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Questioning Misfires

Adapted from articles in Counterman

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Although the spark plug is a wee component, its effect on a properly running internal combustion engine is immense. In fact, this component has “sparked” many a question from professional techs on its use and replacement. We thought we would share with you some technical questions and answers regarding spark plugs and their importance toward a properly functioning engine.

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What Causes a Misfire?

The spark that jumps across the gap between the plug’s electrodes ignites the fuel mixture and releases the explosive power in the fuel. If there’s no spark or the spark is too weak to ignite the fuel, the engine misfires, looses power, wastes fuel and pollutes.

Misfires are bad news because unburned fuel passes right through the engine and into the exhaust. This increases emissions and also causes the catalytic converter to run hotter than normal. A really bad misfire may even damage the converter.

Every engine misfires occasionally, but if the spark plugs are worn or fouled, or the ignition system can’t deliver enough voltage for reliable ignition, misfires may cause hard starting, rough running, hesitation, increased fuel consumption and elevated hydrocarbon (HC) emissions. It doesn’t take many misfires to make a vehicle fail an emissions test.

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What Type of Plug Shall I Use?

One way to prevent misfires is to make sure the engine has a good set of spark plugs. These can be standard plugs, long-life plugs or performance plugs. As long as they produce a hot, reliable spark, ignition misfire shouldn’t be an issue.

To minimize the risk of misfire and maximize ignition performance, today’s spark plugs are designed to resist fouling and wear under a wide range of operating conditions. Improvements in electrode alloys allow many plugs to now last up to 100,000 miles. With conventional plugs, the recommended replacement interval is typically 45,000 miles.

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Why Do Plugs Need Replaced?

Spark plugs need to be changed periodically because of wear and fouling. Every time a spark plug fires, a microscopic amount of metal is lost from the electrodes. As the miles add up, the electrode gap grows wider and wider increasing the voltage required to create a spark. Eventually the point is reached where the ignition system can no longer provide enough volts to fire the plug resulting in a misfire.

Fuel residue and oil ash also build up on spark plugs. Normally, the plugs get hot enough to burn off the deposits, but with frequent short-trip driving, the plugs may not stay hot enough long enough to burn off all the deposits. This can allow deposits to build up and interfere with the spark causing misfires.

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The condition of the spark plug wires is also important. Wires that are damaged, cracked, loose or exceed resistance specifications must be replaced to assure a hot, reliable spark.

What Are Some Plugs Improvements?

Spark plug manufacturers have succeeded in extending the life of spark plugs and fouling resistance by going to high-temperature electrodes made out of platinum, iridium and other exotic metals. Platinum is one of the best conductors of heat and electricity. It also resists chemical corrosion and electrical erosion much better than ordinary steel alloys, making it an ideal material for the electrodes in long-life spark plugs. Iridium is even better. Iridium is six times harder and eight times stronger than platinum. This allows the use of a smaller center electrode, which reduces the voltage required to fire the plug by as much as 5,000 volts compared to a standard spark plug.

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Spark plug manufacturers also use a variety of different electrode configurations to reduce misfires by exposing more of the spark and flame kernel to the fuel mixture inside the combustion chamber. Extended electrodes, surface gap electrodes, multiple electrodes and specially-shaped outer electrodes are all different techniques that are used to improve ignition performance and reliability.

Replacement spark plugs can be any brand and almost any type as long as the thread diameter and length, seat type, electrode reach and heat range are correct for the engine application. Follow the spark plug manufacturer’s recommendations for replacement plugs.

What About Upgrading?

Upgrading from conventional spark plugs to long-life plugs can reduce the need for future maintenance and lessen the need to change plugs as frequently on vehicles where plug access is difficult (like the back plugs on FWD cars and minivans with transverse mounted engines). Today’s tight engine compartments don’t leave much elbow room for changing plugs on many V6 and V8 engines, which is one reason why the OEMs have mostly gone to long-life 100,000-mile spark plugs. The other reason is to reduce the risk of misfires, emission failures and possible converter damage caused by worn or fouled spark plugs.

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What are “Performance” Spark Plugs?

Performance spark plugs are primarily designed to reduce misfires. Some are also designed to last longer than standard plugs, but others are not.

Contrary to what you may have heard, performance spark plugs can’t create horsepower that isn’t there. What they do is maximize engine horsepower by reducing misfires.

When the engine is under load or when the pedal is to the floor, a much denser air/fuel mixture is present in the cylinders and cylinder compression pressures are much higher. This puts more of a strain on the ignition system and makes it harder to ignite the mixture each time the plug fires.

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If the mixture doesn’t light, a misfire occurs and the engine loses power. Every misfire is a missed power stroke so it doesn’t take a lot of misfires to hurt performance, fuel economy and emissions.

When a V6 engine is cruising down the highway at 3,000 rpm, each spark plug is firing 25 times per second. At this speed, it’s virtually impossible to feel or hear an occasional misfire, unless the rate of misfire is really bad.

