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Originally Posted by projekt EPiK  the reason for increased octane is detonation resistance slowing fuel burn. |
Exactly right- and it is actually a very serious issue.
"Knocking" and "pinging" are actually detonation and pre-ignition. These are very damaging phenomenon that happen in internal combustion engines that are pushed too hard and/or run on the wrong fuel. They are actually violent miscombustion- when fuel explodes in the cylinder instead of burns smoothly. If allowed to happen, this can quickly destroy an engine.
BMWs (like most cars in this era) are electronically protected from detonation. Small microphones (knock sensors) on the engine block listen for the sound of detonation occurring, and when it does the engine performance is electronically reduced until the issue stops. But of course if knock protection is activated, it means that it is already happening in your engine- and it is an unhealthy thing, especially if you want that engine to last for a while. The driver will never hear an OBD2 engine knock or ping- the ECU will eliminate it long before the driver becomes aware of it. But that doesn't mean it is not happening, if only momentarily.
In short,
you certainly can damage your engine by using a lower grade. If you only occasionally use the wrong stuff (filled by mistake, they were out of the right stuff), it's not a big issue- the engine will do it's best to protect itself. But you really should not use cheap gas in a ti regularly- especially if you're only trying to save a couple of dollars. Engine damage is expensive....