Suggest making a liquid engine's F/O ratio adjustable in the launch assembly by say right clicking the engine during editing and changing it from the default. In real liquid rocket engines usually the ratio is not the optimal ratio due to excessive heat from the burn that cannot be dissipated quickly enough among other factors. In KSP, the adjusting the ratio could boost delta V but at the cost of overheating the engine quicker as a way to add a higher level of later gameplay as players advance and possibly result in some spontaneous rapid disassembles.
