The ignition coil transforms the battery’s 12-volt supply into the high voltage (typically 15,000 to 25,000 volts) needed to produce a spark at the spark plugs. The coil must be matched to the car’s earth polarity, positive-earth and negative-earth coils have their internal primary and secondary windings connected differently, and fitting a coil of the wrong polarity will produce a weak, reversed-polarity spark that causes difficult starting, misfiring, and poor fuel economy.
Upgrades
The standard Lucas coil is adequate for a standard-specification engine. Performance coils producing a stronger, more energetic spark are available for engines with higher compression ratios, modified ignition timing, or electronic ignition conversions. A higher-energy spark provides more reliable ignition of the fuel-air mixture, particularly under conditions where a standard coil may struggle, cold starting, high-rpm operation, and lean mixture conditions.
Ignition Coil Overview
When replacing a coil, the HT lead from the coil to the distributor cap should be inspected at the same time. A deteriorated HT lead can leak spark energy to ground before it reaches the distributor, producing the same symptoms as a weak coil. Silicone HT leads provide superior insulation and heat resistance compared to the standard carbon-core type.