Originally Posted by Megasquirt.info

Injectors are either high impedance or low impedance. High impedance injectors (usually about 12-16 ohms) can take a 12 supply directly, without a form of current control. Low impedance injectors (generally below 3 ohms) require some form of current limiting. With MegaSquirt, you can use resistors to limit current, or you can use Pulse Width Modulation (PWM), which is a software solution built in to MegaSquirt.