[masraum]

Originally Posted by David Inc.

For modern cars, yes. They can watch for knock and pull ignition advance when they see it. It means less power and efficiency, though, and isn't good for the engine itself. I think mine says it can run on regular in a pinch but it's advised to refill with premium ASAP, '17 Alfa Romeo Giulia.

Isn't ethanol less energetic, and has the effect of raising octane which reduces likelihood of knock. If you're going to have knock with 87octane 10%ethanol, you're going to have knock with 87octane 0% ethanol, right. So ethanol vs none doesn't really change knock characteristics for a given octane rating.

So whether running 87 or premium (91, 92, 93) it makes no difference if it's got ethanol or not (from a knock point of view).