An ignition coil is just a transformer with a story: energy has to be stored in a magnetic field before it can be dumped into the spark plug as high voltage. Dwell time is how long the ECU (or old-school points) keeps the primary winding grounded, letting current ramp up and saturate the core. Get it wrong in either direction and the engine either misfires or eats coils for breakfast.

The physics: A coil primary is an inductor with resistance. When you switch it to ground, current doesn't jump instantly — it climbs on an exponential curve governed by the L/R time constant. A typical modern coil has ~0.5 ohm primary resistance and ~5 mH inductance, giving a time constant of about 10 ms. But you don't need full theoretical current — you need enough to saturate the magnetic core, which usually happens at 7–10 amps.

Old-school dwell (points ignition): Measured in degrees of distributor rotation. A small-block Chevy V8 spec is about 30° dwell on a 45° cam lobe. At 3000 RPM engine speed, the distributor spins at 1500 RPM, so 30° = 3.3 ms of dwell. At 6000 RPM, that same 30° collapses to 1.67 ms — and the coil never fully saturates. That's why points-triggered engines lose spark energy at high RPM.

Modern coil-on-plug (COP): The ECU controls dwell directly in milliseconds, decoupling it from RPM. Typical values are 2.5–4.0 ms at 14 volts. The ECU also runs a dwell compensation table that lengthens dwell at low battery voltage — at 10V cranking, dwell might jump to 5.5 ms because the current ramp is slower.

Rule of thumb: Multiply desired peak current by inductance, divide by battery voltage. For 8A peak, 5 mH coil, 14V system: dwell ≈ (8 × 0.005) / 14 = 2.86 ms. That's your ballpark starting point.

Real-world failure: The GM LS truck coils (D585 "square" coils) spec ~5.5 ms dwell. Tuners often copy dwell values from an LS1 tune (~3.5 ms) into a truck coil setup — the spark is weak, misfires appear under boost, and everyone blames the coils. Conversely, running 7 ms dwell into an LS1 coil cooks the internal ignitor within a few thousand miles. The tell: cracked epoxy at the coil base and heat-yellowed connectors.

The trade-off: More dwell = more spark energy = better ignition of lean or boosted mixtures. But excessive dwell wastes current as heat, drains the alternator, and destroys coils. The sweet spot is just past saturation — anything beyond is thermal suicide.