High Octane Fuel vs. Low Octane Fuel : Facts and Myths

Discussion in '80-Series Tech' started by BrettinSanAntonio, Jun 11, 2007.

  1. Several of us 80 series owners have been debating the use of premium vs. regular fuel and the merits of high vs. low octane fuel. Instead of high jacking the original thread where this discussion broke out, I thought we’d start a new one.

    My stand is this: in a properly tuned and functioning 80 series truck, higher octane or premium fuel is a waste of money and provides no additional performance (power and mpg). In fact, it may mean a slight reduction in performance.

    I’ve listed a just a FEW of the websites that I came across providing information on my position. With the facts to follow these links.




    Octane Facts
    • Knock occurs when cylinder pressures are high. It is normal for an engine to ping a little at full throttle
    because cylinder pressures are very high at full throttle. Engine knock, however, should not be ignored
    since it can result in serious damage to the engine.

    • High octane gasoline burns slower than low octane gasoline. The slow burn prevents engine knock when
    cylinder pressures are high.

    • If your engine runs well and does not knock or ping on low octane gasoline, there is no advantage in
    switching to higher octane gasoline.

    • If your engine knocks or pings, it does not necessarily mean something is wrong with the gasoline. It could
    be a problem with the engine’s electronic control systems, ignition timing or exhaust gas recirculation. On a
    high mileage engine, a carbon build-up in the cylinders can increase cylinder pressures and cause knock.

    • Almost all of today’s new cars have fuel-injected engines that need to use gasoline with a detergent additive.
    They do not necessarily need high octane gasoline with a detergent additive. Generally, new automobiles
    need high octane gasoline only if the manufacturer recommends it.

    • Always follow the auto manufacturer’s octane recommendations in your owner’s manual.

    Octane Myths

    High octane gasoline improves mileage.
    In general, if your car is designed to run on 87 octane gasoline, high octane gasoline will not improve
    mileage. If switching to high octane gasoline does improve mileage, you might find that your engine, or its
    control systems, need repair.

    High octane gasoline gives quicker starting.
    No, it doesn’t.

    High octane gasoline increases power.
    If your car is designed to run on 87 octane gasoline, you shouldn’t notice any more power on high octane
    gasoline. Again, if it does make a noticeable difference, your engine, or the engine’s electronic control
    systems, may need repair.

    High octane gasoline has been refined more – it is just a better product.
    Additional refining steps are used to increase the octane; however, these additional steps do not necessarily
    make the gasoline a “better” product for all engines. They just yield a different blend of hydrocarbons that
    burn more slowly. The additional steps also increase the price.
  2. Romer

    Romer fatherofdaughterofromer Moderator

    Centennial, Colorado
    Good data Brett. 3 of the 4 trucks I have run on midgrade which is 87 Octane. Mine has a supercharger and the manufacturer recommends Premium. However, I know folks who run midgrade with no issues. I have run a couple of tanks with no issues as well.

    Still run Premium most of the time due to the supercharger.
  3. Yes, I came very close to adding the supercharger but a buddy beat me to the sale. I was told once you add the supercharger you must run premium. Pretty cool that you've been able to get away with mid grade. Hey, if it's not knocking, go with the lowest possible.

  4. Here's a question.

    Being that higher octane fuel burns slower than low octane fuel; does that mean it releases less BTU? Is there less energy in high octane fuel, thus providing fewer m.p.g. in a normal funtioning engine? My thought has always been, "yes".

    Is it possible for a slower burning fuel to contain the same BTU's as a fast burning fuel?
  5. jonheld


    New Jersey
    Thanks for the 411 Brett. Good stuff.

