energy, lowest RFI release as well as what is best for the longevity of the secondary ignition components (coil, cap, rotor, wires, plugs).
* When checking plug gaps, the correct way is to use ONLY wire gauges, though many of us are still using the slider style gapping tools. These flat or feeler gauge style gauges do not accurately measure true width of spark plug gap (contrary to what we were taught as young blokes).
* When increasing the gap size for your high performance applications utilizing advanced ignition systems such as Mallory, Accel, Jacobs, Crane and Holley, it is important never to go more than plus or minus .008". This will maintain parallel surfaces between ground and the center electrodes.
* With Higher Compression ratios and Superchargers as well as Nitrous, it is actually smaller spark plug gaps that must be used as well as the use of a much hotter ignition system (see above). These higher cylinder pressures require more energy to jump the spark plug gap.
* The rule of thumb on plug gaps in race applications is to open them up in .002" increments at a time. When the motor begins to lose power or slow down then go back .001-.002" and this in most cases is the optimum gap. Remember! The Ignition Unit, plug brand as well as heat range, cap and/or rotor styles and in many cases fuel type or brand will change your optimum spark plug gap settings.

Hey dude, I've ripped the old plugs out but what does their appearance tell me?

I'm glad you asked ...

This is what a normal plug should look like. The grayish-tan to white in color indicates the plug is operating at the proper heat range as well as correct jetting and the cylinder is running healthy.

HORSEPOWER TIP: That vertical color band on the long ceramic insulator shows you where the plug is indexed. Meaning that band should be aimed slightly at the exhaust valve for Optimum placement. If the band is anywhere but there, it means that the plug electrode is not at it's optimum location. There is still discussion as to whether indexing a plug is worthwhile, but on may applications looking for that last horsepower, it doesn't hurt. In fact I've seen Dyno proof to indicate a 1-3% power increase by indexing plugs. Free Horsepower anyone ?

This plug is rooted. It displays excessive electrode wear, will misfire during acceleration and be the source of hard starting.

It looks ok color wise, so replace it with same plug or at least a compatible heat range one. You've all heard the term " If it works, don't fix it". Don't look for flaws with this plug ... just blame yourself for not changing it sooner (D'oh!).

If your plug looks like this you've either just pasted it at the ground in frustration or your motor is in very deep shit. This kind of mechanical damage is caused by foreign objects in the combustion chamber or improper plug reach where it contacts the piston (high dome replacement pistons are often a culprit). A large piece of carbon rattling around in the combusation chamber can do this also.

To solve this, make sure you have the correct length tip spark plug as well as removing any foreign materials in the combustion chamber. In some cases you may have excessive carbon buildup on the backs of the intake valves that will have to be addressed by decarboning your motor.

This plug suffered from detonation. In cases of severe detonation, insulators may become cracked or chipped. Beware though, that improper spark plug gap settings will also cause the insulator tip to crack or chip. Detonation sucks, it shortens your motor's lifespan and robs you of horsepower.

Detonation is tricky ... make sure that you are using the correct octane fuel first and then verify correct ignition timing. Next check for an inoperative EGR system (if equipped) as well as proper function of the Knock Sensor (if equipped). Also, you will want to make sure you are using the correct heat range plug.

This plug has overheated, you will notice a chalky appearance, white insulator, rapid electrode wear as well as an absence of deposits. The actual shell may also be discolored from the heat.

To cure this you must first verify that the plug is the correct heat range, the ignition timing settings are correct, the air/fuel mixture is not too lean, there are no vacuum leaks and that the EGR valve (if equipped) is functioning properly.

Ash Deposits. These are light-brownish deposits that are encrusted to the ground and/or center electrode. This situation is caused by crappy oil and/or fuel additives and can cause misfires.

The cure for this is to check for worn valve guides or valve seals, not using fuel additives, or you might even try changing fuel brands. By the way, a hotter plug is what most people use to try and fix this problem. You need to first understand that the plug is NOT typically the problem.

Here we have an oil fouled plug. The oily coating was caused by poor oil control. Oil can leak past worn valve guides and piston rings, or on some race engines suspect a possible intake gasket leak and then oil entering the combustion chamber.

Check for worn valve guides, intake gasket sealing alignment as well as worn cylinder walls and piston rings. A leak down test is a good place to start for tracking down the source of the problem.

Initial Pre-ignition .This will usually look like a melted center electrode at least, with probably a melted ground electrode as well.

Check for an incorrect heat range plug, over-advanced timing, lean fuel mixtures, inoperative EGR valve or Knock Sensor (if equipped) and also look for hot spots or carbon deposit accumulation inside the combustion chamber.

If you or your engine builder took the time, all areas of combustion chamber should have been de-burred (smoothed off) to eliminate this problem. This includes the sharp edges on the combustion chamber, piston top and piston flycuts.

Sustained Pre-ignition.

This is really bad and will be pretty obvious ... melted and/or missing center / ground electrodes as well as an obliterated insulator.

Check for incorrect heat range plug, over-advanced timing, lean fuel mixtures, inoperative EGR valve or Knock Sensor (if equipped) and also look for hot spots or deposit accumulation inside the combustion chamber.

