Hi, if you look at it from a thermodynamic point of view, all 4 stroke petrol engines have approximately the same efficiency. It is related to the maximum gas temperature (during combustion,) compared to the minimum gas temperature (when the exhaust gas leaves the engine.)
This is why diesels are so good, they have a high compression ratio (say 20:1.) which can be seen as a 20:1 expansion ratio. This lowers the exhaust temperature adiabatically by much more than a 9:1 expansion ratio petrol engine. So the min to max gas temperatures are better in the diesel. i.e. it gets more power out of the fuel than a petrol engine.
Manufacturers can make slight improvements, by reducing friction losses etc. but the 9:1 ratio is pretty well fixed for petrol as any higher causes pre-ignition.
Absurdly an engine at zero rpm has the least amount of friction losses, so it is more efficient to have low engine rpms. Also exhaust valve timing, opening at bdc would utilise more of the gas energy in the expansion ratio. However a 27 litre engine that only revs to 50 rpm would have pistons the size of dustbins, and flywheels of several tonnes... maybe suitable for boats but not aircraft.