Saturday, July 19, 2008

Fuel Injector and Resistor Box Basics - Wrenchin'

For simplicity's sake, let's just say there are two kinds of fuel injectors: peak and hold and saturated. The difference between the two has to do with resistance. Peak and hold (aka low impedance) injector coils exhibit less electrical resistance than saturated (aka high impedance) injector coils. Typically, peak and hold Honda injectors will exhibit between 2 to 3 ohms resistance while saturated ones will measure somewhere around 12 ohms. This is all pretty useless and boring information until you do something like an engine transplant or go out and swap your stock fuel injectors for a high-flow set.

Injectors are really just valves controlled by the ECU by timed electrical signals called pulse widths. Honda fuel injectors rely on constant power for their pulse widths-the ecU supplies the ground-therefore, the injectors open and close based upon ground signals dictated by the computer. When ground is applied, they close and when it's not, they open. Some Honda ECUs are not designed to work with low impedance injectors. An injector or ECU swap or engine transplant can be all it takes to fry an otherwise perfectly good ECU-and that's exactly what can happen given the right circumstances.

The problem has to do with heat, which can be correlated to resistance in this case. For those Honda ECUs not designed for low impedance injectors, their injector drivers can heat up and fail because of the added current. Even the injectors themselves can heat up. With peak and hold drivers, the 12V is still delivered to the injector but current rises. Testing has revealed nearly double the temperature when using low impedance injectors on the wrong application. It's important to note that resistance decreases current and that current creates heat. If current can't be reduced then the chances of the drivers heating up and failing is increased. Of course, some of this depends on driving habits as well. High engine speeds and injector duty cycles can also create added heat, like when driving up a hill or at the track.