Octane use

Discussion in 'Gen 4 Prius Main Forum' started by Fobbers, Jun 16, 2017.

  1. fuzzy1

    fuzzy1 Senior Member

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    From past information, it seems that both 2-tank and 3-tank systems exist for 3-grade fueling stations. It depends on circumstances and locality.

    I've never followed a fuel truck around, but did watch one fill dad's agricultural tanks several times. That truck clearly had multiple partitions, filling dad's old leaded and new unleaded tanks separately. Back when leaded was still available.
     
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  2. wjtracy

    wjtracy Senior Member

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    For USA, 87 Octane (or 85 at altitude) is the recommendation based on USA EPA gasoline regulations...

    International owners should presumably try to determine Toyota recommendation for their specific fuels.
     
  3. DOHCtor

    DOHCtor Junior Member

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    Allready put some 85 octane in a Skyactiv 2.5l Mazda CX5 when i was in a trip to Yellowstone. Boy it didn't liked it. As soon as i could, i put some 91 in it to raise up that octane level. I would'nt do the same error twice... When the manual say "use 87 or higher" and you don't have 87, use the one grade above it that's available. 88, 89, 90, whatever.. over 87 is fine, under is not as it may knock under heavy load.

    Marko.
     
  4. fuzzy1

    fuzzy1 Senior Member

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    It is unfortunate that your Prime didn't like Wyoming's 85 octane fuel.

    It is sold in certain high altitude regions because it causes no problem in a very large number of vehicles that call for 87 octane (at least at sea level). That includes every fuel injected car I have owned (Accord, Legacy, Forester, Gen3 Prius). My only caveat is that my Prii have experienced less than their share of those 85 octane miles, due to the Subarus taking the winter shifts.

    I can't say the same for my last carbureted vehicles, but they developed a dislike of even 87 octane as they aged. That dislike was very strongly linked to road elevation, originally noticed at below 1000 feet , later increasing to somewhat above 2000 feet.
     
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  5. alanclarkeau

    alanclarkeau Senior Member

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    Probably as much to do with more advanced electronics. Knock sensors started implementation (I think in TurboCharged engines?) almost 40 years ago - but I think the first I had in a car was about 18 yrs ago - now I think they've all got them.
     
  6. DOHCtor

    DOHCtor Junior Member

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    Not the Prime, the Mazda CX5 ;)

    I've just had the Prime.

    Marko.
     
  7. fuzzy1

    fuzzy1 Senior Member

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    Sorry for the misread. I must have looked at your profile details after reading the post, and Prime overwrote CX5 on my mental chalkboard.

    I should check with a friend with a CX5 for her experience on this, but she may not drive far enough from home anymore to reach 85-octane territory.
    I suspect my old knock-prone carbureted cars were suffering carbon buildup, either increasing effective compression ratio, or as someone else also suggested, making hot spots that could pre-ignite the charge. Today's Top Tier fuels should be running considerably cleaner, at the very least taking much longer to suffer similar buildup. I'm not sure which of my cars first had the knock sensor.
     
  8. jerrymildred

    jerrymildred Senior Member

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    We had a late 70s Ford Fairmont wagon with a 302 V8 when we lived in Colorado in the late 70s. When we moved to Ohio, it knocked like crazy on regular gas. Rather than re-jet the carburetor and still have to pay a ton for gas, we traded it in. The sales guy was pretty impressed by that V8 in such a smallish car.
     
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  9. fuzzy1

    fuzzy1 Senior Member

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    ^^ A 70s Fairmont sedan with that same 302 V8 was my first knock-prone car. It didn't knock when first new-to-me, but developed it later.

    My next (and last) knocker was an 80s 2.3L.
     
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  10. Colorado Boo

    Colorado Boo Member

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    I've lived at 7,000 feet in Colorado since 2000 and, yes, 85 octane is fine for our Toyota's here.

    Visitors here sometimes have car problems (RPMs too low, car not running right, can cut off when stopped) and it's not because of the 85 it's because the computers in the engines are self-tuned to a different air pressure. The easy fix it to disconnect the negative battery terminal for a minute or two and then the computers will re-learn the proper settings and run fine. (I'm not sure if they need to do it again after they go back down to near sea-level but I'd assume they would.)

    The recommended fuel type should be printed on fuel fill cap, like the oil fill cap.

    True story...my Dad bought a new Tundra in 2003 (it's still running great) but he only filled it with high-octane. (He thought it was "better"...it's not). Anyway, now he can't run anything but the high octane, an expensive mistake but that truck is indestructible.
     
  11. wjtracy

    wjtracy Senior Member

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    In the "old Days" in the USA, octane was important for the car. Also different brands could be better, because in those days the oil companies had control over the gasoline formulation. I used to like the Amoco clear Ultimate in my 1990 Voyager minivan. Long story short, in the USA. EPA basically took control of gasoline formulation and made gasoline a commodity with mandated recipe (eg; 10% ethanol). Car technology was mostly set to take standard 87 octane. Big (and medium) oil downsized and got rid of probably a million of R&D people, no longer needing to vary the gaso recipe for the market. A relatively few cars need the higher octane.
     
  12. MPGboss

    MPGboss Junior Member

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    there's something very satisfying filling up with 87, best deal ever :)
     
  13. koco

    koco Member

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    Use the lowest octane fuel and calculate mpg for a few tankfuls, then try mid-grade and calculate mpg again a few times and see if there is a gain. Variables like climate and altitude make difference as to what will work better. Then there is the issue of Winter and Summer blends...
     
  14. wjtracy

    wjtracy Senior Member

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    Octane does not correlate well with MPG these days, especially in reformulated gasoline areas.
    Your area might not be RFG, so there could be some energy content variations among brands and grades.
     
  15. koco

    koco Member

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    With an old non-computerized fuel and ignition system it was a matter of advancing the timing until it pings under load and backing off just enough to stop the detonation. The knock sensors will take care of that to some degree in the newer cars.

    Maximum advance correlates with more power (ask any engine tuner). Once you find the right fuel for your car's tuning and conditions you will get more power and better mpg.
     
    #35 koco, Oct 27, 2021
    Last edited: Oct 27, 2021
  16. bisco

    bisco cookie crumbler

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    it has never been shown that higher octane results in higher mpg's in a prius. and even when non ethenol gas is used, the mpg increase doesn't justify the cost.
     
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  17. Mendel Leisk

    Mendel Leisk Sand Pounder

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    I tried a tank of Chevron highest octane (94, and it's ethanol-free up here) a time or two, didn't notice much if any diff. I would think the ethanol-free in particular would help, but it wasn't enough to exceed statistical "noise".
     
  18. fuzzy1

    fuzzy1 Senior Member

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    In the old days, when my refinery relatives were early in their careers and were in charge of fuel blending at two of this region's refineries, it was quite possible for the higher octane to produce lower MPG, because it usually contained slightly less total energy than the low octane blend. But the specifics varied from batch to batch, and these batch details are not released to consumers.

    High powered engines use high octane fuels not because the fuel contains more energy, but because it can be rapidly guzzled in a more controlled manner that helps prevent damaging the engine.

    Processes and blending requirements have changed in the decades since, and these relatives have moved up and on, so I don't have current insights.