No one denies that spark plugs are a critical component in getting the most power and reliability out of your engine combination. But, spark plugs are definitely far from one-size-fits-all — are we aren’t just talking about thread pitch here. In the above video, Dart talks about some of the different variables that affect your choice of the correct spark plug.
There are a number of factors that dictate what spark plug is right for your specific combination. The first is the cylinder head itself, which will dictate the spark plug’s required length and thread pitch. With Dart heads, the thread will generally be 14mm x 1.25 (widely known simply as “14mm” plugs), and in one of two common lengths. The shorter plug will be a .460-inch design, while the longer plugs will be a .750-inch design.
Once you know exactly what physical plug size you need, then, your application comes into play. There are a number of plug variables that will be dictated by things like cylinder head material, combustion chamber thermal efficiency, fuel, compression ratio, cylinder pressure, and any form of power adder you might be using. Any change in combination will likely see a benefit from changing at least one of the variables in the spark plug.
Looking at some of those variables in plug design we have protrusion, which is sometimes called “reach.” It is the distance in which the insulator and electrode extend past the threaded portion of the spark plug. Mechanically, you’d like to see the electrode in a position within the chamber that isn’t so extended that it risks contacting the piston, but not so recessed that it inhibits the spark from igniting the flame kernel.
Then, we have heat range. Another of the combination-dependent variables ideally you’d want a plug that is hit enough to make sure it retains its self-cleaning properties but not so hot that it can cause pre-ignition. The “heat range” of the plug is controlled by the shape of the insulator within the spark plug.
By varying the design and physical location of the insulator material, the thermal efficiency of the spark plug can be altered. Not to be confused with protrusion when looking at the plug, you can physically see the difference between a “hotter” and “colder” plug. Generally, as you add cylinder pressure, be it through compression ratio, boost, or nitrous, you’ll want a colder plug.
So now that we’ve thoroughly confused you with a bunch of variables, where do you start? Well, below you’ll find a chart from Dart with NGK part numbers (which you can use as a cross-reference for your particular spark-plug brand of choice, if you favor something other than NGK). Notice the recommendations are based on each particular cylinder head in its lineup, with the recommended plugs for naturally aspirated compression ratios from 8.0:1 to 16.0:1, as well as heavy nitrous-specific applications, blower and turbo applications, and with alcohol-based fuels.
While this article won’t tell you the exact perfect plug to run in your combination, it should give you a better understanding of the variables going into selecting a plug, and if you happen to have Dart heads on your combination, the chart should get you to a starting point that is a lot closer than just in the ballpark.