By on January 20, 2014

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!

 

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.

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44 Comments on “Piston Slap: Knockin’ on Stoichiometry’s Door?...”


  • avatar

    Knock Knock.
    Who’s There?
    Piston.

    Damnit…

  • avatar
    danio3834

    With those vehicles, just run the regular fuel. Anything else is a waste of money.

    • 0 avatar

      +1

      It astounds me when my family members run Premium fuel in cars with “regular” V6 engines simply because they grew up using “high test”.

      Certain cars demand 91 or 93 (like mine), but my uncle’s 09 Navigator was specifically designed to run on regular unleaded. Makes no sense to put Premium in. You increase the probability of building up carbon residue on the pistons.

      It’s amazing that people simply don’t follow the manufacturers instructions. Perhaps gas caps should have a clear and obvious notation that tells exactly what you’re supposed to use?

      • 0 avatar
        golden2husky

        Some do. My Probe’s fuel door has a label inside that clearly states “Premium Recommended”. So did a supercharged Buick that our family once had, only that one said “Required.”

        • 0 avatar
          APaGttH

          I seem to remember the 89 to 92 Ford Probe GT with the Mazda 2.2L 12-valve turbo 4 could run on 87 or 91, with 91 recommended.

        • 0 avatar
          Kyree S. Williams

          The Buick supercharged 3800 V6? I love that engine…

        • 0 avatar

          I have two tiny labels on two of the cars. (premium fuel required) says one. The other says (minimum octane 91 RON).

          On my other car, on the filler door, it says DIESEL ONLY in a small sticker. The fuel cap says, in bright yellow, DIESEL FUEL ONLY. Lastly, there is a yellow on black ring “sticker” around the fuel filler, installed as a recall, which says….DIESEL FUEL ONLY.

          Back on topic, when I had a turbo SAAB, I did a series of experiments with premium vs regular. It commuted fine on Reg, but boost dialed way back, and clearly HP went missing.

          C/D did an article a few years back, comparing a turbo SAAB, a smallblock mustang with premium recommended, and one other car designed for regular. You are wasting money if the car is designed for 87 octane, but losing power if the car is tuned for 91. The turbo car showed the biggest drop, there was a small drop with the NA Ford motor.

          I find it interesting that the Acura dealer insists you show them the gas receipt showing hi test when you get a loaner car. The BMW dealer doesn’t. I could see why when an X3 loaner I had was a total slug. When I refilled with premium the drivetrain woke up….the prior borrower went cheap.

          Likewise my Acura MDX. It will run regular, but never wakes up. With the correct fuel, you can hear the engine note pushing it harder, and feel it in the back.

      • 0 avatar
        danio3834

        It’s amazing how frequently people are willing to second guess manufacturers. Before they do that, they should really make an effort to understand why the recommendations are what they are and the thought that goes into them.

      • 0 avatar
        Kyree S. Williams

        That *is* annoying. Most non-performance engines designed for regular-grade fuel cannot and will not take advantage of the premium stuff. I have an acquaintance who puts 93 octane—which is not widespread here—into his 2010 Malibu LS. Conversely, I have acquaintances who are supposed to be running at least 91 octane in their luxury cars, but who cheap out and use 87. And my friend wonders why her 335i runs so inefficiently…

        • 0 avatar
          HerrKaLeun

          Not sure what you expect. The same way you can fuel your car with airplane fuel and it still won’t fly.

          • 0 avatar
            bunkie

            Not flying is the least of your worries. At my home FBO, 100LL is $6/gallon.

            Then there’re the damage to the catalytic converter from lead contamination.

            And if you use the *other* aviation fuel (Jet-A), you’re going to be walking, let alone flying or even driving.

        • 0 avatar
          CoreyDL

          I put midgrade in my old I30 once, and it didn’t run nearly as smoothly.

      • 0 avatar
        bachewy

        Here in CO Springs my wife uses only the highest octane she can find because she believes it has ‘more chemicals to clean your engine’. No amount of logic will dissuade her.

        • 0 avatar
          bunkie

          My ex-wife fervently believes that higher-octane equals better gas mileage. She actually cited my repeated attempts to dissuade her of this notion in her filed complaint.

  • avatar
    mike1dog

    The air is thinner the higher above sea level you go. It’s like lowering the compression ratio in the engine, reducing the need for high octane. That’s why gas has lower octane at high altitudes.

  • avatar
    RetroGrouch

    Quotes be damned… Higher octane does not mean more energy.

  • avatar
    skor

    Sajeev is correct, in this case, the extra octane point makes no difference. Save your money and fuel up on regular.

  • avatar
    cgjeep

    So the owners manual on my 01 Jeep GC states to use 87 Octane but to use 93 when real hot or towing. I’ve run a few tanks of 93 through it and it feels like it runs a bit better and got slightly better gas mileage. On the forums people claim the fuel economy gains offset the increased cost of the 93 octane. Note where I live(DC area)93 octane still has ethanol in it. So how is the better performance/fuel economy possible?

