I have not fitted it, but here is what I've picked up about it:

Water injection will cool your inlet temperatures, the resulting combustion temps and to an extent the EGTs.

I've been told it can also add pressure to the combustion reaction as the water converts to steam.

It is sometimes used as a fix for other problems- eg high inlet temperatures, and there is a danger in relying on it cos if it fails whilst the engine is running you risk detonation.

I've always been told it is better to fix the problem as opposed to relying on water injection.

.

.however it also used as an additional cooling agent in some cars without problems, so that if it does fail damage does not necessary follow

(for example mapping without it,but running with it), but you still get the benefits.

Some of the better systems have fail safe options so if the water injection conks out you are warned, or the engine goes into fail safe mode.

Like I say I'm not an expert in it, but I think that's what it's about
