#3 - Too complicated to figure out. Easier to just go with what the manual states is required.
Modern engines can detect knocking(detonation).
It can be mitigated by retarding the ignition timing. Ignition timing is usually specified in degrees before top dead center on the ignition stroke, and a vehicle generally has a standard "map" for when that should occur for a given rotational speed and load. With a mechanical distributor, this is done with a centrifugal mechanism that advances the timing as it spins faster, and by monitoring the vacuum signal and advancing under low load conditions. Once this has been set up, it's difficult to change-the timing can be changed globally(i.e. the amount of advance at idle can be changed, and timing under all conditions will be changed the same amount) by physically rotating the distributor, while the characteristics of the advance "curve" requires changing weights and springs inside it.
Modern engines use a crank position sensor, and fire the spark at an appropriate time based on the above factors. In general, the best performance and economy come from having the timing as advanced as possible. With a knock sensor, you can dynamically stay close to this value, and if the ECM senses knocking it can reduce the engine timing to the point where it stops(or at least isn't occurring at a harmful level). In other words, it won't let the engine damage itself.
A slightly more complicated way to do it is by altering the valve timing. Compression ratios are usually quoted as static figures-i.e. the ratio between the cylinder volume at TDC vs. BDC, and this might be something like 8:0:1 for a 1970s emission-choked engine(lower compression, and consequently lower cylinder temperatures, tends to help mitigate NOx formation) or 12:1 for a modern high performance naturally aspirated engine. Thanks to things like valve overlap(times when both the intake and exhaust valve are at least partially open), though, the dynamic compression ratio, or actual ratio when running, is generally in the 5:1 to 6:1 range. Most modern engines can change their valve timing, and increasing the amount of overlap can decrease the dynamic C/R and allow an engine to run safely on lower octane gasoline. This also comes with both a performance and economy penalty, but allows the engine to avoid damage.
If your car says to use premium, then use premium. I won't argue that-both in theory and from my experience in the real world it gives the best gas mileage and performance. A tank of regular mixed in with a diet of premium won't hurt a darn thing, though, in the short term or the long term.
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the one for Premium Unleaded is always the last one on the right, and thus impossible for me to press the wrong one.
Until a station messes with you and puts premium in the center or on the left, which I've seen more times than I care to count(although I'd guess center is the most common "different" placement).
And yes, if you ever travel(I drive about 2.5K miles a month on average) it's easy enough to find yourself at an unfamiliar gas station where they are in different places even if you normally go to the same place around home.
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