Water/methanol injection (WMI) sprays a fine water-methanol mist into the intake to cool the charge and suppress knock, letting a tuned GTI run more timing/boost — a bit like a chemical intercooler-plus-octane-boost. It's an enthusiast mod with real benefits and real responsibilities: it must be tuned for and fail-safed, because if the spray stops while the map expects it, the engine runs unprotected
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