6.8 Ohm 50W easy-to-mount resistor.
Note: instead of series power-resistor LowZ extension (active flyback) is recommended whenever there is the slightest chance of using lowZ injectors (possibly in the future) so order assembled v3 with that option when in doubt (with the low-Z extension, low-Z injectors can be used with only simple configuration change, by enabling injector PWM-ing).
While this 6.8 Ohm power-resistor works OK with many low-Z injectors (when connected in series), sometimes lower resistance (like 4.7 Ohm, or even 2.2 Ohm) is better, requires try-and-see, your milage may vary (unfortunately without low-Z extension, eg. with the 30V flyback option, PWM-ing duty must be 100%, PWM-time=25.5 msec in software configuration ).
Low-Z extension allows to configure from software (eg. PWM-duty 35-40-45% can be good start for 2-3-4 Ohm injectors) conveniently.
Useful in series with low-impedance injectors. The 30V transient diode flyback solution gives very fast injector closing and good injector control (mostly noticable at idle), but incompatible with PWM-ing, so one power resistor is needed in series with each low-impedance injector to limit peak current.6.8 Ohm 50W easy-to-mount resistor.
Only needed for low-impedance injectors below 5 Ohm.
6.8 Ohm series resistance is usually too high resistance for the 1680cc bosch 5.9 Ohm "medium impedance" injectors: there a lower resistance eg. 2.2 Ohm min 20W series power resistor is recommended (currently not available here), but some use it directly without a series resistor.
Useful in series with low-impedance injectors. The 30V transient diode flyback solution gives fast injector closing and good injector control (mostly noticable at idle), but incompatible with PWM-ing, so one power resistor is needed in series with each low-impedance injector to limit peak current.
For the curious:
Because the series resistor only drops voltage in proportional with current, injector opening is reasonably fast with the series resistor. And injector closing is actually faster with the series resistor than without.
Some of our users claim that with some (most?) low-impedance injectors they reached best success (very good idle with huge injectors on small cylinder displacement) with self-controlled flyback (click here). In fact v3 was designed with this possibility in mind. Since people like the results with that circuit, batch-manufacture is planned in late 2010.