I am aware of one case where someone tried injecting water pre-turbo. It worked, but after a while the compressor wheel developed pitting. I just think it's a bad idea to inject before the turbocharger. On superchargers, I've read (Forced Induction Performance Tuning by A. Graham Bell) that injecting before the supercharger helps to seal the rotors (assuming it is a rotors type supercharger, and not a centrifugal one!), plus you get the added benefits of injecting 'pre boost'.
Keep in mind that water reaching the combustion chamber in liquid form is a real benefit to reducing detonation.
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