Variable compression? So logical next step is variable octane, right? One tank of 87 and one tank of 93, dynamically mixed in response to changing compression ratio. :)
A common way to run lots of boost on turbocharged cars is to use 87-93 normally, and E85 under high boost. You get the high octane when you need it, without the 30% hit to fuel economy when you don't.
I've seen this done before, but I wouldn't consider it common. In my experience, the two most common approaches for mitigating knock on high boost street cars involve either running E85 all the time (or a reduced mixture like E30), or using a water/methanol injection system. A secondary fuel system with race gas is more common on nitrous cars though.
Anecdote: I'm running a water/alcohol injection system on my 335i (tuned on higher than stock boost). It really helps on the hotter days. Currently, I'm running a mixture of ethanol (everclear) and distilled water. Most people use methanol, but it's not really compatible with the viton gaskets inside of my pump.
I think the point of the person you replied to is that any variation in octane is handled by the fuel injection system. This renders having two tanks unnecessary (unless you require more distance between fill ups).
It can adapt to lower octane fuel by adjusting the timing, it's far from optimal, and at high enough boost level you need higher octane regardless. At non boost levels, you need less octane rating to prevent knock, but under boost you need higher octane rating. So you run the higher octane fuel all the time, and that is less efficient.
Correct. This involves relying on feedback from knock sensors which literally "listen" for the specific tone that a detonation event produces (usually somewhere in the 6-7 kHz range depending on cylinder geometry). You will sometimes hear this phenomenon referred to as "pinging", and it’s actually audible to the driver if severe enough. There is also another method of knock detection which involves using the spark plugs to detect the resistance of the air inside the combustion chamber immediately after an ignition event, which I believe is an indirect way of measuring cylinder pressure. I’m not positive on this, but I think the idea is that you want peak cylinder pressure at the moment of time where the piston is like 20 degrees after top dead center. Earlier than that, and the majority of the force is wasted pressing directly down on the crank shaft. This is more advanced and is used only in specific cars such as the E90 BMW M3.
If the ECU detects knock, it will retard timing. This works to an extent, but you’re ultimately worse off than if you just ran premium fuel in the first place. The other issue is that (on systems with conventional knock sensors) this causes you to basically “bounce” off of the knock threshold. Knocking is terrible for the bearings inside of the engine and should generally be avoided, but running regular fuel in a car that calls for premium results in the ECU constantly trying to creep up timing (or switch to the high octane map), only to be confronted with knock again.
