Typically, the J-strap electrode...
Typically, the J-strap electrode is welded to the metal casing. It’s along this weld that cracking—and eventually breakage—occurs in Cup motors. When the electrode falls off, it not only eliminates the spark but the electrode bouncing around inside the combustion chambers usually leads to damaged valves and scored cylinder walls.
But that's not too often a problem in circle track racing where engine builders often recommend a plug on the cold end of the heat range. The trouble comes more often from running a plug that's too hot and heats up beyond the range it's designed to operate within. This can happen quite easily in performance engine packages like the race engine in your stock car.
Engines in OEM vehicles don't typically see super-high rpm levels or run for extended durations under load and use a much colder plug. In fact, even drag race engines with much higher horsepower numbers will usually run a colder plug than a stock car because they are run for much shorter periods.
A plug that is too hot will pre-ignite the air/fuel charge in the combustion chamber before it actually fires. This happens because the high heat-range plug is too well insulated and the electrode gets hot enough to ignite the fuel without the help of a spark.
Pre-ignition leads to detonation, which is also called knock. It's deadly to an engine because it begins the combustion process too early and builds pressure in the cylinder while the crank is still trying to push the rod and piston up to TDC. The result is pounded bearings, broken pistons and bent rods—and various other problems. To avoid this, you have to find the spark plug that is hot enough to burn off the carbon buildup but not so hot that is causes detonation.
The Right Plug
Because the right plug will vary depending on many variables, including horsepower, combustion chamber design, cylinder head material, and other factors, we can't simply recommend a specific plug. Instead, you'll have to find what works best for you.
Here’s an example of a spark...
Here’s an example of a spark plug that was too hot for the application. You can see the damage to the lower electrode from excess heat in the plug. Had it not been caught in time it could have led to engine damage.
Begin by talking with your engine builder. If you're building your own engine, ask around at your track with people racing in your class. That should at least get you going in the right direction.
If you're starting completely from scratch, it's always best to err on the side of caution and choose a plug that is probably too cold rather than one that's potentially too hot. After all, the worst case scenario is a fouled plug rather than detonation and a broken engine. If the plug is too cold, move up one heat range at a time until the plugs stop fouling and instead has a slight gray coloring after a long run.
If you're confident you can detect pre-ignition early enough to avoid engine damage, keep working your way up the heat range ladder until you detect the start of detonation and then back off a step. This process is obviously best done during testing and not the night the night of a race.
One area where spark plugs have evolved greatly over the years is the electrode design. The traditional spark plug uses a "J-strap" design for the electrode that extends over the tip of the plug. These are the plugs that need to be gapped for proper clearance between the tip of the plug and the electrode.
But having the J-strap extend all the way over the end of the tip can actually shelter the spark from the air/fuel charge. As a result, most performance plugs now feature an electrode that is still the J-strap configuration but ends nearly lateral to the tip. Now the spark no longer extends directly up to the electrode, but out laterally. This exposes more of the spark to the air/fuel charge for a larger flame kernel which can help power. In high compression engines it's also more difficult to ignite the air/fuel charge and this design—along with a high-power ignition—helps improve firing efficiency.