In general, if you use a lower Octane fuel, then you will possibly encounter detonation or "Knock", especially on warm days. The engine has a knock sensor, which is a kind of microphone that listens for knock in the engine. If the sensor hears knock it will retard your timing advance and to eliminate the knock and as a result your performance will suffer during acceleration.

If your knock sensor does not hear the knocking then you can fairly quickly trash your engines moving parts, especially the cylinders and rings as the detonation occurs well before top dead center is achieved, on the compression stroke. I have seen holes in pistons the size of quarters caused by knock erosion and detonation. Not good.