I'm new to the forum and new to Jeeps (just bought an '84 CJ7 with 71k miles) but this is a topic that I can speak with a little authority on. I've been modifying cars and building engines for myself and others as a hobby, and sometimes for a little extra $ in my younger days, since the 1970's. I went to engineering school and have spent 20 years of my career in powertrain engineering in the automotive and off highway industries, most of it in automotive. Going back to my college days, I was running a SB Chevy with small chamber double hump heads and flattop pistons - with a small cut on the heads, this was around 11:1 CR, on 91 octane gas, which was what you could get back then. One of my friends was running a 9:1 400 SB with a homemade twin turbo setup (Gale Banks was just getting started back then, and Jeff struck up a friendship with him via phone, they spent hours on the phone when Jeff built his first system) running around 7-9 PSI boost, maybe more (hey, that was 1980-something).
We were both running water injection to hold down pinging, mine was Holley, his was Edelbrock. We had access to a dyno cell at school and plenty of SB Chevy parts, so we decided to make water/alcohol injection experiments our senior project. We set up another carb'd SB with 12.5:1 CR and went to work. We ran racing gas, 91 octane pump and 87 octane pump in a matrix with variable levels of water/alky injection, with timing and knock as the variables. We would increase timing up to the max recommended for SB Chevys (14 initial/36 all in as best I recall) until knock started with each combination. Here are the basics of what we found:
Race gas/max timing was 100% power
Methanol injection with 91 Octane was maybe 2-3% down from this
50/50 mix of water and meth about the same as methanol. maybe 1% down
Water only injection cost another 2-3%
Retarding the timing with no injection & 91 RON cut power 17-18%
I may be a little off on the exact numbers, but not the order of findings and the delta between race gas and no injection. I could go on for a while about the thermal profile we measured from point of injection to EGT. Interesting stuff to an engineer, would put most car geeks to sleep. Suffice it to say that the injected liquid didn't get anywhere near vaporizing completely in the intake tract. Much later, when I tore down my own engine at 70k miles, the chambers and pistons were very clean with no deposits whatsoever. Those deposits are knock initiation sites because they get very hot, so they will cost you power and longevity in the long run. In this respect, water injection definitely delivers. If you don't have the CR, you won't get a power benefit though, not anything really measurable. The utility on a modern injected engine with factory knock sensors is pretty questionable.
Now, here's a secret you won't get anywhere else. We noticed that isopropyl alcohol, the same stuff you buy at the drugstore, had similar properties to methanol. Methanol can be a PITA to obtain, store and handle anyway, so we tried isopropyl. The thermodynamic performance - HP, EGT, intake temps, etc - was not measurably different from methanol. Most of the stuff you buy at the drugstore is 33% iso/67% water, but if you read labels, you can find 100% iso. Back then, the iso was $0.33/bottle on sale at Walgreen's. Guess what we ran? Jeff still does in his car that is now a Pro Touring monster. I will again when I build a high CR stroker I6 for the Jeep.