    • 0 avatar

      You can find some gas stations that sell 93 with a lower ethanol content, or possibly, none at all. You need to do some googling or look at the 93 pump. Some Sunnoco’s in my area sell a good quality 93.

    • 0 avatar
      danio3834

      I assume you have the 4.7L V8 in there. In the case of some engines as the design and calibration permit, igintion timing can be advanced to make use of higher octane fuel than normally required by the compression ratio and regular spark mapping. Igniting the fuel earlier in the power stroke has the potential to make more power, but the danger in doing that is detonation. If your manual says that higher octane can be used to make more power, then the ECU contains a calibration that can use the knock sensors to advance the timing a little further than normal to make use of the extra octane and give a bit extra power.

      Using it under all circumstances may not net you a benefit as you don’t always need the extra power, but there are many variables. The best way to find out would be track the mileage as scientifically as possible on your own to determine if it makes a difference for you.

      • 0 avatar
        cgjeep

        Danio, thanks for the explanation, it is the 4.7L. I found it made a slight difference but not enough to offset the cost. Funny the ECU calibrates fine going from 87 to 93, but doesn’t adjust well going back to 87. Would run rough for a while then smooth out. I chalk it up too old age and poor Mopar electronics.

        • 0 avatar
          cdotson

          Also, with the 2001 Jeep 4.7L V8, depending on what trim level you got, you may have the “HO” motor that has a different pair of cams and maybe some other minor differences.

          I believe all 4.7L engines were tuned for 89 octane, with the HOs tuned for 91+ octane, but due to the level of programming around the knock sensors use of 87 octane was acceptable with the understanding that economy would suffer slightly. This is why yours would run better on 93, especially if it’s fitted with the HO parts.

          The 4.7L in my Ram has only ever used 87, which was only ever a problem before they started putting ethanol in the gas as I would have to run fuel injector cleaner every now and again. Now the doctored gas does it for me with every tank.

  • avatar
    B Buckner

    Some cars adjust to lower than recommended octane better than others. I ran an 03 Audi A4 on 87 octane for years when the manual recommended 91 to 95. No problems whatsoever. But when I eventually experimented with the higher octane, my gas mileage improved almost as much as the increase in gasoline cost, with a noticeable increase in power, so I switched.

  • avatar
    jim brewer

    I don’t think you can generalize about this anymore.

    My manual (Ford 3.7L) advises to use 87, evidently even in high altitude locations such as New Mexico. “Some stations offer fuels posted as “Regular” with an octane rating below 87, particularly in high altitude areas. Fuels with octane levels below 87 are not recommended.” Notice how it doesn’t say you can use lower octane rating at higher altitudes like in the old days. It seems to say the opposite. Its possible that was just an inartfully drafted passage, but its the owner’s manual, meant to be taken literally.

    As to the Ecoboost the Ford manual says: “Premium fuel will provide improved performance and is recommended for severe duty usage such as a trailer tow.”

    Ford 2012 F-150 manual, p. 397.

    My prior car, a BMW used high octane but had a knock sensor. Are BMW owners foolish to use high octane when the knock sensor will allow lower octane?

    • 0 avatar
      HerrKaLeun

      always use what manual says, never higher. While higher doesn’t hurt, it costs $. If your car has certain compression and the manufacturer adjusted the program to work under these conditions. It isn’t like when you use premium gas the piston and crankshaft will magically change geometry to compress higher….

      The absolute compression won’t change. What changes ignition will be moved up to ignite before compression is completed to be ahead of self-ignition due to heat. But ultimately if the piston moves to decrease combustion chamber from 130 cm³ to 10 cm³ compression always will be 13:1, no matter what gasoline you use.

      As for using less than recommended, it won’t hurt the engine due to knock sensor. i think every car nowadays has one. But you have less mileage and less power. so cost-wise it may be a wash. Do you notice less power on a 300 hp car? Probably not… Most drivers of expensive cars can’t even parallel-park and don’t know what RWD actually….

  • avatar
    PrincipalDan

    Towing this summer with my 2004 F150 Heritage over 2000 miles from the Midwest, Plains States, and Rocky Mountains.

    I did my best to find 100% gasoline as I was filling up and I did use some high test on the portion of the trip going from flat Kansas to Colorado Springs.

    Otherwise, I’m a “whatever the owners manual says” kind of guy.

  • avatar
    jimbob457

    Just run regular and don’t worry about it. Octane ratings of gasoline sold at the pump are slightly lower at higher altitudes because, as the BRB point out, the air is thinner.

