Why did ExxonMobil premium fuel drop the performance on my '12 Honda Civic Si?

My car has had a very significant drop in performance due to use of ExxonMobil 91 fuel. Why would this happen?

Possibly bad gas. It's possible that where ever you bought the gas that it was contaminated a little. There was certain brand of gas in Albuquerque, NM I would try, and my bike would ALWAYS run terrible afterwards. So I never bought that gas and it worked fine.

Also, getting hi-test premium isn't always the best. Sometimes the cheapest gas, or lowest octaine is best. I doubht that a Honda Civic needs expensive premium gasoline.

Goto Walmart or Dollar General, get their cheapest bottle of gas additive (like STP), actually buy 2 bottles, and put into your gas tank. If the gas is contaminated, this will help clean it out.
When you burn the gas with the gas additive in it, it will burn hotter and faster. You won't get good gas mileage during this time, because you're burning hotter. But run a couple tanks thru with the additive in it, and it should help to bring your performance back up.

We tend to think that premium is just a matter of octane, but that difference in octane is produced by the addition of aromatics and those chemicals change the fuel/air mixture needs. The difference is enough that many Toyotas will set the check engine light on fuel trim if you put premium in them.

You should use the previous fuel, maybe ExxonMobil is not compatible with the engine of the Honda Civic Si.

Or maybe it was a bottle with flaws in it… Impurities etc. Have you checked the expiration date?

What performance(s) you're talking about? I can tell you right now what ever it was: The Premium fuel produces by major companies are the same when it come into the combustion chambers.

Go back with the "Other" premium fuel if that will please you.

Actually the 2012 Civic Si does require premium gas. Sounds like bad gas. Maybe you filled up while the fuel truck was filling the tanks.