If you're serious about water injection, and would like to read an excellent thread on pre-compressor injection, sign up on Aquamist's forum. http://www.aquamist.co.uk/phpBB2/index.php
There's SO much good info there, it's almost impossible to sum it all up here in this thread. Go there, you'll see.
Also, as Paul said, there are a few caveats to injecting pre-compressor. I mentioned them earlier, but it was kind of muddled in the middle of a paragraph and not 110% clear. Here's a breakdown:
1) Inject directly into the compressor wheel if you can, not at a 90* angle to it.
Injecting directly into the compressor, at the correct distance from it, will keep water off of the walls of the intake pipe and mouth of the compressor cover where it can converge into larger droplets and wreak havoc on the blades of the compressor wheel. It's not cheap or easy to mount the nozzle pointed directly at the compressor wheel nut, but it's the best way to do it.
2) Inject with as fine a mist/fog as possible.
Again as Paul already mentioned, droplets in the 10-50 micron range are preferred. If they get larger than that they can start to really chew up the compressor wheel. The Bete PJ nozzles
I mentioned earlier in this thread are perfect for this application.
3) A high pressure pump (100 psi+) is very preferable.
The higher pressure will contribute to greater atomization and smaller droplets. You might
be able to get away with a 60 psi pump, but I wouldn't recommend it. And don't even think of running a 45 psi Shurflo pump!
Hope this helps...