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The great "octane" experiment

Discussion in '2nd Gen. Tacomas (2005-2015)' started by MaineYota, Jun 9, 2010.

  1. Jun 9, 2010 at 4:41 AM
    #1
    MaineYota

    MaineYota [OP] Well-Known Member

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    At the outset, I know I'm going to get flamed for putting crap gas in my engine.

    That said, I want to know how much my truck's fuel economy is affected by different grades of gasoline.

    Here are the experiment parameters:

    - I live in Maine, and there's NO 93 octane in the state (which was a real PITA when I was driving my WRX)
    - Maine uses 10% ethanol
    - Fuel grades are fairly uniform: 87, 89, and 91 octane
    - I'll be tracking my mileage at fuelly.com
    - This phase of the experiment will last 2 months or 500 miles, whichever comes first
    - Next phase will be 89 octane
    - Current fuel economy is 19.7, and that's over 28 fuel-ups covering 7,305 miles
    - Truck in question is a 2005 4x4 SR5 Doublecab (short bed) with 38,229 miles at the start of the experiment, and the only mods that would affect aerodynamics are rain guard things on the windows, tube steps, and a tonneau cover (all installed by the previous owner.)
    - Engine is bone stock, save for new plugs installed at 30k, and a Goodyear Gator serpentine belt installed at 38k

    I ran the last tank of 91 octane (what I normally use) down as far as I dared, and this morning I deposited 16.904 gallons of 87 octane on top of it. I know that the residual 91 will *slightly* increase the overall octane rating of the fuel in the tank, but seriously - this "experiment" is quasi-scientific at best already. I'm not worried about confounds from a gallon of the good stuff.

    I'll update the thread with issues, noises, fuel-ups, and the like. My hope is that nothing explodes and I see only a minor drop in fuel economy. Worst-case scenario would involve my engine blowing up and me needing a tow from one of you guys.
     
  2. Jun 9, 2010 at 4:59 AM
    #2
    sooner07

    sooner07 1/2 man 1/2 amazing

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    Good luck man. I did a similar comparison with my truck. About 5 tanks of each grade. I found that my overall $/mi were slightly better with 91. But, the over all benefit was smaller than my driving style. If I were to accelerate a little more gently and was more cognizent of my speed, I could increase my 87 enough to make it better.

    Same could be done with the 91. I also noticed that with 91, the truck seemed a little more responsive.
     
  3. Jun 9, 2010 at 5:07 AM
    #3
    Taco-NB

    Taco-NB MMMMM Taco's

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    What is your hypothesis for this experiment?
     
  4. Jun 9, 2010 at 5:13 AM
    #4
    MaineYota

    MaineYota [OP] Well-Known Member

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    My hypothesis is that our engines are more octane-tolerant than most believe, and that the loss of fuel economy that people are worried about is negligent at best.

    I'm definitely not an engineer, mechanic, or anything of the sort...so the results I come up with will easily be dismissed, and I'm okay with that :D

    I do think, however, that unless you're running a crazy high-compression motor (which we aren't), there's no MPG or engine longevity benefit to running fuel above 87 octane.
     
  5. Jun 9, 2010 at 6:21 AM
    #5
    BRP27

    BRP27 When I grow up I want to be just like Me

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    You won't see much diff in mileage running diff grades of fuel. The energy in 93 oct and 87 oct is about the same. 93 has anti knock aditives that keep high performance engines that have the timing more advanced from knocking but the energy in the fuel is about the same.
     
  6. Jun 9, 2010 at 7:18 AM
    #6
    mjohn617

    mjohn617 Well-Known Member

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    Hey keep in mind your engine runs on an adaptive fuel map. Everytime you disconnect your battery you basically go back to the factory base map. Your engine will have a maximum allowed timing advance for each "quadrant" of your fuel/load map. 500 miles might not be enough for each grade if you decide to reset your ECU each fuel grade change. And with out reseting your computer your engine has kind of already set it's maximum allowable parameters so your just burning expensive fuel. It will begin to "test" itself and creap back up the advancement with higher grade octane but that also takes time.
     
