Effect of water's expansion on combustion pressure

[Dust]

It's been said that water, when it phase shifts, expands to 1800? times it's previous size. I have seen some mention that this increases combustion pressure. Last week, I was told by a member of another board that the psi increase is negligible. Not being an ME, I don't know what it would take to work out the formula. But lets say at 15% of fuel flow, what would water actually do to the overall pressure inside a combustion chamber?