[DENN_SHAH]

i had 2 of the Edelbrock water injection systems, or Vari-jection as they called it on my 76 ford truck.
IIRC, the first one used a vacuum switch for when to start injecting water and different size orifices for the amount of water it injected.
the second one used manifold vacuum ran to the module and had an adjustment for when to start injecting water and had a variable adjustment for the amount of water.
they injected the water above the carb thru a nozzle.
with them, i was able to advance the timing a little more without getting ping which gave me a little more power and a little better fuel mileage.
without advancing the timing, i noticed no gains.
performance wise, i was able to get just enough more power from my motors to beat my friends that could just beat me without it. usually i gained about a truck length or so in the quarter mile, so it wasn't a huge performance gain.
plain tap water would clog the nozzle after a little while.
i used a mixture of rubbing alcohol and distilled water.
i ran the second Vari-jection for sometime and on tear down the chambers were clean.
i never had problems with water in the oil or hydrolocked the motors.
the motors i used it on were a 360, 390 and a 428.

im not sure what gains could be gotten with water injection on modern NA FI motors.
probably some, but i really doubt a 25% mileage gain.
but then again they talk about replacing the O2 sensor with the kit. if your O2 sensor is weak, more than likely you would see a nice MPG gain, mainly from a good working O2 sensor.