Hi - I drive an MR2 with water injection. I don't think you will want to install the nozzles before the intercooler. I would install them after the IC, but before the throttle body. Is this possible on the WRX?
While water injection has benefits of cooling down the intake charge, a major benefit is if water in liquid for reaches the combustion chambers. When it state changes from a liquid to a gas it absorbs (takes) much heat away from the combustion process. This really reduces the chances of detonation. In general you want to run about 25% of fuel flow as water. When you are calculating the size/number of nozzles to use, keep in mind the pressure of the water at the nozzle and then subtract the amount of boost pressure you are running at that time.
Do you know if you will potentially be running lean with your setup? If so, you may consider running up to a 50/50 mix of distilled water and denatured alcohol. This will help richen the mixture - however to get the most prevention of detonation, nothing seems to beat straight water.
Good luck on your project - hope this has helped.
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