A spark plug does two jobs: ignite the mixture, and shed combustion heat into the cylinder head. Heat range describes how fast that heat travels from the firing tip up through the insulator nose to the shell, and ultimately into the head's cooling jackets. Get it wrong and you either foul the plug or melt a piston.

A cold plug has a short insulator nose — heat takes a short path to the shell, so the tip stays cool. A hot plug has a long insulator nose that retains heat at the tip. Why does this matter? The plug needs to stay above roughly 450°C (the carbon self-cleaning temperature) but below about 850°C, where the tip becomes a glowing pre-ignition source that lights the mixture before the spark fires.

Rule of thumb: for every 75–100 hp added per cylinder via boost, nitrous, or aggressive tuning, drop one heat range step colder. NGK and Denso number their plugs opposite each other — higher NGK numbers (e.g., 8, 9) are colder; higher Denso numbers are hotter. Always check the manufacturer's chart.

Real-world example: A stock Subaru WRX EJ255 ships with NGK SILFR6A (heat range 6). Owners running an E85 tune at 22 psi commonly swap to SILFR7A or even an 8-range plug. Skipping this step is a known cause of detonation and ringland failure on tuned EJs — the stock plug runs too hot under elevated cylinder pressure and acts as a glow plug between cycles.

Electrode design is the second variable. Older plugs used a nickel alloy ground and center electrode about 2.5 mm thick — cheap, but they erode quickly and demand more voltage as the gap widens. Modern iridium and platinum plugs use a fine-wire center electrode (often 0.4–0.6 mm) for several reasons:

• Lower required voltage — sharper edges concentrate the electric field, so the spark jumps at lower kV. Important when coil energy is marginal at high RPM.
• Less flame kernel quenching — a thin electrode shadows less of the early flame front, improving lean-burn and idle stability.
• Long service life — iridium melts at ~2,450°C and resists erosion, so gap growth over 60,000–100,000 miles stays minimal.

Gap setting matters too. Wider gaps (1.0–1.1 mm) light a bigger flame kernel and help fuel economy in NA engines. Boosted engines tighten the gap to 0.6–0.7 mm because cylinder pressure raises the voltage required to bridge the gap — a worn coil simply can't punch through a wide gap at 25 psi, causing misfires that feel like fuel cut.