My car (a 2000 Celica GTS) has been painfully sluggish in first gear lately. And not just 4-cylinder-pseudo-sports-car sluggish. We're talking having-difficulty-pulling-into-20-mph-traffic sluggish. I finally brought it to the dealer the other day, and they stared at it for most of a day.
Part way through, they called me up and asked, "What grade gas do you put in it?" "Medium", I said. I knew that the car says it wants premium, but I figured it couldn't matter that much. Octane is all about preventing engine knock; as long as you don't hear knocking, you're fine, right?
Apparently not. I appears modern cars have knock sensors that detect engine knock long before you can hear it, and then retard the spark timing to prevent further knocking. They're actually pretty clever, rolling the timing back until the knocking stops, then advancing until it starts again, to find the sweet spot. Or something. I don't know the details, but the point is that the system settles into a new "learned" configuration, rather than the ideal one.
This means that in a recent car, a knock situation due to lower octane just saps horsepower. The dealer's best guess is that that's what's going on in my car. It's strange that the system would be willing to retard timing almost to the point of unusability, but I guess the priority is to prevent damage.
At their suggestion, I dumped a bottle of Octane Boost into the current tank of medium-grade gas, and will start using premium next time. The Boost seems to have helped--I can merge into standing traffic now!--though it'll take a couple days of responsiveness before I'm fully convinced.
Knock sensors! Who knew?