The Unsung Hero of the Ignition System
Tucked away in the engine bay, often overlooked and underappreciated, is a device that’s been quietly making the magic happen for over a century - the ignition coil. In the old days, they came in metal canisters, oil-filled and mounted somewhere on the firewall, intake manifold or the side of the engine. These oil-can coils might look simple, but they’re doing something nothing short of incredible.
Let’s break it down: In a 6-cylinder engine running at 5040 RPM, you’re getting 252 sparks per second. That’s one spark every 3.97 milliseconds. That’s relentless. Now spin that same engine up to 6500 RPM, and you’re looking at 325 sparks per second—a spark every 3.08 milliseconds. And worse still, if it’s a V8 at 6500 RPM, that coil has to throw out 433 sparks per second - that’s one every 2.31 milliseconds, all from a single coil. It means the coil has to charge, saturate, fire and cool in under the blink of an eye - and then do it again and again, hundreds of times per second. That’s a brutal pace for any electrical component, especially one built before digital controls were a thing.
To pull this off, the coil has to charge up, store the energy, and then dump it through the ignition circuit fast enough to keep up with the firing order. And it has to do it cleanly, consistently, and hot enough to jump the plug gap and ignite the air-fuel mixture. No room for a weak spark at high revs. At higher RPM, that time window shrinks even further and the margin for error disappears. If the coil doesn't saturate fully, the spark gets weak. If it overheats, resistance goes up and the output voltage drops. Either way, misfires creep in.
How It Works: The Heart of the Spark
Inside that metal can is a simple transformer. It has two windings, a primary winding with a few hundred turns of thick copper wire, and a secondary winding with thousands of turns of much thinner wire. When current flows through the primary winding and is suddenly interrupted (usually by mechanical points or an ignition module), it collapses the magnetic field in the core. That collapse induces a high-voltage surge in the secondary winding - often 20,000 to 40,000 volts or more - which then arcs across the spark plug gap.
In older systems, this process was entirely mechanical, triggered by the points opening and closing as the distributor rotated. Later systems used transistors and modules for more precise control, but the core principle hasn’t changed much.
Weaknesses of a Single Coil Setup
The old-school single-coil system was never designed for sustained high-RPM abuse. As RPMs rise, there’s less and less time for the coil to recharge between sparks. If the voltage output drops even a little, the engine can start to stumble. Heat builds up, and over time, internal insulation starts to fail. Add 40-year-old wires, heat cycling, and maybe a cracked case, and you’ve got a recipe for misfire. Throw in some resistance from long ignition leads, a worn distributor cap, or poorly gapped plugs, and you’ve got a recipe for misfires at high revs.
Enter Coil-On-Plug: Modern engines solved this by utilising coil-on-plug (COP) systems. Instead of one coil working overtime, each spark plug gets its own mini coil sitting directly on top of it. Now the coil only has to fire once every two revolutions of the crankshaft, and only for one cylinder. That’s a massive reduction in workload. In a 6-cylinder with COP at 6500 RPM, each coil is only firing 54 times per second instead of 325. That’s roughly once every 18.5 milliseconds. Way more time to charge up, way less heat, and a far more consistent spark. It also eliminates spark loss from long ignition leads and allows better control over timing.
Still, there’s something special about those old oil-can coils. No microchips, no fancy circuits - just wire, iron, oil, and a whole lot of volts. They powered everything from tractors to high performance cars, in heat, cold, rain, and dust. They might not hold a candle to modern systems when it comes to efficiency or precision, but they earned their place under the bonnet.