One way to reduce misfires under load and improve ignition reliability is to expose more of the spark to the air/fuel mixture. Standard electrodes tend to shield the spark somewhat and can actually quench the initial flame kernel under some operating conditions. To open up the spark, some spark plug manufacturers split the tip of their outer electrodes. Others use smaller, thinner electrodes or an outer electrode that comes to a point rather than having a square end. Some plug manufacturers add additional electrodes (two, three or four) and/or use a surface gap electrode to expose more spark to the mixture and reduce misfires as well as electrode wear.

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Performance spark plugs cost more than regular spark plugs because of these added design features, and they generally give the best bang for the buck. Performance plugs are a good upgrade for any high-performance vehicles as well as those used for towing or sustained high-speed driving.

How Do I Choose Spark Plugs with the Correct Heat Range?

Follow the spark plug manufacturer’s recommendations. Spark plugs come in a variety of different thread diameters, pitches, reaches and configurations to fit different engines. Spark plugs also have different operating temperatures or heat ranges that vary from one engine to another. The heat range depends on the type of electrode material (copper and platinum help carry heat away from the center electrode) and the length heat has to travel down the center electrode to the steel shell.

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It’s important to get the right heat range spark plug because this affects the plug’s ability to burn off fouling deposits and to resist preignition and detonation.

If the heat range is too cold for a particular engine, the spark plug may have fouling problems at idle or low speed. If the heat range is too hot, the spark plugs can run dangerously hot and cause engine-damaging preignition and detonation.

Most spark plugs today have broad enough heat ranges to satisfy almost all driving conditions. But for some performance applications it may be necessary to select a plug that’s one or two ranges colder to prevent preignition and detonation when the engine is working hard. Switching to a hotter plug in a high-mileage engine that is burning oil may help reduce fouling and extend plug life.

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Can a Set of Spark Plugs Really Go 100,000 Miles?

Yes, but not if the engine is using oil or the vehicle is used only for short trips. Spark plugs have to get hot to burn off normal fuel and oil deposits. If the engine is never run very long or under heavy enough load to heat up the plugs, they may not get hot enough to burn off all the deposits. Over time, the deposits will accumulate and eventually foul the plugs causing them to misfire.

Electrode wear can also cause plugs to misfire. Every time a spark plug fires (which is thousands of times per mile), a few molecules of metal erode off the electrodes. As the miles accumulate, the electrode gap widens and requires more firing voltage from the ignition system to make a spark. Eventually the gap is so wide that the spark can’t jump across it to ignite the fuel. A misfire occurs, and the engine wastes fuel and pollutes.

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The lifespan of standard spark plugs is typically around 45,000 miles. Long-life plugs, which typically use platinum or iridium electrodes, resist wear much better than standard plugs and can theoretically last much longer, up to 100,000 miles or more under ideal operating conditions. But as was stated earlier, if the engine is burning oil because of worn valve guides, piston rings or cylinders, the plugs will probably foul out long before they ever reach their upper wear limit.

Some spark plugs have multiple electrodes to improve ignition reliability and reduce wear. When a multi-electrode plug fires, there’s still only one spark and it jumps from the center electrode to one of the outer electrodes. The next time it fires, the spark may jump to a different electrode and so on, spreading the wear around and extending the life of the plug.

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Long-life plugs reduce the need for changing plugs and are a good choice for engines where plug access is difficult (which is most V6 and V8 engines these days!) Long-life plugs cost more than standard spark plugs, but more than pay for themselves in their greater longevity and performance.

Steps to Choosing Racing Spark Plugs

Do you like to race? Selecting the proper race plug for your customer’s engine can mean the difference between front of the pack and a DNF. When using this guide, understand that race plugs are usually of a much colder heat range rating than automotive plugs. Colder plugs must be used in engines with increased cylinder pressures, higher temperatures and greater BHP. Other factors such as fuel delivery (turbo, supercharged), fuel types and piston-to-head clearance will also affect proper plug selection.

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Step 1: Shell Design – The first step in choosing the proper race plug is determining the plug type that your cylinder head/piston will accept. Thread diameter and pitch, thread length and shell seat, as well as hex size are all factors that will define what shell type works best for your engine.

Step 2: Electrode Design – The second decision is electrode design and configuration. Is it a fine wire center or standard electrode? Projected or non-projected? Full coverage ‘J-Gap’ or perhaps a cut-back or angled ground wire? A good rule of thumb is to attain as much projection into the cylinder as possible. But be aware of piston clearance that could prohibit projected designs from being used.

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Step 3: Heat Range – The third factor in choosing a race plug is heat range. Correct heat range is critical in maintaining peak performance throughout the duration of your race or event. Switching to a colder or hotter plug will not increase horsepower, but could affect engine performance. Choosing a plug that is too hot can result in preignition or detonation. A plug that is too cold could cause an engine to stumble, misfire or foul.

The main factors to consider in selecting the proper heat range are: type of race, methanol, specific output, nitro-meth, compression ratio, nitrous oxide, horsepower, super or turbo charging and racing fuel.

Source: Champion Spark Plugs

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