    In olden days, you occasionally might drive from Santa Fe (8000+ ft.) to somewhere east of the Llano (1000- ft.) and experience a little engine knock from low octane gas purchased in Santa Fe. I don’t think even this is true anymore, but if so and it happens to you, just fill up with the local stuff.

  • avatar
    matador

    I run an Audi A6 in Wyoming and Montana. Here, our Regular Grade Gasoline is 85 Octane. The car runs fine. My other cars (Chrysler New Yorker, Buick LeSabre, Chevrolet Impala) also got 85 Octane and all made it over 200k without a single hitch in the engines.

    You’ll be fine on regular.

  • avatar
    SCE to AUX

    What everyone has said is true about preventing knock, but the knock sensor will also permit the timing to advance to help develop more power if higher octane gas is used.

    This can result in better fuel economy. If the engine can develop say 5% more power at a given RPM, then a smaller throttle opening will permit better fuel economy, and thus pay for the 3% cost adder of premium.

    I have observed this in my V6 Sedona minivan while fully loaded (or towing).

    • 0 avatar
      Power6

      Have you been able to verify the actual savings? Every semi-scientific comparison I have seen shows that you can’t overcome the cost of the premium. Maybe at WOT on a track. Higher throttle opening actually is more efficient, paradox of a throttled gas engine you have to choke it off to reduce power (well pre Valvetronic style anyways).

      • 0 avatar
        bigdaddyp

        On my Georgia to Michigan run in my Kia Seona, I have observed some difference in economy between regular and premium. On the Tennessee/Kentucky portion of the run, the most difficult and fun part of the trip, I average about 18.4 mpg on regular and 22 mpg on premium. To me it’s worth it simply for the improved throttle response and punchier engine performance between 2500 rpm and 4500 rpm..
        Keep in mind my van is loaded to the brim with passengers and cargo and I am usually trying to exceed the sound barrier when maki g this trip.

        Now my boss, who makes the same trip, has found on his mid decade Ford Focus, he saves a few pennies per gallon on using premium. The improved mileage just offsets the increase in price paid for premium. As always Y.M.M.V..

    • 0 avatar
      HerrKaLeun

      The throttle is electronic anyway, so it does whatever the computer tells it. the accelerator you put your foot on only tells the computer hat you want to drive faster – there is no direct linkage to throttle. Injection, ignition and throttla all will be manipulated based on the interpretation of the “want to go faster” signal from your foot.
      If you fill in fuel with higher rating than the manufacturer recommends (and designed for) it wil do nothing for power and mileage. Of course, if you use lower than recommended and go back to the recommended fuel, you see better power and mileage. I don’t think a manufacturer sells a 210 hp car at 91 octane and sells it as 190 hp car to run on 89 octane. If that was the case, then indeed you coudl gain power by over-octaning. but thsi is unlikley.

      • 0 avatar
        bigdaddyp

        Some Kia’s and I think some Huyandi’s are sold with their engines rated on both regular and on premium.
        My Sedona is rated 242hp on regular and I think it was 250hp on premium.

        • 0 avatar
          HerrKaLeun

          Interesting, which is the official gasoline?

          • 0 avatar
            bigdaddyp

            From the manual…”this vehicle was designed to run on fuel with a minimum octane rating of 87. For improved performance 91 octane or better is recomended.”

            And in my experience this statement holds true. Runs fine on regular, feels more responsive with premium. Under normal circumstances I run regular.

  • avatar
    Power6

    I may have missed this but I don’t think OBD-II has anything to do with anything, other than denoting the current technology of the post 1996 era. OBD-II is a diagnostic and interfacing standard, it does set forth emissions requirements (second o2 to verify the cat works) but not engine control strategy. Given that, anything 96 on will have a knock sensor.

    There are a couple problems with the explanation there, first most of the time you are going to have knock, you are in enrichment mode, the A/F is not going to be stoich. Also the response to knock is not to richen mixture, it is to pull timing. But that is academic…

    The interesting thing here is the knock control has come a ways since pre-OBD days. My old 1987 Grand National would respond to knock, by pulling timing, read from a static table in the ECM. Every time you lay the throttle down, the process starts again. If the gas is low octane, this feedback cycle continues until you fuel up with good stuff. With a turbocharged car, you are taking a chance on the system reacting quickly every time you plant the throttle. With an NA car, typically who cares if it knocks a bit, not a big deal.

    Now your 06 and 08 cars are likely more advanced than that. There is a “knock learning” capability. If you put the 86 octane in, and it does cause a little knock, you are affecting a long term trim value. It will learn the low octane gas over time by pulling timing and losing a little HP. After a while the learned timing has been adjusted to where it doesn’t knock in the first place.

    You really have nothing to worry about with a modern car as long as you are happy with the way it drives on the gas you put in it.

  • avatar
    jim brewer

    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.

  • avatar
    sastexan

    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.

    • 0 avatar
      burgersandbeer

      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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