For many racing engines, spark plug color is a window into the engine combustion process that can be used for tuning. Without knowing your system inside and out, however, tuning with spark plug color alone can be misleading.
Exposure to heat and combustion byproducts changes the colors on the various parts of the spark plug. Heat is the catalyst of spark plug color changes in any of these sections. Excess heat can come from the combustion process or the engine cylinder head temperature. However, a heat indication in one spark plug section may not produce a heat indication in another section of the spark plug.
The spark plug has several sections to read to determine the engine tune-up: the porcelain, the housing ring around the porcelain, the strap or outer electrode, the center electrode shape, and the threads. The porcelain is the insulator between the center electrode and the spark plug housing. On a new spark plug, it starts out white. After a test run, the spark plug can be examined with the naked eye or under a magnifying glass for minute particles, cracking, or surface bubbling.
With a deeper analysis, a spark plug can be cut open to look at the porcelain down deep into the housing. However, for this article, we will just discuss discoloration of the visible part of the porcelain.
Spark Plugs and Gasoline Fuels
For gasoline fuels, the residuals of carbon compounds from rich mixtures of gasoline fuels can cause discoloration. Proper gasoline fuel mixtures for long endurance racing usually leave only a faint porcelain discoloration. However, discoloration can increase with more racing mileage.
In racing engines with carburetors, excessive accelerator pump volume can send a surge of rich mixture into the engine that may or may not clean up with higher engine speed. Repeatedly rapping the throttle is a tease to the carbon build-up on the spark plug porcelain.
Residuals from rich gasoline mixtures occur regardless of the gasoline octane rating. Low-octane, as well as high-octane gasoline fuels in a rich mixture leave carbon compound deposits on the spark plug porcelain. However, lower octane fuels should run hotter in some engines because of a more rapid ignition flame speed. This leaves more heat indications in spark plugs from lower octane fuels. Engines that run hotter due to lower-octane gasoline may burn off more residuals in some cases, whereas more residuals may build-up from an engine using higher octane fuels.
Rich and lean gasoline mixtures
As you can guess, rich gasoline mixtures will discolor the porcelain. Besides being an indicator of mixture content, excess carbon build-up is electrically conductive and drains the strength of the spark, reducing power. Some tuners offset that effect with increased spark advance. We’ve seen ignition advances of 10- to 15-degrees more than baseline values to offset a weak ignition. Performance was about the same with the increased spark advance compensation in several of our drag racing setups.
Stoichiometric mixtures of gasoline do not provide much discoloration. Gasoline mixtures up to a certain point on the lean side may increase engine operating temperature and show signs of heat on the plug. Without an adequate cooling system, a lean condition can cause overheating, pre-ignition, or even detonation, which could cause porcelain failure. If engines are run even leaner beyond that point, the engine may be showing signs of running cooler because of inadequate fuel burning with all of the excessive air going through the engine. Reading the combustion temperature from a plug is not necessarily a good indication of fuel mixture leanness, by itself.
Spark Plugs and Alcohol Fuels
For alcohol fuels, there are essentially no fuel residuals on the porcelain from rich mixtures, including pure ethanol (99-plus-percent) or pure methanol. This means that porcelain color remains white after running with ethanol or methanol. These fuels do not have carbon deposits from normal combustion like gasoline mixtures. When porcelain discoloration occurs with pure alcohol fuels, it is from oil burning deposits.
E85 is becoming more common in racing. It is reported to eventually build-up gasoline deposits if the mixtures are rich, although the excessive alcohol content tends to clean the spark plug porcelain, making it tough to read the color of the porcelain as a tuning guide.
Rich and Lean Alcohol Mixtures
Lean mixtures of alcohol fuels can provide misleading results. Excessively lean mixtures may result in cold combustion from inadequate fuel. This can leave a wet, uncolored spark plug housing looking like a rich mixture, when the opposite is actually true.
Slightly lean mixtures of alcohol fuels should leave no color from the fuels as well. If engine temperature is excessive from the lean mixture, porcelain failure may occur. This includes surface failure, cracking, and peppering of the surface from burnt metal particles from the engine.
Of note, is that most racing engines running alcohol fuels run very high compression ratios. Supercharged engines usually run high compression and high boost. To prevent detonation and overheating, overly rich alcohol fuel mixtures are anywhere from 15-percent rich to 100-percent rich. Richer mixtures are also used in engines with marginal cooling systems, as they work well at cooling an engine. In these cases, running without enough fuel enrichment can cause overheating. These overheating engines may still be very rich from a stoichiometric standpoint, just not rich enough to maintain the cooling effect desired.
Conversely, lean spark plug indications from excess engine heat can actually be made with a rich alcohol fuel mixture. Record-running sprint car engine temperatures are typically run at 220°F cooling system temperature. In comparison, high-performance racing engines on the street are usually not run over 200°F cooling system temperature. The sprint car with good oil control and the street engine should both have white spark plug porcelain.
