My car is european version and has octane corrector. US version cars probably don't have it. With octane corrector you can set the ignition timing according to the grade of fuel you use. Mine has four positions. Below is a picture of the octane corrector located nearby the fusebox(under hood, on driver's side).
While using 87 grade fuel I tried switching between super and normal positions but never heard an engine knock when driving under load. So, I concluded that my car probably has a knock sensor that is doing a good job or else timing is already retarded way back by the timing management because of some faulty sensor.
If a person don't hear a nock while driving using low grade fuel under load does that mean that it is safe for the engine to use lower grade fuel?
1989 230E, 8v, 166.000 km, updated to 94/95 trunk & hood
2002 Daewoo Nexia 50.000 km
1987 VW Jetta GLE 16V, Recaro seats
1982 Volvo 240 DL (lovely car!)
and few more american cars.