The main components of any water injection system are a pump, nozzle, and a trigger device. All other parts make the system more automated, complex and expensive. You need to decide what you need and can do without when you build your system. My goal was to design a system that would work during most driving situations with the least amount of driver interaction.
The system described here has 2 stages with three injectors. Although three injectors proved to be too much for my application, it may be not in yours. For most situations one main injector and a RPM injector will work well.
This system will inject based on throttle position and rpm's. A variable voltage regulator is attached to the throttle. The voltage range is 0v at idle to 7v at WOT. That voltage is then fed into a control box. The control box uses the regulators voltage as a reference to control a 0-12v output to the pump to vary the water pressure for injection.
The control box that I use is from Snow Performance. It was originally attached to the MAF sensor which has a variable voltage of 0-5v. My GM MAF sensor uses frequency rather than voltage output so a converter had to be used in-between which didn't’t work. So, I modified a voltage regulator and fixed it to the throttle.
An RPM switch is used to open a second solenoid for an additional injector. Keep in mind that you will want to be either in passing gear or near WOT when that solenoid is opened. The pump should be as close to full voltage as possible to overcome the loss of overall pressure in the lines when that solenoid opens.
A lever switch attached to the throttle activates the main solenoid. It is wired to a 12v relay using the 87 and 87a outputs. When you are not on the pedal, the 87a output of the relay opens a solenoid that relieves pressure in the lines. When you are on the gas, the lever switch opens and the 87 output of the relay closes the pressure relief solenoid and opens the main feed solenoid to the injectors. The reason for this set up is that after I came to a stop light or slowed down in traffic, there was still pressure in the lines and the water would still be spraying into the intake. It caused stalling and bogging. Using a relay to control the solenoids solved the problem.
I used 14ga wire for most of the wiring to the switches, relays, solenoids and control boxes. All connections are soldered together and heat shrink tubing covers each. I cannot stress enough the importance of good solid connections. Take your time and be meticulous. It will save you a lot of trouble in the end and will ensure a long lasting, reliable system.