There is a lot of debate about the best place to position the injectors, so this will cover the different options, along with either the pros or cons of each.

Before the Air Intake Temperature Sensor:

This is the most common place because it is usually the easiest.  Make sure you never inject pre-intercooler, unless you are injecting pre-blower on a supercharged engine.  The idea is that if you have an engine that adjusts timing on its own, then the computer will see the cooler air charge and advance timing, allowing for more power.  By placing the injector as far from the intake manifold/air intake sensor as possible, it allows for the water/methanol mixture to be absorbed into the air charge to allow for the best possible even distribution to each cylinder.  In theory, since the air charge will be mixed for a longer distance, and therefore time, allowing for the moisture to be absorbed by the air, creating the coolest possible air charge going into the cylinders.  If you have multiple nozzles to install, try and space them out by a couple of inches, rather than have them mounted at the exact same location.  This will provide better dispersal of the mist into the aircharge.

If your engine is computer controlled, it is probably programmed to reduce boost when it sees a lower air charge temperature.  It will do this because it is programmed to output a given horsepower, and since the air charge is colder, it is therefore denser, resulting in less boost being required to provide that given horsepower.  If you have a manual or electric boost controller, you may or may not be able to bypass the computer’s control.  This area is a common area of discussion among tuners, so we wanted to pass on the information so you can monitor your own personal results.

After the Air Intake Sensor:

This is also a very common place to mount the injector.  You can usually mount it either before or after the ‘butterfly’ and still be after the air intake temperature sensor.  The main difference when mounting the injector before or after the butterfly is there is less vacuum before versus after the butterfly when idling or driving in vacuum.  Translation, when in vacuum, there is more vacuum after the butterfly than before it.  Here is why this is important:  If your solenoid is placed very far from the injectors (more than a couple of feet), then the vacuum can actually suck just a little bit of the water/methanol.  This small amount of moisture ingestion during vacuum is not a problem, but the next time you boost, but not enough to trigger the injection system, the water/meth sucked out will now be replaced with air.  It will be a minimal amount, but what happens is the next time the water/meth injection system is triggered, there will be a small amount of air to expel before the water/meth reaches the injector.  This is a minor issue, as the injectors are primed very quickly again, but it is something the user needs to know about.

Summary:

Every engine will react differently.  What works great for one, might not wors as well for another.  A true tuner will take what knowledge is available, and mount the injectors in the location they believe best.  After thorough testing, you might want to move the injectors to see if your results improve.  This can be challenging for the average user, as there will be a hole where the injector was previously mounted.  It is recommend that you contact people with your same setup (engine, modifications, etc…) and see what has worked best for them.  If you are unsure how to contact people, go to www.google.com and search for ‘your car’ and ‘forum’, and then surf the results.