The Effects of Spark Plugs on Emissions

The federal government tests for three basic types of emissions: unburned hydrocarbons, carbon monoxide and nitrogen oxide. Unburned hydrocarbons are just that; fuel that passes through your engine without getting burned. High carbon monoxide also happens because of incomplete combustion, and very nasty nitrogen oxide emissions occur when the combustion chamber gets too hot. The air is about 80 percent nitrogen by volume; while nitrogen oxide isn't normally involved in the combustion process, temperatures exceeding 2,500 degrees Fahrenheit can cause the nitrogen oxide molecule to break apart, combining with oxygen to form oxides of nitrogen.

Any kind of defect in your ignition system can result in a complete or partial cylinder misfire, meaning that some or all of the cylinder's fuel will go through unburned. Fouled, damaged or mis-gapped plugs will cause just this sort of misfire, since they'll fail to ignite the fuel mixture. But even a perfectly clean and undamaged plug can cause high hydrocarbon emissions at start-up. Different spark plugs have different heat ranges; a "colder" plug will better resist damage through high temperatures, but won't work as efficiently as a "hotter" plug at low temperatures. If you fail the cold start-up test on hydrocarbons, it may have something to do with your plug's heat range.

High carbon monoxide levels tend to accompany high hydrocarbon levels, since they're both a result of incomplete combustion. As such, bad or inappropriate spark plugs can have all the same effects on carbon monoxide as hydrocarbon. The good news here is that replacing your stock plugs with a set of performance fine-wire platinum or iridium plugs can help to reduce both carbon monoxide and hydrocarbon output by encouraging a more complete combustion cycle. While you shouldn't expect performance plugs to perform any miracles relative to properly functioning stock plugs, they can help to give you a bit of an edge on test day.

This is something of a sticky subject, since what's good for power isn't always good for nitrogen oxide emissions. Big power means big pressure in the chamber, and that means heat. A high-performance plug could, hypothetically, increase combustion chamber temperatures simply by dint of increasing the amount of fuel burned. But, realistically, the plugs in and of themselves are extremely unlikely to affect nitrogen oxide emissions in any significant way unless you have additional contributing factors. Common causes for excess nitrogen oxide production include an excessively lean fuel mixture, a defective exhaust gas recirculation system, a bad catalytic converter and consistent engine overheating.

Bad spark plugs -- plugs that are fouled, damaged or too cold for the engine -- are going to increase your hydrocarbon and carbon monoxide emissions, while a decent set of aftermarket plugs should reduce them. nitrogen oxide emissions, on the other hand, tend to work in inverse proportions to hydrocarbon and carbon monoxide emissions. More nitrogen oxide means more heat in the combustion chamber, and that's usually a good thing as long as it doesn't indicate an excessively lean fuel mixture. If in doubt, buy the best plugs you can afford and gap them properly; they can only stand to help by getting your engine closer to its ideal operating conditions.