Forget the musclecar era, this is the golden age of horsepower. Never in the history of high performance has it been easier and less expensive to make killer power. Face it, 500 hp is the new 400 hp, and it escalates from there. All this comes down to what the internal combustion engineers call BMEP: brake mean effective pressure. This cylinder pressure is what drives the pistons down. Pressure comes from heat, so that means with all this newfound power, engines are cooking up more heat in those cylinders.
There's more to choosing the optimal spark plugs than choosing ones that just screw into the hole.
When building a custom engine combination, it becomes a challenge to choose the right spark plugs, since there are often no factory reference points from which to start. And this is much more than just "if the spark plug fits, wear it." There are questions of heat range, extended nose versus standard plugs, copper versus platinum versus iridium electrode, and the incredibly diverse world of specialty or "designer" spark plugs. We'll shed some light on all these factors, but expect to do some homework to intelligently choose the right plugs for your application.
In Car Craft's own dyno testing, we've recently been victimized by overheated and pre-ignition-plastered spark plugs, and that never used to happen to street-friendly engines. This sparked an investigation into the world of fire-starters, and we learned some tricks you should know before you light a fire under your latest horsepower effort.
Where To Start
It seems every engine requires its own uniquely designed spark plugs, but despite the fact that spark-plug catalogs' pages now number in the hundreds, for the sake of domestic car-crafting needs, we can condense spark-plug construction to four separate areas: hex size, sealing seat, thread size, and thread reach. Older '60s and early '70s engines generally used 131/416-inch hex sizes, but the industry has been gradually making spark plugs smaller to fit increasingly tighter spaces, so the 51/48-inch-hex plug shell is the most common. As long as the other requirements are fulfilled, you can easily exchange 131/416-inch-hex plugs (like for the '60s big-block Chevy) with 51/48-inch-hex versions.
Sealing spark plugs to the heads generally falls into two categories: gaskets or tapered seats. Generally, iron heads use tapered seats to seal in cylinder pressure, while aluminum heads generally use gaskets. But even this isn't set in stone, since the GM Gen III small-blocks with aluminum heads use tapered-seat plugs. Some aftermarket cylinder heads are now machined for either, but gaskets are still a better idea for aluminum heads, since they prevent metal transfer between the soft aluminum and the steel plug shell.
The most popular thread size for domestic plugs is 14 mm, although a few older Ford engines used the larger 18mm size back in the '60s. Thread reach does vary, but the two most common flavors are the short 0.460-inch and the longer 0.750-inch versions. The longer thread reach is generally used most often with aluminum heads, while the shorter versions are used with iron heads.
Here is an example of a long-thread-reach (0.750-inch) versus a short-reach (0.460-inch) plug. Both are 51/48-inch-hex with the long-reach plug using a gasket seal style compared with the tapered seat of the short-reach plug. | 
Most performance cylinder-head companies recommend some kind of antiseize thread coating to prevent galling, but use this stuff sparingly. Antiseize is also a great electrical conductor, which can cause shorted-out plugs if it gets anywhere near the center electrodes. | 
Most Gen III GM V-8 and some Ford engines use tapered-seat plugs with an in-between 0.0708-inch thread reach. If you use slightly longer 0.750-inch thread-reach plugs, a portion of the threads will protrude into the chambers and cause all kinds of grief and possible pre-ignition problems. This is why the right spark-plug application is so important. |