We don't have a water/methanol section like some other forums, and therefore, we don't see as many threads/posts about it. This is only a "work in progress" currently; I'll add more as I think of it. Any suggestions/comments are welcome.
What is water/methanol injection?
Water/methanol injection is a system that injects a fine mist of water, methanol, or a combination into the air/fuel mixture along the path to the combustion chamber, creating a chemical intercooling process. It does so because as the water evaporates, it dissipates heat. When methanol is used, it also provides some chemical intercooling, as well as a fuel octane increase. Utilizing water/meth injection not only gives you more power, but the increased octane and cooler charge provide a much higher threshold for detonation, meaning knock events are nearly a thing of the past.
Does water/meth injection require a tune?
Water should only cool the intake charge, but the use of methanol will affect AFRs. Regardless, if you're going to invest in this type of setup, retuning is needed.
Who would benefit from water/meth injection?
Everyone! With the chemical intercooling and octane increase, you can run more boost, more timing, and leaner AFRs, all which lead up to more power. You could easily run 1-2 more PSI, 2-3 degrees more timing, and .5 leaner AFRs, while having a "safer" tune. Power gains of 20whp/wtq aren't unheard of, even at StageII STi power levels.
Other than a water/meth injection kit, what should I have?
You will want some type of failsafe in case of an issue with the kit. Some kits include them, some don't; some manufacturers offer an optional failsafe you can purchase as well. Alternatively, you can just buy simple 3-port solenoid and fittings from Grainger to drop you to wastegate pressure in the event of system failure.
Also, you'll want quality gauges to monitor what's going on in the system; an inaccurate gauge is worse than no gauge at all. At a minimum, you'll want a boost, EGT, and WBO2 gauge.
Where do you buy methanol?
You can buy methanol from many places. A very popular methanol vendor is VP Racing. Some manufacturers sell bottles of pre-mixed fluid (e.g., Snow Performance's Boost Juice). Lastly, you can buy denatured alcohol at any home improvement store (e.g., Home Depot, Lowe's, etc.).
How much is this going to cost me?
This is quite dependent on your tune. On average, expect to get two or three full tanks of gas before having to refill the water/meth reservoir. It's also quite common to create a mixture (e.g., 50/50) of water/meth to help prolong the meth supply. If a gallon of meth/alcohol and a gallon of distilled water cost you $15 total, and lasts you 4-6 tanks of gas, you're really only spending about $.25/gallon (or less) extra for fuel in the 105+ octane level.
Who makes Subaru water/meth injection kits?
Of all the systems out there, how do I choose?
There are four styles of operation for water/meth injection kits:
Which kit you select should depend on the recommendation of your tuner, the features of the kit, and your budget.
For more information, the following write-ups of water/meth injection systems are quite good:
How to choose a WI kit - NASIOC
Indepth study of WAI injection systems -restart - NASIOC