The biggest improvement in spark plug technology came in 1985 when the first generation "long life" plugs with platinum or gold-palladium electrodes hit the market. Up to this point, electrode wear usually dictated when a set of plugs had to be replaced. With standard nickel alloy electrodes, the spark gap between the center and ground electrodes grows about .0002" to .0006" for every 1,000 miles of driving. After 35,000 miles of driving, the gap can grow as much as 0.015" or more.
Every time a spark plug fires, the hot spark blasts a few molecules of metal off the electrodes. Eventually the point is reached where the ignition system can't generate enough juice to jump the gap, causing the plug to misfire.
With platinum, gold-palladium and other exotic metals, electrode wear is greatly reduced. Most platinum plugs can go up to 100,000 miles before they have to be replaced. The same is true for plugs that use other exotic metals, such as iridium for their center electrode. Plugs with platinum on both electrodes ("double" platinum plugs) experience even less wear than plugs with only a single platinum or platinum-tipped electrode.
Long-life spark plugs drastically reduce the need for maintenance while helping to maintain like-new performance. Not having to change the plugs so often is a real savings, but it's no guarantee the plugs will go the distance.
Electrode Magic
Spark plug manufacturers tout the advantages of their unique electrode designs, but regardless of the design, the purpose is to make it as easy as possible for the plug to fire reliably. A spark jumps more easily from a sharp edge than a rounded, blunt edge. So the more sharp edges it has to jump to, the better the odds of the plug firing under all types of driving conditions.
One thing to keep in mind with respect to performance claims is that no spark plug creates horsepower out of thin air. A special electrode configuration can reduce misfiring and the voltage needed to fire the plugs, but the spark only ignites what is already in the combustion chamber.
Fouling Resistance
To burn off carbon deposits, the center electrode needs to reach about 700° F quickly. But if it gets too hot (above -1,500° F), it may ignite the fuel before the spark occurs, causing preignition and detonation. For most plugs, the ideal operating temperature is around 1,200° F.
The temperature of the electrodes is controlled by the length of the ceramic insulator that surrounds the center electrode and the design of the electrode itself. Ceramics do not conduct heat very well, so an insulator with a relatively long nose will conduct heat away from the electrode more slowly than one with a relatively short nose. The longer the path between the electrode and the plug shell, the slower the rate of cooling, the hotter the plug.
A spark plug's "heat range" (heat rating), depends on the length of the ceramic insulator and the design of the center electrode. The heat range must be carefully matched to the engine application otherwise the plugs may experience fouling problems at idle or run too hot under load causing preignition and detonation.
For performance applications, switching to a slightly cooler plug can reduce the risk of pre-ignition and detonation at high rpm and loads.
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.
It's important to remember that 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. Some 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.