If you or your engine builder took the time, all areas of combustion chamber should have been de-burred to eliminate this problem. This includes the sharp edges on the chamber, piston top, and cylinder wall valve reliefs (if applicable).

If you ever see plugs like this this you'd better look for possible internal engine damage as soon as possible... we're talking things like pistons, cylinder walls, valves, rings, big and little end bearings, etc.

Here we see splashed deposits.

These look as if they are small islands of contaminants on the insulator nose. This usually indicates dirty carburetor bores, air intake and rooted air filter as well as the possibility of a dirty or faulty injector in vehicles so equipped.

You must use aggressive carb and choke cleaner or other solvent cleaner (a pressurized fuel injection service on fuel injected vehicles or injector removal and cleaning) before installing new spark plugs and changing the engine oil and filter (probably contaminated).

Carbon Fouled.

This is a very common visual condition to see on hard driven motors and race engines. Soft, black, sooty, dry-looking carbon indicating a rich mixture, weak ignition or wrong heat range plug (too cold).

You will first need to verify the plug heat range as correct. On carbureted engines, check the choke action as well as choke butterfly off-position for proper function and adjustment. On fuel injected engines, check for clogged injectors and the cold-start valve and circuit. You also need to check for correct fuel pressure settings.

As a general rule on all computer-controlled engines, you need to also make sure that all input signals to the computer are working and accurate. This includes, but is not limited to, all temperature and pressure sensors (mass airflow meter, throttle position sensor, air temp sensor, coolant sensor) as well as the EFI system components.

Lastly on all engines, check for vacuum leaks and weak spark or low voltage output from the coil.

Here we see the difference between a standard and a fine (hi-po) electrode

So Dude, what kind of plugs do you use on your high perfomance gear?

Well after 20 years of owning hi-po cars and bikes, doing most of my own wrenching and trying just about every kind of sparkplug out there I've come to a few conclusions...

Anything Autolite is pretty much shit, Champion not far behind. Splitfire plugs are a gimmick and Halo plugs give you a fucked up flame front, robbing you of horsepower. Fine electrode, precious metal plugs are where its at.. better flame propagation and greater longevity are well worth the extra dollars. Chuck in added reliability and easy starting (really applicable if you own a kick start bike) and you have a winner.

NGK plugs are what I've used and still use today in vehicles like; Ford / Chev V8's (Both Iron block and Alloy), Turbo Kawasaki, Turbo Nissan Engined Commodore and my Harley. I find their quality, power production and lifespan always excellent, even in forced induction environments.

This is an original advertisment for sparkplugs around the WW1 era ...
Seems like they havn't changed that much huh?

Enjoy this article ? The site ? Would you buy me a beer for my efforts?



Plug Manufacturer TC88/95 Engine
Plug Gap .040"
H-D Standard 6R12
H-D Gold 6R12G
H-D Platinum 6R12PP
Champion RA8HC
Autolite 4164
Autolite Platinum AP4164DP2
ACCEL 2418
Splitfire SF416D
Screamin Eagle EX12S
ACDelco no data
Bosch no data
Denso Iridium IXU22


Plug Manufacturer 80 CID Evolution 1984-up
Plug Gap .040"
H-D Standard 5R6A
H-D Gold 5R6AG
H-D Platinum 5R6APP
Champion RN12YC
Autolite 4265
Autolite Platinum AP4265DP2
Splitfire SF6D
Screamin Eagle EVS13S - Hot
EVS13P - Cold
ACDelco 44NS
NGK Platinum BPR5EVX-11
Bosch WR8D
Denso Iridium IW16


Plug Manufacturer Sportster 86-up
Evolution Engine
Sportster 80-85 Sportster 72-79
Plug Gap .040" .040" .040"
H-D Standard 6R12 4R5 4-5
H-D Gold 6R12G 4R5G 4R5G
H-D Platinum 6R12PP    
Champion RA8HC RL82YC H8C
Autolite 4164 4123 996
Autolite Platinum AP4164DP2    
ACCEL 2418
2413 2402
Splitfire SF416D SF21F SF21F
Screamin Eagle EX12S
Denso Iridium IWU22


Plug Manufacturer Shovelhead 82-84 Shovelhead 80-81 Shovelhead 78-79 Shovelhead 75-80
Plug Gap .040" .040" .040" .040"
H-D Standard 5RL 5R6A 5A6A 5R6A
H-D Gold 5R6AG 5R6AG 5R6AG 5R6AG
H-D Platinum 5R6APP 5R6APP 5R6APP 5R6APP
Champion RN13LYC RN12YC N12YC RN12YC
Autolite 2585 4265 4265 4265
Autolite Platinum   AP4265DP2 AP4265DP2 AP4265DP2
ACCEL 2417 2410A 2403A 2410A
Splitfire SF426E
Screamin Eagle EVS13S - Hot
EVS13P - Cold
EVS13S - Hot
EVS13P - Cold
EVS13S - Hot
EVS13P - Cold
EVS13S - Hot
EVS13P - Cold
NGK Platinum   BPR5EVX-11 BPR5EVX-11 BPR5EVX-11