Discoloration from engine oil usually shows as a dark build-up, ranging from a slight discoloration to a completely dark color. This occurs with carbon build-up from excessive oil consumption, coming from one or several sources:
- Engine oil blow-by from rings
- Leaking around valve guides from excessive valve guide clearance or failed valve guide seals
- Engine oil leaking into the intake tract such as from:
- Bad head gasket
- For engines where pushrod guide tubes run into the intake ports: unwanted leaking between the tube and the port casting, forging, or billet
- Intake port casting failure.
Spark Plugs and Nitro Fuels
Nitromethane and mixtures of nitromethane and methanol do not leave carbon deposits on porcelain from rich mixtures. Other colors may appear such as peppering or cracking due to excess temperatures, from detonation, or inadequate cooling from combustion temperatures.
Spark plug porcelain indications can occur when running low percentages of nitro with methanol. In our drag race engine testing, we ran a test program for various percentages of nitro ranging from 6-percent up to 30-percent. With good air/fuel ratio mixture control, we saw porcelain indications similar to spark plug reading from straight methanol. Although, starting at 6-percent, porcelain cracking and peppering occurred in a couple of cylinders due to uneven fuel distribution from our manifold and blower arrangement, even though the overall air/fuel mixture was well-defined and set up. This prompted the addition of port nozzles to even out the fuel distribution. Spark plug readings from then on were quite even across all eight cylinders and similar to those of straight methanol.
We did not see this porcelain sensitivity on straight methanol mixtures. Overall air/fuel ratio was rich, but because of fuel distribution problems, it was just not rich enough on a couple of cylinders to prevent porcelain failure with even low percentages of nitro. Nitro is sensitive if it is wrong!
Other events that discolor spark plugs
Regardless of the fuel, detonation can cause porcelain failure. Surface failure and cracking may appear along with metallic peppering on the porcelain (metal flakes) from overheating the pistons.
Lean mixtures including those that are stoichiometricly rich, but simply not rich enough, can cause overheating. This can cause failure of one or more engine parts. Pistons can melt, ruining the piston ring seal, causing excess engine oil consumption. In more dramatic failures, a burned hole in the piston can ignite engine oil causing a fire. If either occurs and is caught before engine meltdown, spark plug porcelain should show excessive carbon compound buildup from oil consumption. Where alcohol or nitro fuels are used, porcelain color change would not be from the fuel deposits.
Spark Plug Readings from Air Density Changes
When racing with gasoline at lower air densities, porcelain will show richness without fuel mixture adjustment. At higher air densities, porcelain color may show leanness without a fuel mixture adjustment. It is possible with a lean mixture to not make enough heat to show any heat indication on the spark plug. Often, this is mistaken as a rich condition where the tuner leans the engine further making it colder yet. Exhaust temperature sensors can produce misleading results in this case as well. Controlling the actual air/fuel ratio is a good way to avoid this.
When racing with alcohol fuels at low or high air densities, porcelain color should not indicate a difference. Other indicators such as spark plug housing discoloration from engine heat, engine temperature, and sometimes exhaust thermocouples are needed for tuning. However, the exhaust thermocouples may produce misleading information when engine temperatures are high if it is from after-burning in the exhaust. This can cause a misleading indication of an overly lean or rich engine.
For gasoline fuels, porcelain color is a good indicator of a rich or lean mixture as well as tuning needs from air density changes. For alcohol fuels, however, porcelain color is not a good tuning indicator for different mixtures or different air densities. It must be remembered that porcelain color can be masked by excess oil consumption leading to carbon compound build-up and loss of ignition strength.
Peppering on the porcelain is a good indication of an engine that is overheating and causing metal particles to break away from the piston, appearing as burnt specks on the porcelain. Finally, porcelain cracking is a good indication of engine overheating and/or detonation. So, while a useful tool in the toolbox for determining the engine’s tune-up, reading plugs can be tricky, and should not be your sole source of information in tuning your engine.
Spark Plug Indicators By Fuel
- Stoichiometric mixtures offer little to no color. However, slight discoloration of the porcelain can be expected with a correct air-fuel mixture over extended periods of use. The longer the period of use, the darker the color.
- A rich mixture results in a dark porcelain coloration from carbon buildup. The darker the color, the richer the mixture is.
- A lean mixture can be indicated by excessive heat indicators on the plug and ground strap. Excessively lean mixtures will show a “peppering” of carbon on the insulator from detonation, and in even more extreme cases, cracking of the porcelain.
- No porcelain discoloration should occur in normal use with alcohol-based fuels, making porcelain color a poor indicator of air/fuel ratio. Porcelain discoloration in an alcohol application indicates the presence of oil in the combustion process.
- A rich mixture can show similar heat signs to that of a lean plug, without the peppering or porcelain damage. Extremely rich conditions leave a wet plug with minimal signs of heat throughout the plug and ground strap.
- A light to moderate lean condition will show signs of excessive heat in the plug, moving into peppering and porcelain failure from detonation. Extremely lean conditions can show misleading signs of being rich, with a wet plug and lack of heat, similar to an excessively rich mixture.
- Nitro, like alcohol, doesn’t leave carbon deposits so the color of the porcelain is not an indicator.
- A rich mixture of nitro can result in porcelain peppering and cracking from excess heat and detonation, or inadequate cooling.
- A lean nitro mixture can also result in cracking and peppering of the porcelain.