I know of at least one system that uses the ground from an injecter wire. That would be kinda rpm based by using the increase in pulse width, but I don't know how they are converting the signal. Right now, I am using my engine controller to turn it oon based on any boost or any rpm or a combo of the two. My current turn on point requirements are 3.4 psi and 2500 rpm....both must be met. This contoller can also run an extra injector so I have the ground output/pulse width with total control but I am not sure how to use that signal. To chatter a relay or solinoid or some electrical way to vary the voltage.
This is on a screw type s/c that has full boost at 2500 rpm, that's why boost reference won't help me vary the waterflow. I have two nozzles and could stage them.....one to come on at full throttle (throttle switch) and the other could be rpm activated by the engine controller.