Yes ron97 can handle more pressure but who puts the pressuee? The engine right? Then the compression ratio is determined by the engine, so back to my main question, is the engine compression ratio variable for normal msia cars

The difference in octane generally indicates the resistance to detonation. It has nothing to do with compression ratio. So throw the idea of the compression ratio out the window.

A car mostly cannot change its compression ratio, but it can advance or retard the ignition. It does this in concert with the O2 sensor for AFR and knock sensor.

eg; If a car senses it is using a lower RON than what is recommended due to the detection of knock and pinging via the knock sensor it starts to retard timing, which lowers the power. And if it detects that knock is 0 and AFR meets the target it gets more aggressive with the timing advance.

But, no. On normal cars, the car will go no more aggressive on the timing than it's stated RON target. If your car maker recommends a RON91 you gain nothing by going to 98 as the ECU will go no higher to preserve the engine from blowing holes in your piston.

I heard on bfm last week or so some caller said she sometimes fill up with ron 97 to 'treat' her car.. Got such thing ah? Car will be happier if use expensive petrol?