What is a spark plug heat range?
The heat range is probably the most misunderstood — and the most dangerous to ignore — spec of a spark plug. It has nothing to do with how "strong" the spark is, and everything to do with heat management. Here is what you actually need to know to never get it wrong.
What the heat range really measures
During combustion, the firing end of the plug (the centre electrode and the ceramic insulator around it) reaches several hundred degrees. The heat range describes how fast the plug sheds that heat into the cylinder head and, from there, into the cooling system.
It all comes down to the length of the insulator "nose":
- A long insulator nose has a long path to release heat: the plug stays hot. This is a "hot" plug.
- A short insulator nose dumps heat into the metal almost immediately: the plug stays cool. This is a "cold" plug.
Counter-intuitive but essential: "hot" and "cold" do not describe the temperature of the spark — they describe the plug's own ability to cool itself.
There is an ideal "thermal window", roughly 450 °C to 870 °C at the firing tip. Below it, carbon deposits don't burn off and the plug fouls. Above it, the tip glows and can ignite the mixture before the spark fires (pre-ignition) — the exact scenario that melts a hole through a piston.
Hot or cold: which one for which engine?
| Use case | Recommended plug | Why |
|---|---|---|
| Small engine, low revs, short trips (city) | Rather hot | Reaches its self-cleaning temperature despite light load. Prevents fouling. |
| High-output engine, high revs, motorway, sport | Rather cold | Sheds excess heat quickly. Prevents overheating and pre-ignition. |
| Normal road use | Medium | The factory compromise — perfect for 95% of cases. |
Workshop rule: you don't go "up" a heat range to gain performance. You match the heat range to the engine's actual thermal load. A tuned engine that runs hotter needs a colder plug; an engine that mostly idles in town and fouls needs a hotter one.
Why brand numbers mean nothing across brands
This is the classic trap. Every manufacturer has its own scale, and they don't even run the same direction:
- NGK: 2 to ~13. The higher the number, the colder the plug. (BPR6ES → medium; BPR9ES → much colder.)
- Denso: ~14 to 37. Higher number = colder plug.
- Champion: reversed scale — the higher the number, the hotter the plug.
So an NGK "6" has nothing in common with a Champion "6". Comparing raw numbers between brands is the number-one cause of wrong fitment.
That's exactly why TheSparkTwin never shows the raw number, but a universal category: Very hot → Hot → Medium → Cold → Very cold. Two genuinely equivalent plugs always share the same heat range, whatever the brand — and that equivalence is what matters, not the number.
The symptoms of a wrong choice
Plug too cold for the engine
- Rapid fouling (black, wet or oily deposits on the tip);
- Cold-start misfires, hard starting;
- Higher fuel consumption, rough idle.
Plug too hot for the engine
- Very light, whitish or "glazed" electrode and insulator;
- Knock, loss of power under load;
- Worst case: pre-ignition and a melted electrode or piston crown.
Tip colour is a real thermometer. After a few hundred kilometres, a correctly-rated plug shows a light brown / tan tip. Too black = too cold (or too rich). Too white = too hot (or too lean). This "plug reading" is exactly what mechanics and tuners rely on.
In short
- Heat range = rate of heat dissipation, not spark strength.
- Hot = short trips / small engines; cold = high-output engines; medium = the majority.
- Never compare numbers across brands: trust the heat-range equivalence, not the digit.
- When in doubt, keep the factory heat range. Only change it for a real reason (chronic fouling or engine tuning).