> requiring 4x the flow rate to do the same work. I strongly suspect that this is incorrect. A given amount of oxygen and fuel releases the same amount of ener

> requiring 4x the flow rate to do the same work. I strongly suspect that this is incorrect. A given amount of oxygen and fuel releases the same amount of energy at any (reasonable?) pressure. 4x oxy (and fuel) --> 4x energy... this is the principle of turbochargers and superchargers (although the limiting factor is compression). Interestingly, and paradoxically, I suspect that at an appreciable depth you mitigate the "air pump" effect and you don't need the compression stroke (or as much of one) to achieve a particular fuel mixture density prior to ignition so you'd lose "compression braking" and gain some efficiency. (I also suspect that what happens if your vessel implodes and you're suddenly subject to 6,000psi is that the now superheated oxygen reacts with any oxidizable material at hand in an explosive exothermic reaction.)