Fuel octane requirement is much more complex than simply what compression ratio is present. The timing and mixture condition have much more to do with it. Also the head material, an aluminum head can conduct more heat away from the combustion chamber, thereby allowing a higher compression ratio, all other things being equal.

With the engines utilizing a knock sensor and the associated controls, you CAN use low octane fuel with negligible engine damage, but it is usually false economy. When the knock is sensed, the timing is retarded and possibly other actions taking by the engine management system that result in poorer fuel economy. So, it might cost less per gallon, but require more of those gallons to get the same distance down the road.

Good luck,