Symbol Z A measure of the total opposition to current flow in an alternating current circuit, made up of two components, ohmic resistance and reactance, and usually represented in complex notation as Z = R + iX, where R is the ohmic resistance and X is the reactance.
Injectors are DC. However, since they are pulsed on and off, inductive reactance plays an important role.
Low Impedance injectors are generally 3-5 ohms. Roughly 1/4 of the impedance. This means with B+ applied across the injector you will have 4x the current and therefore 4x the coil "power". This means that the injector will have more "strength" to pull the pintle open.
High Impedance injectors are cheaper to make and much cheaper to control. They operate in saturated mode. This means full B+ is applied across the injector and the higher impedance of the injector is what limits the current. Most PCMs use inexpensive N-channel enhancement mosfets that just turn on and off gating current to the injectors.
High Impedance injectors have been proven to be reliable up to 42.5lbs. Meaning there is enough coil strength to control the pintle properly. Beyond 42.5lbs in High Impedance there just isn't enough coil strength to control the larger pintle properly. Spring pressure must be reduced in order to even move the pintle. This results in losing linear response, slow reaction time, and dribbling (leaking). This is why many people using 50lb High Impedance injectors have found that their gas mileage gets even worse. Very few manufacturers even make a 50lb High Impedance injector because of these issues. At 57lb High Z, these problems are further exacerbated. Its a poor solution, but for many, including myself it was the only option we had at the time.
These issues do not exist in Low impedance injectors at the larger sizes. There is plenty of coil strength to control the bigger injectors. They are available in sizes exceeding 155lb/hr. They also have a faster response (typically 1ms) than a High-Z of the same value. This means you will crouch your fuel window even less. The downside is that we cannot control Low-Z injectors from the PCM directly. The higher current left unchecked would burn up the mosfets in the PCM and shorten the injector life.
Low impedance injectors normally work in what is called a Peak and Hold mode of operation. This means that full B+ is applied until the injector opens. When the driver CKT sees that the injector has opened it reduces applied voltage to 1/4. This is because not much current is needed to hold the injector open after its been moved. At the end of the pulse, the driver shuts off all the way and the injector closes.
Many import folks install large resistors in series with low impedance injectors and run them in saturated mode with their high impedance ECUs. This is silly because you are externally reducing the injector current all the time in order to save the drivers in the ECU. You would lose many of the benefits you were trying to get in the first place.
