A supplemental fuel system that sprays a fine mist of methanol or water-methanol mix into the intake charge, cooling the intake air and resisting knock on turbocharged engines running pump gas.
Methanol injection (also called water-methanol injection or meth injection) sprays a fine mist of methanol, water, or a water-methanol blend into the engine's intake stream, typically just before or after the throttle body. The evaporation of the methanol-water mix pulls heat out of the incoming charge air, reducing intake air temperature (IAT) significantly. Cooler, denser intake air is more resistant to detonation, allowing more aggressive ignition timing and boost pressure on pump gasoline than would otherwise be possible.
Methanol is also itself a fuel with a high octane rating (around 100+ on the RON scale) and high heat of vaporization. The methanol portion actually burns in the combustion process and contributes to power output, not just cooling. This makes meth injection a dual-purpose performance tool: charge cooler and supplemental fuel. The result on a turbocharged vehicle can be an additional 20-60 or more horsepower over what the same tune makes on pump gas alone, depending on the specific application.
Common meth injection systems include Snow Performance and AEM kits. They consist of a reservoir, pump, controller (often boost-triggered), and nozzle(s). Meth injection is popular on the Subaru EJ257, Mitsubishi 4G63, and boosted domestic V8 platforms. The key limitation is the requirement to keep the reservoir filled; an empty tank without failsafe tune reverts to the more aggressive timing without the charge cooling benefit, which risks detonation.