Autozilla’s Ultimate Guide to Spark Plugs

The spark plug is the middle man between your ignition system and power stroke of your standard gasoline engine. It consists of a center electrode and ground electrode placed just above the center electrode, and both attaching to the body of the spark plug. In between the two electrodes is what is called an air gap, which is where a high voltage spark (40,000 to 100,000 volts) travels to ignite the air/fuel mixture.

Once the mixture ignites, an explosion is created that drives the pistons down, turning the crankshaft, which subsequently turns the driveline and powers your vehicle either forward or backward. With so much at stake for every revolution of the engine, it is easy to see why the spark plug has been the brunt of a lot of improved formulations, one being the composition of the plug.

Copper, Platinum, or Iridium?

The basic spark plug is made of copper, which is a pretty good conductor of electricity. The problem with copper is that it is not as hard as some metals, and therefore, wears down with high pressure and heat over time. This wear eventually causes the plugs to wear (increase gap), foul, and work less efficiently, which can cause a loss in vehicle fuel efficiency and even misfiring in extreme cases. This is because it takes more voltage to jump between the electrodes as the air gaps widens. One way around this is to use a harder metal, like platinum.

Platinum is a harder metal than copper and has a much higher melting point. This gives platinum the advantage of lasting longer than conventional copper plugs. One such spark plug is the Autolite Double Platinum, which is claimed to last up to 100,000 miles. Besides longevity, platinum plugs in themselves do not make the spark better or more efficient than copper. The main advantage of platinum is its ability to heat better, which subsequently burns off deposits, enabling it to reduce wear and the amount of air gap.

There is one way a platinum plug can be considered better at fuel efficiency, and that is if it reduces the size of the center electrode. This reduces the chance of quenching (heat loss), because it leaves less area for the heat to be reabsorbed by the plug. This design also needs less voltage to jump the gap and burn completely. This can be a particularly nice benefit for older engines with less efficient ignition systems. Both benefits give the combustion process the potential to create a more complete burn and greater fuel efficiency.

Iridium is said to be six times denser and eight times stronger than platinum. This dictates that a spark plug can have even finer electrodes (less quenching) than ever before and still maintain excellent wear characteristics. Iridium is also a more precious metal than platinum, which means you could pay as much as double for a spark plug ($12-$14). Is it worth the extra cost? Yes and no.

If you are willing to change your plugs once every year or two (depending on your driving habits), copper plugs offer good performance and efficiency. However, if you would rather install your plugs and then forget about them for up to 100,000 miles, iridium or platinum are the best choice. One such example is the Autolite XP (Extreme Performance).

It is also worth mentioning, that with platinum and iridium plugs you generally get what you pay for. The more expensive the plug, the more platinum (double/triple coated) or iridium is used. A thin coat of these precious metals will not fair as well as a thicker, so in general, you won’t want to go too cheap, as otherwise you’d have been better off going with the good old copper for the maximum value.

Do It or (Not to) Do it Yourself
Spark plug installation are generally fairly simple for anyone with a minimum of mechanical experience. Below is an instructional video by Autolite giving you an idea of what the project will entail. If you are at all hesitant, however, you can also bring your vehicle into your local shop and have them swap out your plugs for a nominal cost.

Shape and Configuration

Splitfire spark plugs have been around for many years. While they have been around a long time, there has never been any research that has proven its design is any better than the standard electrode configuration. In fact, in the late 1990s, Splitfire lost a law suit stating that there was no proof to all to their claims of increased horsepower and fuel efficiency.

Today, although there has been the occasional vehicle that has worked well with this spark plug configuration, chances are these plugs are no better than your standard inexpensive coppers. These plugs work in theory by splitting the ground electrodes, thereby opening the air gap to where the flame kernel is hottest to help reduce the amount of quenching (heat loss).

E3 is the relatively new plug on the market for automobiles. They have a pretty good explanation for the theory of how these plugs reduce emissions, increase gas mileage and performance, but there has never been any conclusive evidence. Horsepower TV dyno-tested these plugs and found an increase of 5 horsepower from a 383 cubic inch V-8, which is interesting, but certainly not the gospel.

At $6 a spark plug, these may just be worth the extra money to see how they work with your configuration. They do currently have a five year/100,000 warranty, which shows some confidence. The E3 works by creating a sort of halo ground electrode around the center electrode where the spark has a variety of places it can form. This leaves the very center open to reduce quenching, as well as create a hotter burning spark.

Multi Electrode Configurations-
The multi electrode configuration plug has been around a long time, and was particularly made popular by the rotary engine, which is known for being hard on the typical single electrode setup. Besides offering an open center, similar to the E3, the multiple electrodes offer several choices from which the spark can appear, therefore reducing the amount of wear on any one electrode.

Unlike what some people think, the multiple electrodes do not each offer spark, but instead just one spark appears at one of the electrodes at any given point during the combustion cycle of the engine. There is no advantage to multiple electrodes, other than the reduced quenching and wear on the spark plug itself. These generally come in platinum or iridium and offer a variety of configurations from two, three, and even four ground electrodes per center electrode.

Multiple electrodes tend to work better with newer cleaner-burning fuel injected engines, as the small center electrode can get fouled rather quickly in a carburetor engine setup. There is evidence that these plugs last longer (especially in a rotary engine) than conventional copper plugs and in some cases offer improved fuel economy and power over the convention single electrode.

Cut-back Spark Plugs-
Drag racers have been cutting back their spark plugs since the sport began, and for one good reason… it works! Similar to all the other specialty plugs on the market, the cut-back plug opens the flame kernel to reduce quenching.

These plugs are basically a standard plug with the ground electrode cut back until it is just over the edge of the center electrode. With the center electrode open to the combustion chamber a more complete burn can be had and therefore, a light increase in horsepower (1 to 5 horsepower) and fuel efficiency (0.5 mpg improvement on up).

These plugs can be made by cutting back a standard set of plugs with a grinder or dremel tool. The one drawback of these plugs as compared to some of the more specialty designs, is they tend to foul and not last as long as a standard plug.

Indexed Plugs-
While not a specialty plug per say, indexing is a technique of installing your plugs that has also been proven to add performance and fuel efficiency. The basic idea behind indexing spark plugs is to draw a line down the spark plug body, so you can position the open-air gap of the spark plug so that it is facing the center of the cylinder in favor of the exhaust valve.

Indexing can be done in a variety of ways, such as either buying several plugs and trying them all until you find a plug that twists-in close enough to the correct angle, or using what is called, index washers. These washers are special copper washers that you can place over the plug, that compress as you tighten, allowing you to set your plugs at just about any angle you want.

Indexing your plugs, reduces quenching and opens the air gap right to the sweet spot of the engine where the most complete burn and best fuel efficiency can be had. If the firing electrode is in the way, as when you just twist spark plugs in without trying to match it, you may not get as complete of a burn as your engine has the potential for. This is why indexing offers such a great advantage. You can expect anywhere from 1 to 5 horsepower, and an increase of 0.5 mpg and up with this project.

A Final Word
While a spark plug alone cannot create an efficient engine as some of the hype might suggest, there are spark plug designs that offer some slight improvement over others. Enough improvement to spend the extra dollars? We’d say yes…