Although using injector resistors can be used to switch low impedance resistors some implications of using serial resistors in conjuction with low impedance resistors in batch fired configuration should be understood.
It takes a certain amount of time (+-1.2ms)to open an injector before it starts squirting fuel. Not all injectors open in the same time, there are small differences between them. Eg, the opening times on my Rover V8 high impedance injectors vary from 1.2ms to 1.45ms. Even with a constant injector pulse, these differences in injector opening time produces differences in the AFR resulting from the different cylinders.
When wiring a current limit resistors in series with the injectors the net effect is that the opening times are increased but more importantly, the DIFFERENCES in opening times are INcreased (by limiting the current it takes longer to establish the current needed to open the injector). This means that the differences in the AFRs resuling from the different cylinders are increasing as well.
When you build a race engine which basically only operates at full throttle this issue is a non-event since the opening times are a fraction of the times the injectors are open and the differences that do result are minor.
On a road going car, the situation is very different. I found on my Jag V12 that
a) using current limit resistors made it impossible for me to achieve a smooth and stable idle even with an AFR of around 13!! ;
b) it became impossible to achieve lean mixtures (leaner than 14) at partial throttle and this was accompanied by a rough running engine clearly not happy despite mixtures that still seemed rich. This is due to different cyls having widely varying AFRs under these circumstances.
My conclusion is that when using low impedance injectors,
current limit resistors should be avoided (except in race applications) by employing PWM either with the piggy back board on the V2.2 board or the on board components on the V3 board. Also take note of the comments at par 16 here:
http://www.megamanual.com/ms2/V3trouble.htm
PS: This is where Megasquirt has an advantage over may big$$$ programmable ECUs, in that it can effectively switch LoZ injectors without additional electronics.
