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I've finished putting together and installing my own water injection setup. It uses a pressure tank that contains a water bladder. The tank is 2 gallons. I fill the water bladder with one gallon of water, and then pressurize the tank with an air compressor to 100psi. The setup uses two pressure switches to control two misting water nozzles (mcmaster car mini-mister nozzles). The first nozzles comes on at 5psi and sprays water onto the outside of the intercooler. The second nozzle comes on at 10psi - currently using 189cc/min nozzle at 100psi, and injects right before the throttle plate. The pressurized water is controlled via two solenoid valves. This allows the not only a fail-safe for the throttle body injector (in case one solenoid fails), but also permits the use of a second nozzle for the intercooler mister.
Advantages:
No Pump - no pump delay - injection starts immediately.
Doubles as intercooler mister.
High pressure - excellent atomization (at 40-100psi).
Turn on pressure easily adjustable via adjustable switches.
Able to run with denatured alcohol.
Disadvantages:
Filling the tank is somewhat cumbersome, but not too bad. Have to drain air out, open valve, and gravity feed the tank occasionally letting excess air out. After filling - close the valve, and pressurize via air compressor.
On a two gallon tank, with one gallon water, and one gallon 100psi air, after all water has been used, pressure will have dropped to 50psi. The user can just repressurize the tank - but that leads to -
Knowing how full the tank is. I'm still working on this one - can use pressure reading to see how full, but would be nice to use an additional pressure switch to turn on an LED when the tank is empty.
Observations:
Water injection really does work - I've noticed at least a 100 degree drop in EGTs, and I can run more boost without the ECU going into the bad gas mode. This can be observed via two LEDs connected to the turbo VSV, and the TVIS ports on the ECU. When the ECU goes into the bad gas (knock) mode, it will open the TVIS valves under any throttle position other than closed, and the ignition timing seems to be severely retarded. Since I changed the headgasket (and had the head resurfaced) the ECU has been going into this mode under moderate (14.2psi) with PT upgraded 50 trim turbo - I believe this is due to increased compression. I've been able to run 15.6 psi with water injection without problems, and also tried 17.1psi briefly without issues. I did some GTech runs with and without water injection, and also with a 50/50 mixture of water and denatured alcohol - results are averaged:
Just distilled water:
14.2 psi without 225
14.2psi with 222
15.6psi with water 242.
50/50 mixture of distilled water and denatured alcohol:
14.2psi with 225
15.6psi with 235
17.1psi with 242.
I attribute the lower numbers with the alcohol to the engine running too rich - but that's just a guess.
I also did a my personal best 0-60MPH time with distilled water at 15.6psi - 5.22seconds.
Pics are at:
<http://josepho.www4.50megs.com/WI> Sorry they are not labeled.
Oh - BTW - the nozzle for the intercooler is 315cc/min. So one gallon of water will last about 7.5minutes - course that would be under boost conditions - with just the throttle body water injector it would last 20 minutes.

DSC00001: Tank mounted in trunk (really bad mounting job - want to use metal bracket in future). You can see the first solenoid - comes on to spray water onto the intercooler.
DSC00002: Second nozzle on throttle body, and second solenoid valve.
3: Nozzle mounted on greddy IC.
17: Another outside view of IC nozzle.
18: Another view of second solenoid - can see pressure switches as well on right.
19: Close-up of pressure switches.
20: Another view of second solenoid.
21: Yet another - hey it was my first time with a digital camera! :-)
22: Close up of first solenoid valve on tank.
23: Another view of tank, can see valve to fill tank on left.
25: Picture of two LEDs - RED is for TVIS, green is for turbo VSV.
26: Two LEDs, EGT gauge, and fuel pressure gauge (green LED comes on when intercooler mister sprays, and red when the throttle body injector comes on. EGT gauge shows #3 and #4 EGTs.
27: Water injection enable/disable switch, fan switch, and turbo timer.
28: Dash shot - Apexi AVC-R boost controller.

joeo