TTAC Commentator dastanley writes:
I have a Piston Slap question: Most of the gasoline sold here in New Mexico is high altitude gas, meaning that Regular is 86 octane, Midgrade is 88, and Premium is 91. The owner’s manuals for my ’06 Corolla and my wife’s ’08 Hyundai Tucson (try not to be jealous :-) ), calls for 87 octane fuel. Am I OK to use Regular or should I take the manuals at face value and pay a dime more per gallon for Plus?
My understanding with OBD-II vehicles (1996 to present) is that the engine doesn’t “know” what type of gasoline is used unless the knock sensor registers a knock and sends a signal to the ECM to momentarily retard spark timing and richen the mixture beyond stoichiometric. If that doesn’t happen, then the engine runs normally with the timing and mixture programmed for Regular at open loop (real-time feedback) factory specs, with adjustments made for density altitude and load. Thus the engine won’t know or care what octane fuel is run.
So Sajeev, what do you and the Best and Brightest have to say? Thank You Sir.
What do I say? I love it when people correctly answer their own question!
Many OBD-I cars also use knock sensors like this, but that’s not the point: run 86 octane and you’ll be problem free for life. Even worse, the more expensive 88 octane is pointless as the knock sensor doesn’t give a crap about higher octane. That’s the job of the fuel tables, which do not consider something with more “energy” than 87 octane. Unless it’s been reprogrammed, ‘natch.
So either you get the fuel tables tweaked for 88 octane (likely a fruitless exercise) or just enjoy 86 octane and the magic powers of Ye Olde Knock Sensor.
Go ahead Best and Brightest, find a case that proves me wrong!
Send your queries to sajeev@thetruthaboutcars.