NACA ran a WI with just water rate of 1/2 lb for every lb of fuel all the way to 10:1 A/F ratios or so. That's a pretty good amount of water.
Most modern cars have ignition systems designed with the exact voltage required to ignite the mixture in mind. So if you make it too hard to ignite the ECU has trouble. Also the NACA engine was 7:1 compression, higher compression engines will be even harder to ignite. Theoretically I'm not sure what the limit is. Diesels often run more water than fuel.
I guess the limit is wherever you end up when you can't get any more power by adding more, no matter how you tune/upgrade, and I'm not sure that point has been reached.
Adrian~
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