Don't forget you need a controlled high pressure fuel supply, and atomised fuel is extremely (as in extremely) combustable - so this can be a pretty hazardous experiment and a little 12v spark would make it even more exciting .
You can look on line for a given injectors flow rate characteristic ,which is easier. Some injectors ( usually diesel) have a coding for their individual performance ( often loaded into the ECU when an injector is changed)
If you don't use fuel remember a liquid of different density and viscosity will flow at a different rate
I have a regulated fuel supply, I can either use the fuel system that is on the racing sidecar that this engine is connected to, I am mainly worried about finding 4 similar injectors (in fuel flow rate) at present. If I find the injectors are very different I will be making something that can supply cleaning solution to the injectors so it can be pressured through while being pulsed by the Arduino.
I have been testing the injectors today, manually connecting and disconnecting a 12v circuit to the injector, but obviously they can be pulsed much faster by an Arduino and I can also measure the difference in flow rates if the pulses are precisely controlled,
These are petrol injectors, of a 600cc motorbike engine. I have been looking for the specs of them but have been struggling to find out what flow rate they are rated at, and what duty cycle they are run at.
I take your point about the danger, I will keep that in mind.
I have ordered some of the MOSFETs I linked in my original post, they should arrive on Monday so will start experimenting. I have some of the 1N4007 diodes and so will be adding a flyback diode.