I use an old Varijection that I modified a bit for instant on under boost. I spray 50/50 isopropyl and water (less corrosive) onto the turbine inlet. Works great, I have been running it for over 20 years on the TT340.

I don't agree with the statement that water injection will negate the need or usefullness of an intercooler. They do some similar things, like cool the charge air, but the water and alcohol also displace air and can impede fuel vaporization. The best setup is to have just enough of the water injection to allow higher compression and boost levels, and do the rest of the cooling with an intercooler. I actually use very little water and probably don't go through 2 gallons a year in normal cruising with occasional romps.