As mentioned in my thread on water/methanol injection, I have reconfigured the sprayer on my supercharged jeep to spray 100% distilled water that I can get off the shelf in a supermarket. And, FWIW, I am also running 91 octane fuel from the local supermarket.
I have mentioned in other posts that when tuning for HP, it is the ignition timing advance that can have the most impact - at least as compared to AFR (unless AFR is way off). This is why you see decent horsepower gains when you get a tuner that has tunes for higher octane fuels like 91 or 94. Sure these tunes have some AFR tweaks but the HP is coming from the additional spark advance as afforded by the higher octane fuel.
What I started with was an Edelbrock supercharger with the canned tune which requires 91 octane. Then I got a custom tune for the same 91 octane fuel and in that tune, I picked up more ignition timing and some AFR tweaks and there is a noticeable (yet unproven on the dyno) gain over the canned Edelbrock tune.
In the custom tune, we were watching/logging (among other things) Long Term and Short Term Knock Retard (AKA: LT KNK RETARD and ST KNK RETARD) and we got as much advance as possible on the 91 octane fuel in the custom (Johan - Diablotoona) tune.
Now that I am on straight water, I am able to increase the timing further over what John did in the custom tune but not as much as with the water/methanol spray.
Timing is tweaked in three RPM ranges, 1k-3k RPM, 3.1k-4k RPM and 5k-7k RPM (or something like that). With straight water and 91 octane gas, I am getting 2, 4 & 6 degrees of advance respectively with no timing being pulled by the PCM. I think I feel that I can add a degree or two here and there and get a little more out of this.
Not sure how to equate number of ponies (HP) per degree of ignition timing advance but I have read that it is on the order of 2-3HP per degree of advance. Perhaps a little more. I have even read about much more lofty numbers per degree of advance on the LS forums but 2-3 seems reasonable to me on a Pentastar. I do hope to hit the dyno eventually. Have not so far. It would be good to dyno what I am getting out of the custom tune and compare that to custom tune + water injection with my tweaks. I am hoping to demonstrate a 12hp-20hp gain by using just water injection and advancing the ignition timing.
For anyone thinking that water puts out the fire, I ask you this.... Can you use water to put out a gas station fire? Why not?