While 91+ octane may be recommended, the car must be mfgd to run acceptably well on regular 87 octane fuel.
Modern engines w/electronic engine control units have knock sensors and good knock-avoidance adaptive strategy algorithms to back off ignition advance (and perhaps richen mixture at slightly higher loads & RPMs) - and stay backed-off if knocking persists. Eventually the system 'adapts' to fuel you use and you have no problems.
I used to reverse-engineer EFI/ECU software for a variety of non-US brands. Knock sensors were pretty good - we could even jack the timing up ridiculously and the knock sensor would hammer some sensibility back into the ignition timing curve. Base tuning curves are conservative, however, in many brands so we could get some extra horsepower by redoing the advance curve and sometimes ever so slightly *leaning* the mixture at given load/RPM points. (Richening mixture often *reduces* horsepower!)
[BTW - I have little experience with Fords, but an ME/racer guy with whom I worked and who was an acknowledged engine control expert, said that from his racing experience, stock Fords are mfgd with a pretty untuned base advance curve that is adaptively put into shape thru a knock sensor + algorithm. Thus putting 'power chips' into stock Fords doesn't make too much sense until there are major engine changes (intake, headers, exhaust, stroker job, etc.) deserving of a full retuning.]
[BTW#2 - in our estimation, Shell 91 (in Kalifornia) did not seem up to snuff. For our Dinan turbo BMWs we did some controlled tests w/o knock sensor engaged and we could see earlier knock onset + elevated CHT with Shell than other fuel brands. Other brands were largely the same and OK, but Chevron came out on top with best knock performance/lower CHT. While base fuels are supposedly 'fungible' (the same, or exchangable w/no perceptible difference) different additives, detergents, etc. are added by various brands. This prob has something to do with differences.]
Bill Wiese
San Mateo, CA