I think you can go about 17.5:1 before it knocks without the water. Then the trouble is getting an even burn. Best economy is supposedly around 16:1 due to power dropping off like a stone after that. There's a region in between stoich and 16:1 though where NOx spikes, so you can use water for combustion cooling to prevent NOx in transition, and to keep it lower when you're at around 16. When ratios as low as 25:1 are talked of in lean burn modes, what's really happing is that the charge is stratified such that there's a richer pocket that burns completely and evenly, and the remainder is air that is heated by the burn for extra piston thrust. A homegenous 25:1 charge would not burn at all well, unless it was plasma jet excited or had other exotic means of ignition (Microwaves, UV burst, other ionising radiaton etc..*)
(* oh yeah, that's what I need to invent next, pulse laser spark plugs)
Edit: We might also regard HHO as an "exotic" method of ignition promotion, due to flame front propagation speed.
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I remember The RoadWarrior..To understand who he was, you have to go back to another time..the world was powered by the black fuel & the desert sprouted great cities..Gone now, swept away..two mighty warrior tribes went to war & touched off a blaze which engulfed them all. Without fuel, they were nothing..thundering machines sputtered & stopped..Only those mobile enough to scavenge, brutal enough to pillage would survive. The gangs took over the highways, ready to wage war for a tank of juice
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