Ding ding. Calling it "premium" is a silly marketing gimmick. Auto ignition is when the air/fuel mixture ignites without a spark, due to too much heat and compression. This is bad in ICE engines. If it occurs too early, it will try to push the piston down as it's traveling up, causing damage. Higher octane resists this. Higher compression means more power, so running higher octane enables the engine to have a higher compression ratio. However, the mixture needs to burn so that maximum cylinder pressure occurs at the optimum moment of the stroke. Higher octane fuel takes longer to burn, so it requires more advanced timing so the mixture ignites earlier. If it doesn't ignite early enough, then the mixture will burn after top dead center, wasting energy and losing power. So 93 isn't better than 87. Which octane is best is entirely dependent on the design of the engine because it's a balancing act. Although, anyone who says the engine was designed around 87 octane is kidding themselves. The engine isn't designed with a fuel's octane rating as its primary influence. It would be the compression ratio, with changes in timing made to accommodate different octane fuel. If the engine were designed around 87, then the jump to e85 wouldn't be so substantial. In fact, e85 providing such a nice power bump implies that 93 is as much of a compromise as 87 is. An example of an engine preferring 87 would be the pushrod 5.0l with its 9:1 CR. Running 93 would just result in wasted energy and unburnt fuel.
This leads me to the following. The 12:1 compression ratio is a theoretical number because the CR is dynamic. There are several factors that influence this but, for the sake of 87 vs 93 octane, I'll focus on the cam timing (hello VCT). The sooner the intake valve closes, the more cylinder pressure occurs and the more low end power is made. This hurts top end power because the engine also has to work harder to compress the mixture, and this is why vct offers so much flexibility (like running 87 octane). If the intake valve closes later then there's less cylinder pressure and reduced risk of 87 octane fuel auto igniting. Direct injection further helps because of the cooler charge and more controlled fuel delivery because it's independent of the valve timing. But you're giving up some good low to mid range power. Also, at high engine speeds, the amount of time the mixture is fully compressed is very short, so auto ignition is less likely to occur. All the above is why peak horsepower is not affected as much by running 87. Oh if only peak horsepower were everything...
I guess that's a bit off topic, but there was plenty of arguing about the power benefits so... ya. As for MPG, a richer mixture is cooler and resists auto ignition so if ecu richens the mixture for 87 then it would get worse gas mileage. Although, if I had to guess, it wouldn't be by very much when cruising around and would only show if driven hard on a regular basis. I, personally, wouldn't use mpg as my reason to pick 87 or 93.
That’s all great info.
The part about the piston being pressed down is slightly misleading though.
The part of higher compression detonation that causes damage is the combining flame fronts. The energy pushing piston to push down the other way is not what causes the problem.
This is why a lower compression engine can knock(ping) its whole life and a higher compression engine will start to eat itself up over a shorter amount of time.
A higher compression engine can have multiple flame fronts of detonation inside the cylinder. These flame fronts can combine into a point to create serious damage inside the the cylinder. It’s the multiplying effect that causes problem.