Water/methanol injection is used to suppress detonation and to safely allow much greater amounts of boost and/or compression ratios. When implemented correctly it provides benefits very similar to unleaded racing fuel. However, it costs pennies per gallon instead of the typical $12 or more. Furthermore, it allows the tuner to keep the boost turned up all week long instead of only at race weekends.
Water/methanol injection works in three ways. Firstly, when the fine mist is injected into the intake system (after the intercooler), the small droplets absorb heat from the intake air. It takes heat energy to evaporate a substance from liquid state to vapor. The required amount is measured as "latent heat" and the units involved are "kiloJoules/kilogram" (kJ/kg). This means that to convert 1 kg of water at 100 ºC to 1 kg of steam at 100º C, the water must absorb 2260 kJ of heat. This takes time. Methanol requires only 1099 kJ/kg to completely evaporate. Thus, methanol absorbs less heat (cools less), but it also cools much faster than water -helpful in cold weather and with very short intake tracts.
As the tiny drops start to evaporate into gas, it takes additional heat energy to sustain the process. So, the intake charge is cooled further. Finally, the remaining droplets and steam reach the combustion chamber and make the combustion process smooth and even. (The piston is accelerated with a sustained shove vs. a sharp and short "bang".) The super-heated steam also melts away carbon deposits and eliminates the glowing hot spots which lead to detonation. On top of typical 91 - 93 octane pump fuel, water/methanol injection can raise effective octane to over 100. This allows the enthusiast to daily drive on 100 octane files and more.
Whether you're running huge boost, want to tame a nitrous oxide system, or are towing uphill through the mountains in summer, these systems allow for maximum power output while substantially increasing overall engine reliability.
A 50/50 mixture of water/methanol is generally advised to provide the best compromise of detonation control and increased torque. In usual operation, the alcohol flashes away almost instantly. This leaves behind a pure water aerosol that evaporates more slowly (but pulls more heat while doing so). These remaining water droplets will now be half the original size but yet have 4.84 times the original surface area ratio. With this much more surface area exposed to the hot air, the water evaporates at a 4.84 faster pace! This makes it relatively dry by the time it reaches the throttle which it will NOT attack.