So, going backwards. One lb of fuel can produce X amount of energy. It takes a given amount of oxygen to burn that fuel. In an internal combustion engine, we use pressure to create motion. We know the practical highest compression ratio. We also know how much pressure one lb of fuel will produce.l when burned starting at the pressure within the cylinder at TDC at the max practical compression ratio. Nothing in the above is variable, you could argue highest practical compression ratio, but not greatly. The rub! In a NA engine the amount of oxygen you can get into the cylinder is limited to a little over 100% VE. So the amount of oxygen is limited. Which limits EVERTHING else. We really argue about what mechanical way we convert that pressure. Small bore/big stroke or big bore/small stroke or big bore/big stroke or small bore/small stroke.