Spark Plug Heat Range & Gap
"Heat range" describes how quickly a spark plug transfers combustion heat to the cylinder head. A colder plug pulls more heat out — needed for high-compression, high-rpm, or supercharged engines. A hotter plug retains heat to burn off deposits — needed for low-load, low-rpm street use.
Typical Gap by Ignition Era
| Ignition System | Years (typ.) | Gap |
|---|---|---|
| Points / breaker (Kettering) | pre-1975 | 0.025–0.030" |
| GM HEI (high-energy) | 1975–early 1980s | 0.045" |
| Ford Duraspark / DSII | 1976–1984 | 0.044" |
| Mopar electronic (orange box) | 1972–1989 | 0.035" |