Piston Slap: Knockin' on Stoichiometry's Door?

by Sajeev Mehta

Updated: December 26th, 2019 Published: January 20th, 2014

piston slap knockin on stoichiometry s door

TTAC Commentator dastanley writes:

Hi Sajeev,

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.

Sajeev answers:

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!

[Image: Shutterstock user ThePowerPlant]

Send your queries to sajeev@thetruthaboutcars.com. Spare no details and ask for a speedy resolution if you’re in a hurry…but be realistic, and use your make/model specific forums instead of TTAC for more timely advice.

#AirFuelRatio #Detonation #FuelMixture #FuelTable #Knock #Lighten #LowOctane #Octane #Rating #Retardation #Spark #Richen

Sajeev Mehta

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3 of 44 comments

Jim brewer on Jan 20, 2014

Well again, there is considerable waffling in some of the owner's manuals these days. Ford substitutes for the usual statement that lower than 87 octane fuel is acceptable at high altitude to a "Watch out for lower octane fuel often sold at higher altitude!" warning. I understand from the Mustang forums that the 3.7L puts out abut 10 extra hp with premium. A manual that says "premium fuel will provide improved performance." Probably means that premium fuel will provide improved performance. Think about it: A middle-class automobile that requires premium is at a considerable competitive disadvantage to one that does not. With knock sensors its possible for the manufacturers to make cars that run on regular--but really prefer premium.

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Sastexan on Jan 20, 2014

When I was out test driving with my mother for her '10 RX350 (which she ultimately purchased), she asked the salesidiot about fuel. "My Infiniti said 'premium fuel recommended' but I always put 87 octane in and had no issues in 14 years. Can I do the same thing with this car?" "Oh, ma'aam, the Lexus is a PREMIUM vehicle and requires PREMIUM fuel." Which prompted me to start questioning the salesmoron: "What is different about Lexus's tuning of this engine over the Camry and Highlander? Doesn't it share all the same parts? Isn't the power essentially the same, only different for packaging purposes?" Salesquack then states, "Oh, sir, you don't know how much Lexus engineered this vehicle." To which I replied, "You, sir, don't know squat about the product you are selling and I suggest you do a little reading before you hand out comments that make you look stupid." My mother told me that I was mean to him. Not the first time, and won't be the last time that I call someone out for trying to convince someone with a complete lack of facts. Result - she has had the car for 3 1/2 years and only uses 87 octane - mostly from Costco and Sams Club. No knocking or even perceived down on power.

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- Burgersandbeer on Jan 21, 2014
  
  Why wouldn't he just say "sure! fill with 87" to help himself make the sale? That's the worst part; not that he didn't memorize every nugget of Lexus/Toyota trivia.
  
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