Hmm, my head hurts now.

Say you have your 14.6/1 ratio fuel mixture in a cylinder, basically a b0m-b, but one that is strong enough to contain the energy from the perfect fuel mixture, at atmospheric pressure. Ignite it, the fuel uses up the oxygen and converts to heat, increases the pressure and everything eventually cools back down to ambient temp. I'm thinking there'll be a slight vacuum because the volume/mass of fuel and oxygen that was expended and has been taken out of the cylinder.