Mine's OEM too. Just got bits cut out.

 

Water injection into the engine has been a big source of debate for many years. It may work on certain applications (think drag cars) especially if mixed with alcohol, but on the street it is almost completely useless, often causing more problems (if the amount is incorrect, or the water not completely vaporised then you end up with rust in your engine).

Also I wouldn't worry about the quality of the water, after all how many of you cover your intercooler when it rains?? The water bouncing off the road, or kicked up by cars in front of you is much worse than the water coming out of your tap. Not to mention do you use filtered water to rinse your intercooler off after washing it? If you are really worried I would instead just suggest installing a filter at the neck of your water champer to filer out any large materials, but again totally not necessary.

Or if you are really worried about it, and like the diy approach, get a cheap Brita filter pitcher and cut the inner neck that holds the filter and platicweld or superglue that into the neck of your water chamber. That way you have a replaceable water filter that will last for years instead of having to buy water.

Another quick note, don't worry too much about the nozzle. The idea is that you want as much of the intercooler to get wet as possible. It is the evaporation of the water that is causing the metal to cool and the air to compress, so more wet = more cool up to the point where there is too much water and wind/gravity is pulling it off of the intercooler before it evaporates. Personally I would suggest more of a wide angle spray nozzle instead of a mist unless you are going to be able to mount the misters infront of the entire intercooler (like Auss but preferably with 2 more rows). If you intend to spray from 2 nozzles on the side/top/bottom then mist would not have enough mass/force to get to the middle of the intercooler and you would end up with less coverage.

 

If you spray hard water on your intercooler for long enough you will get deposits. I would use distilled or RO water myself since the tap water here is very hard. You may use whatever you like.

Water/methanol injection is not only useful on the drag strip. It is very good at preventing detonation. Of course you have to have a tune that takes advantage of it to realize the full potential.

And if you're getting rust in your engine then you're injecting WAY too much water or not using a good atomizing nozzle.

The disadvantage is of course that the bottle will run dry so you have to have a safety that either switches to a different map or backs down the boost, or at least gives you a visual warning so you can let off the throttle.

 

Really?? And where did you get this bit of mis-information?