Can my MegaSquirt ECU control low impedance (low-Z) fuel injectors?

Yes, it can! There are several techniques for controlling low impedance fuel injectors, each with its own pros and cons.

1. The most basic method for safely controlling low impedance fuel injectors is to add inline resistors to your fuel injectors. These resistors limit the total amount of current through the injectors, but unfortunately the injectors won’t open or close as quickly as they were designed. Sluggish injectors cause sloppy idle control.

Resistors are fairly inexpensive and work well with all versions of MegaSquirt hardware. The MegaManual has information on selecting inline resistors here:

2. A more advanced method for controlling low impedance fuel injectors is pulse width modulation (PWM). PWM quickly turns the injector on and off thousands of time per second limiting the average current draw. All MegaSquirt processors can control fuel injectors this way, but v2.2 boards require a special add-on called the “flyback board” to suppress high voltage spikes generated by low impedance fuel injectors:

Depending on how it’s built, the v3.0/v3.57 board is usually already equipped to handle these spikes.

PWM allows low impedance fuel injectors to open and close much more quickly, and you generally don’t need to purchase any new hardware (except for the flyback board for v2.2 boards). Depending on a variety of factors, PWM can sometimes cause undesirable electrical noise, which affects sensor readings, and the PWM settings must be fine-tuned for optimal results.

3. The ultimate solution for low impedance fuel injector control is peak-and-hold drivers. SymTech Labs’ PH Driver board is a small, integrated package that contains four peak-and-hold drivers – one for each injector.

Peak-and-hold (P&H) drivers are “smart” devices that implement an active feedback loop to monitor and adjust current through most low impedance fuel injectors. There are no settings to experiment with, virtually no electrical noise, and and the injectors open and close very quickly. The only drawback is the added cost and somewhat greater complexity.

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