Water spray (in the intake) will not do much to cool the pistons. It would affect the burn of the air-fuel mixture though with potential marginal increase of power. But with only normal aspirated engines, the power gains is very small. With too much water, it would just cause rust inside the cylinders.
If you want to cool the pistons more effectively, spraying oil from the crank side of the pistons and adding an engine oil radiator would be more effective.
If you want to produce a lean burn engine, you will need to increase the compression ratio, adjust the ignition and maybe also change the piston head shape too (shape of the piston quench also shapes the flame's shape & propagation).





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