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.
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 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:
To remedy pre-ignition:
Proper Plug Gapping:
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.
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).
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?