Originally Posted by BowerR64
If lower grade fuel causes knock and retard timing how can it be better to use that fuel?
There is much more to fuel than octane. Different brands use different additives, such as detergents, lubricants, surface protectants, etc. 'Top Tier' brands use much more cleaning agents than required by law, and they will keep the engine running better, longer.
Thus, using a good (e.g., 'Top Tier') 87 octane fuel is better than using a crappy (e.g., oxidized, off-season, no-additives) 93 octane.
You are incorrect in your statement about causing knock. The Focus' engine does not knock when using 87 octane because it adjusts the timing. Of course, it is not getting as much power as it could get with a higher octane fuel, but that is of small importance compared to the cleanliness and life of the engine.