Impedance measures how easily a circuit conducts current when a voltage runs through it. Measured in ohms, impedance is a way of telling you how much of the voltage introduced at one end will really make it to the other end. Impedance depends on other qualities of electricity, such as resistance, reactance, inductance, and capacitance.

Most injectors fall into two categories: High or Low Impedance

High Impedance.

High Impedance fuel injectors  have a typical coil resistance of 12 – 16 ohms and are the most commonly used. Drive circuitry for this type of injector is simple and sometimes referred to as a saturated driver circuit. The injectors them self are also referred to a saturated injectors.

Low Impedance.

Low Impedance Injectors has normally a coil resistance of 4 ohms or less and is typically found in larger sized or high performance injectors. Driver circuitry for these types is called a Peak – Hold type and is more complex and costs more than the saturated type.

In a Peak – Hold system, the driver circuit allows a high current for a short time to help turn the injector on quickly. The current is then reduced to a lesser value to keep the injector open. Peak current may be 4 amps to open the injector. Once open, the current might be reduced to as little as 0.75 amps.
With the lower coil resistance, the internal spring pressure that holds the pintle closed can be increased to shut the injector quicker. This allows both faster opening and closing times and makes tuning large injectors easier.

Peak – Hold systems reduce the power requirements to the injector coil thus keeping the coils from overheating.