Originally Posted by JasDmw
...To no ones real surprise, if you car doesn't need it (per the manufacturer), you are wasting your money....
Fortunately for us the Focus owner's manual specifically states that higher octane fuel improves performance. Easiest way to do that isn't all that high tech. Just advance the timing until knock is sensed then retard it a bit. Higher octane resists detonation as it can handle higher pressure and heat.
In other words, with premium gas the combustion process can start earlier.
Knock is relatively easy to detect due to its almost universal sound spike at around 6400 Hertz.