  7. Jun 9, 2010 at 7:21 AM
    #7
    steve o 77

    steve o 77 braaap

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    Cool idea.

    The only reason I use mid grade 89 octane is in my state, it has no ethanol. At least I think it has no ethanol.
     
  8. Jun 9, 2010 at 7:39 AM
    #8
    kingston73

    kingston73 Well-Known Member

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    I don't know about NM, but here in all the new england states I'm pretty sure every grade is 10% ethanol, you can't get away from it.
     
  9. Jun 9, 2010 at 8:13 AM
    #9
    Lurkin

    Lurkin Well-Known Member

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    Given this then, his best method would be to run premium for 500-1000 miles, get his premium data, then start going down the octanes; 91 -> 89 -> 87. This would force the ECU to set the fuel/timing adaptives at each change since he's moving down. Other thought is to run a tank or two and be pretty hard on the accelerator, that way the ECU again is forced to setting the max timing given the knock sensor data for the fuel being used.
     
  10. Jun 9, 2010 at 8:32 AM
    #10
    OCTaco

    OCTaco Well-Known Member

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    I like where you're going with this. Keep us posted.
     
  11. Jun 9, 2010 at 9:01 AM
    #11
    BRP27

    BRP27 When I grow up I want to be just like Me

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    So far south in Texas my feet are in salt water
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    04 4X4 Tacoma SR5
    3 in Ultimate ToyTec, set @ 2.5 in. AAL PINCH Weld Mod ( beat the pinch weld over inside the fender) Painted the pinch weld on bottom of cab Black Hi Lift Knock off

    Same in Texas. The ethanol is the anti Knock replacement that proved to be toxic and leaking into the soil around gas stations
     
  12. Jun 9, 2010 at 9:05 AM
    #12
    MaineYota

    MaineYota [OP] Well-Known Member

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    I'm willing to change up the experiment if it makes good sense to do so. I know nothing about the black magic that takes place in the ECU, so I'll defer to others to tell me what and how to do this the smartest way possible.
     
  13. Jun 9, 2010 at 10:26 AM
    #13
    JKD

    JKD Well-Known Member

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    Contrary to 4th grade science teachers, not all experiments require an hypothesis.
     
  14. Jun 9, 2010 at 10:38 AM
    #14
    AeroCooper

    AeroCooper Half the strength of ten (microscopic men)

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    I use 87, and get 20.9 mpg consistently with mixed city/hwy driving.
    (V6 Access cab)
     
  15. Jun 9, 2010 at 11:51 AM
    #15
    Evil Monkey

    Evil Monkey There's an evil monkey in my truck

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    The energy may be the same but if your truck knocks, it'll retard the timing to prevent it. So it's conceivable that it will run better on an octane higher than 87.

    I remember reading an NHTSA bulletin about using premium that stated if your engine's compression is greater than 10:1 you should run premium and if it is less than 10:1, you should run regular. Our trucks are right at 10:1.
     
  16. Jun 9, 2010 at 11:53 AM
    #16
    Evil Monkey

    Evil Monkey There's an evil monkey in my truck

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    Ethanol or MTBE?
     
  17. Jun 9, 2010 at 12:04 PM
    #17
    MaineYota

    MaineYota [OP] Well-Known Member

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    Maine's definitely ethanol.
     
  18. Jun 9, 2010 at 12:09 PM
    #18
    Lurkin

    Lurkin Well-Known Member

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    MTBE has been outlawed everywhere I believe.
     
  19. Jun 9, 2010 at 12:11 PM
    #19
    Lurkin

    Lurkin Well-Known Member

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    Depends on the engine, if it has knock sensors it should adjust to a lower timing level if the fuel isn't sufficiently knock-proof.
     
  20. Jun 9, 2010 at 12:16 PM
    #20
    MaineYota

    MaineYota [OP] Well-Known Member

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    All our 2nd gens should have knock sensors, as they were mandated as part of the OBDII spec from '96 on.
     
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