Unleaded fuels do not color plugs as clearly as leaded fuels, so you have to look closely for the A/F ratio ring at the bottom of the center porcelain. The big things to look for are tiny flecks of aluminum that have transferred to the ceramic from the piston or perhaps the combustion chamber during an oh-too-lean condition that has caused aluminum to melt.
Ignition timing plays a huge part in terms of spark-plug temperature. The best place to read this is on the spark plug's ground strap. The tip of the ground strap will tend to color first, and then, as temperature increases, this coloring (called annealing) will travel down the length of the strap toward the shell. An ideal ignition-timing temperature range for a given spark plug will create a light blue ring around the strap about halfway down its length. If the ground strap shows no coloration, the engine could respond to more timing. If the ground electrode begins to color in blues and greens or the tip begins to melt, the timing is over-advanced. Of course, if you are tuning at the dragstrip, these conditions will have raised red flags in terms of lost e.t. or trap speed as well.
The Demands of Superchargers, Turbos, and Nitrous Power-adders do a fantastic job of increasing cylinder pressure and heat. This also places much greater demands on the spark plugs. Nitrous is perhaps the most abusive because of its reputation for extremely fast cylinder-pressure rise time. Some rules of thumb include running a colder heat range and using nonprojected-nose plugs. The general consensus among power-adder tuners is that it's best not to use platinum plugs, since this material acts as a kind of heat sink that will cause pre-ignition problems. Other tips include running a tighter gap, since high cylinder pressures place much greater demands on the ignition system. The best plug gaps are the largest ones that will survive; start with 0.030 inch.
For nitrous, a safe recommendation is one step colder for every 75 to 100hp increase in power. So, for a 300hp engine adding 200hp of nitrous, this would mean using plugs that are two steps colder. This is also a good starting point for similar power increases either normally aspirated or with a supercharger or turbo.
Specialty Plugs
This is the age of designer spark plugs. SplitFire rode the first wave of specialty plugs with its V'd ground strap, and now it seems that every spark-plug manufacturer has its own version of what's hot. While the debate will continue to rage over whether diamond-shaped or split-V ground straps are worth more power/mileage/emissions, we do know that plugs using more exotic metals enhance durability. Platinum arrived a few years ago, with Bosch leading the charge with three different versions of its platinum-tipped spark plugs. Then NGK upped the ante with its Iridium IX series of plugs. The plan with virtually all these designs revolves around reducing tip or ground-strap erosion that increases the gap between the center electrode and the ground strap. Some plugs, like the Bosch Platinum +4, use four ground straps to also minimize the overall gap to ground.
Learning to read spark plugs can start with looking at the three tiers of engine operation as indicated on the plugs' porcelain insulators.
The ground strap is a great heat indicator. Over-advanced ignition timing can cause the ground strap to discolor and in extreme cases can cause the strap to melt.
Our buddy Troy LeCrone tells us he tunes his nitrous small-block Chevy with cold AC spark plugs because the ground strap burns easier than with the more durable race plugs. LeCrone views these plugs as having a much quicker "fuse" that will burn more quickly under marginal-lean conditions and perhaps save a head gasket or a piston. Plus, he says, "They're cheaper!"
High-compression engines that use domed pistons can run into mechanical trouble when the piston domes hit the spark plugs. That's what happened here. This required nonprojected-nose spark plugs that still had to be indexed to place the ground strap away from the piston dome.