Misfire Mayhem
The voltage required to produce a spark can range from as little as 5,000 volts to as much as 30,000 volts or higher. The actual firing voltage will vary depending on operating conditions such as engine load, rpm, temperature, compression and the richness or leanness of the air/fuel mixture. The wider the electrode gap and the greater the load on the engine, the more voltage it takes to fire the spark plugs. Likewise, the higher the resistance in the spark plugs and plug wires, the higher the required firing voltage.

A cylinder may misfire if the spark never reaches the plug due to excessive resistance or breaks in the insulation in the plug wires, or a buildup of oxide, cracks or an excessively wide air gap inside a distributor cap. A weak coil or a faulty ignition module that doesn't give the coil enough time to fully charge between firings also can reduce the available firing voltage to the point where the spark may be too weak to jump the electrode gap. Of course, worn or dirty spark plugs also can make an engine hard to start, idle roughly, lack smoothness, waste fuel and pollute, too.

The most common cause of ignition misfire at the spark plug is fouling. A buildup of fuel and oil residue or other contaminants on or around the plug's electrodes can short out the spark before it reaches the gap. Contaminants also can form a barrier that blocks the gap or requires more voltage to punch through than the ignition system can deliver. The contaminants come from fuel additives as well as oil that gets past the rings and valve guide seals. A high-mileage engine with worn rings, cylinders and/or valve guides will often have a plug fouling problem.

On 1995 and newer vehicles with OBD II, ignition misfire will usually set a fault code and turn on the Check Engine light. Fuel that isn't burned causes a huge increase in hydrocarbon emissions and will usually cause a vehicle to fail an emissions test. Unburned fuel also can damage the catalytic converter by causing it to overheat. So if you find a cylinder-specific misfire code such as P0302 (indicating cylinder #2), check the spark plug, plug wire, coil (if it's a DIS or coil-on-plug system), fuel injector and compression to isolate the cause. On the other hand, if you find a random misfire code (P0300), the problem probably is not the ignition system. It's likely a lean fuel mixture caused by a vacuum leak or dirty injectors.

Plug Service
As a rule, replacement plugs should have the same or better service interval as the original plugs. Long-life platinum and iridium plugs cost a little more than standard spark plugs and are an excellent upgrade for engines that were not originally equipped with these types of plugs. So replace same with same or better - unless an engine that was originally equipped with long-life plugs has a plug fouling problem due to high oil consumption. In this case, it doesn't matter what type of plugs you install because they won't last as long as normal. If the customer can't afford to fix the oil burning problem, switching to a slightly hotter plug may help reduce fouling.

For performance applications, switching to a slightly cooler plug can reduce the risk of pre-ignition and detonation at high rpm and loads.

Choosing Racing Spark Plugs

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.

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.

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



Spark Plug Replacement Tips


Before you install any spark plug, compare the old and new plugs to make sure the replacement plugs have the same thread diameter, pitch (SAE or metric), thread length and seat configuration as the original plugs.

On engines with aluminum heads, let the engine cool before you attempt to loosen and remove the plugs. This will reduce the risk of damaging the threads in the cylinder head.

Always inspect the old plugs after they have been removed and note any conditions that would indicate a cylinder is running rich, lean or is burning oil.

Plug wires should also be inspected - and replaced if the insulation is damaged, if resistance exceeds factory specifications or the boots are loose.

Torque and Gap

How much the plugs should be tightened depends on the size of the plugs and the type of plug seat. Spark plugs with gasket-style seats require more torque than those with taper seats. Always follow the vehicle manufacturer's torque recommendations, but as a general rule:

14-mm plugs with a gasket-style seat should be tightened to 26 to 30 ft.-lbs. in cast iron heads, but only 18 to 22 ft.-lbs. in aluminum heads.

18-mm plugs with gasket-style seats should be tightened to 32 to 38 ft.-lbs. in cast iron heads, but only 28 to 34 ft.-lbs. in aluminum heads.

14-mm taper seat spark plugs should be tightened to 7 to 15 ft.-lbs. in both cast iron and aluminum.

18-mm taper seat spark plugs should be tightened to 15 to 20 ft.-lbs. in both types of heads.

As for setting the plug gap, always follow the vehicle manufacturer's recommendations - unless you are installing a set of Bosch's Platinum+4 spark plugs. These plugs are pre-gapped at the factory to a standard 1.6-mm gap. This is necessary to achieve maximum plug performance and longevity, so don't change the gap.

The reason is that the multiple ground electrodes in the Bosch Platinum+4 and Platinum+2 series are made from a special, wear-resistant nickel alloy, enhanced by the addition of yttrium-alloy. Not only do these plugs utilize factory set "surface air gap" technology that doesn't need gapping prior to installation, the manufacturer also said that the special alloys resist erosion so the gap maintains its correct setting for the life of the plug.

"Every time a spark plug fires, a tiny bit of metal is lost in the electrical discharge. Over time, this can degrade a plug's performance," said Otto Stefaner, general product manager, consumer products, for Bosch. "Multiple electrodes and precious metals minimize this degradation from metal transfer."

Stefaner also explained other benefits to their pre-gapped design. "Surface air gap technology also increases the electrode gap for better ignition, without increasing the voltage requirement, enabling the production of a larger flame core in the combustion chamber. Tests show that the four-electrode, platinum-core plugs transfer more energy efficiently to the air/fuel mixture. And plugs with four ground electrodes have up to 33% better cold restart reliability than conventional plugs," he said.


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