87 vs. 91 octane

Discussion in 'Fiesta ST Chat and Discussion' started by MLKN, Apr 14, 2014.

  1. wash

    wash Active Member

    Octane rating has nothing to do with unburned fuel or carbon deposits.

    Also that test showing 5hp change does not tell the whole story. Since we see maximum boost down near our torque peak, that is where we are going to feel the difference the most.

    Higher octane gives you maximum power and efficient operation. If you drive for economy the efficiency pretty much offsets the cost premium and that means whenever you use it, that extra torque and horse power is essentially free.
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  3. quiznos

    quiznos Member

    More octane, more compression, more complete burn, less deposits and more power.

    More octane, same compression, no additional burn, more deposits, no additional power.

    What I failed to mention is that the additional buildup of deposits in very minor (and you can burn them off easily enough). Especially since this engine is optimized for higher octane fuel.

    But if you want to experiment yourself though take an older car with a low compression ratio, like a 1999 Saturn SL1, put in new spark plugs and run through 5 tanks of regular or premium. Change the plugs, do the same thing with the other fuel. And compare, you'll not only waste money for no power gain (because the engine only compresses so much), but the plugs will be darker on the premium. So you'll want to run something like gm top engine clean slightly more often, every 150k instead of 200k (not a big deal at all).

    Unlike the Saturn
    With our cars it would be pretty difficult to find at the pump gas with an octane rating high enough, compared to our high compression engines, to see/have any deposit difference between the available fuels.
    But you will see improved performance and fuel efficiency in the FiST, whereas with the Saturn, you'd just be wasting money.

    *I'm a more of science guy and not a mechanic. Sorry if I over do it. There are a lot of variables to this, but I was generalizing.

    Sent from my HTC One using Tapatalk
    Last edited: Apr 22, 2014
  4. quiznos

    quiznos Member

    Long story short. Use higher octane. There is no downsides worth worrying about in our car.
    over 120 (in a stock FiST engine), that would be dumb $ and will have some slight downsides.
    240 octane fuel would have apparent downsides, no upsides as compared to 120.

    VP race fuel has ranges from 87 to 118 octane (Mon) for recommended rally use. Their site (and presumably staff) can answer a lot of octane (and other fuel) questions.

    Sent from my HTC One using Tapatalk
  5. Neely2005

    Neely2005 Member

    So why does the Owners Manual say to use 87 Octane?
  6. BlueBomber

    BlueBomber Active Member

    Did you read that in the regular Fiesta manual or the ST supplement?
    Because what I read says that it will run on either but for best performance use premium fuel.
  7. Neely2005

    Neely2005 Member

    It's in regards to the Fiesta ST 1.6L Ecoboost Engine:

  8. wash

    wash Active Member

    I think I'll skip the Saturn...

    A low compression naturally aspirated engine vs. EcoBoost is not a valid comparison because the Saturn was designed for 87 and it probably couldn't advance the ignition enough to burn 91 clean. The EcoBoost is coming from the opposite direction and has more knobs to adjust for the fuel. Its always going to burn clean, output and fuel economy are the only things that change and 91 will beat 87 in both.

    If you drive 15,000 miles a year, the cost is about $1 a week and if your fuel economy is 5% better its essentially $0. I don't think its 5% So the actual cost is somewhere in between.
  9. MacDubois

    MacDubois New Member

    Do we know where the upper limit is?

    If 91 produces appreciably more horsepressures than 87, does 93 produce appreciably more than 91? Is the engine capable of taking advantage of 100 octane fuel?

    I assume there would be diminishing returns at octane levels above 91, but does anybody know how far we can go?

    Edit: Follow up question:
    Does the engine compensate in real time? If I have only run 87 and I switch to 93 on an empty tank, will the car immediately adjust or does it have a learning period?
  10. WScottCross

    WScottCross Well-Known Member

    It definitely takes a bit of time for the ECU to advance the timing. I don't know exactly how long, but I would imagine it would be settled after one tank.
  11. mkeperson

    mkeperson Member

    The Ford claimed horsepower figures clearly denote they were achieved on 93 octane. Whenever available, I would suggest 93 octane.
  12. wash

    wash Active Member

    Race gas with very high octane tend to have slightly lower energy density. If ethanol based, much lower.

    At some point maximum boost with full ignition advance and the TiVCT set for optimal cylinder filling could be achieved and then there is no adjustment left.

    We will probably find the limit of the fuel system before we get there.

    I ran some 98 octane and it felt particularly strong that day but at twice the cost of 91, it doesn't make $ense.

    If you live in a place where you can get a good deal on an unleaded avgas, that might work well if not exactly legal.
  13. reddog99

    reddog99 Active Member

    Based on my own experience, I'm afraid that I have to disagree with you regarding the statement that gas mileage is better with premium. At about 1,000 miles I changed from premium to regular and I have seen NO change in my mileage, and I track every tankful.

    I've got nothing against premium, and if you mash the gas pedal a lot, then premium is the only way to go. But 99% of my driving is freeway commuting and for me, premium is a waste of money, however little it may be. :cyclops:
  14. Neely2005

    Neely2005 Member

    Yes but then the Owners Manual and the Fiesta ST Insert both say to use 87.

    Talk about sending mixed messages.
  15. Neely2005

    Neely2005 Member

    This is from the Ford Fiesta ST Supplement:



    Regular unleaded gasoline with with a pump (R+M)/2 octane rating of 87 is recommended. 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. Premium fuel will provide improved performance and is recommended for severe duty usage such as trailer tow.


    So it says the exact same thing as the Owners Manual.
  16. Ca5p3r

    Ca5p3r Member

    It's basically like going to a grocery store and looking at getting Mac'n'Cheese.

    You have several random brands of mac'n'cheese available and Kraft mac'n'cheese. All of them are mac'n'cheese so any will be fine. But the Kraft probably will taste a little bit better.
  17. Neely2005

    Neely2005 Member

    That's a good question how much of an improvement would we see with 89, ,91, 93 or 94 Octane compared to 87 Octane?
  18. Slowclimb

    Slowclimb Active Member

    Back in 2004 I had a friend with a WRX who insisted on running it on 87. He was getting knocking (detonation), but thought it was no big deal. Eventually (at 55K) it resulted in the head gasket prematurely failing and getting coolant in the cylinders...ended up having to pay to replace the motor. I still wonder if he saved enough on cheap gas to offset the price of his motor. Generally on high compression or forced induction motors I figure you are better off "splurging" for the octane...the difference is probably nominal since the motor efficiency and mpg will be better with the premium gas anyway.
  19. Neely2005

    Neely2005 Member

    But what octane did your friends owners manual recommend? Our owners manual and our Fiesta ST supplement both say 87 octane. Also there is very little evidence to prove that higher octane will give you significantly better mileage.
    Last edited: Apr 25, 2014
  20. Slowclimb

    Slowclimb Active Member

    91...just saying...the savings doesn't seem worth it.

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  21. Neely2005

    Neely2005 Member

    Doesn't seem worth what? The owners manual and the Fiesta ST supplement both say 87.

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