In internal combustion engines, water methanol injection, also known as anti-detonant injection, is spraying water into the cylinder or incoming fuel-air mixture to cool the combustion chambers of the engine, allowing for greater compression ratios and largely eliminating the problem of engine knocking (detonation). This effectively reduces the air intake temperature in the combustion chamber, meaning that performance gains can be obtained when used in conjunction with a supercharger, turbocharger, altered spark ignition timing, and other modifications. The reduction of the air intake temperature allows for a more aggressive ignition timing to be employed, which increases the power output of the engine. Depending on the engine, improvements in power and fuel efficiency can also be obtained solely by injecting water.[1] Water injection may also be used to reduce NOx or carbon monoxide emissions. The horsepower you gain from methanol injection in other words is from bieng able to add timing and boost without cylinder detonation. You wont be able to nail down your horsepower gains until it is tuned without and with injection.