    FWIW I have run 87 in my 3FE since new. Purrs like a kitten caught in a sewing machine.
  6. '74 UA FJ

    '74 UA FJ

    There is slightly fewer BTUs of energy in premium gas than in regular. This does not corrolate to the speed of combustion for other fuels though. Diesel fuel burns much slower than gasoline but has a higher BTU content.
  7. Ivan80


    Omaha, NE.
    It has always been my understanding that higher octane fuel is slower to ignite, thus less prone to pre-detonation (detonation from compression, rather than spark). Which is why it is required in high compression ratio scenarios (which would explain the need to run it in forced induction 1FZ-FEs).

    This also begs the question, if you advance the timing on your engine, then run higher octane (which ignites and burns slower), are you not, in effect, chemically retarding the combustion.

    FWIW, I run 87 octane with advanced timing.
  8. I wonder if the higher BTU’s in diesel and significantly higher compression accounts for the increased efficiency of diesel engines. My thought would be “yes”. On an interesting side note, I own an early 80’s Mercedes Diesel that I often run Bio Diesel and SVO (straight vegetable oil) on occasion. In our hot south, Texas summers I’ll run the SVO without any modifications and run the bio diesel in the cooler months. In both applications my engine runs smoother and quieter but looses almost 2 mpg on the non petro-diesel. A BTU difference between petrol and bio products?

    My LX450 has always run of regular, 87 Octane and never had an issue until recently. I’ve noticed a very subtle, pinging on hard accelerations. Either I didn’t notice it before or something is out of adjustment.

  9. alvarorb

    alvarorb Color Geek in Charge

    Sacramento, California
    Another interesting tidbit: As altitude rises, you can use lower octane fuels.
    In many parts of the world 83 Octane is the lower octane gas available. My old FJ40 used to run on 83 Octane fuel.


  10. knorrena


    So what differentiates knock from ping? I always thought that these are the same.

  11. Romer

    Romer fatherofdaughterofromer Moderator

    Centennial, Colorado
    Knocking sounds like someone repeatedly rapping the engine with a hammer, and the quieter pinging resembles marbles being shaken inside a tin can.

    From a google search

  12. First, I am not disagreeing with any of the information you posted. I have no expertise or knowledge in this area other than I consume vast quantities of petrol.

    We have a conundrum here.
    * The LX450 Owner's Manual calls for premium fuel.
    * The Toyota Land Cruiser Owner's Manual calls for regular fuel.
    * We all know the engines are identical as is the complete drive train (96 & 97 US models)

    What do we make of this?

    Edited. Above information is not correct. See post #21.

    Last edited: Jun 11, 2007
  13. Grench

    Grench SILVER Star

    Omaha, NE
    Lexus owners feel better about their purchase the more they have to spend? There is a status feeling to running more expensive gas.

    Here 90 octane ethanol mid grade is the cheapest so I run it the most. It is 87 gas mixed with 10% ETOH to give 89-90.

    87 isn't a problem, just more expensive. I've run 92 octane before when I thought I had fuel issues just to eliminate fuel quality from the possibilities.

    I think the Lexus Premium / FZJ80 Regular difference is more marketing than fact.

  14. CJF


    It's Doug's fault.

    I'm not kidding; look:


  15. Hmmmm...I just checked my owners manual, '97 LX450 and it clearly states that Lexus recommends 87 Octane fuel for my truck. I couldn't find anything in the owners manual recommending premium fuel. Are you sure you're not looking at the 1998 model LX470? I know the requirement changed when they went to a V8.

  16. dixie_cragger


    Ditto. My Pink Panty manual only recommends 87 octane for my truck too :)
  17. I'm not sure how this result came to be.

    I looked over the various websites to see what makes gas "high octane" vs. "low octane". They said nothing about detergents or cleaners but stated octane is a chemical component of the fuel and adding more of it only effects how easy the fuel/air mixture ignites. Low and High octane fuel should have the exact same amount of detergents and clean the same.

  18. CJF


    From Chevron's site:

    5. I have seen your advertisements that say your Supreme gasoline has "more Techron®." What does the higher dose of Techron® do for Supreme customers? The higher dose of the Techron® additive in Chevron Supreme can clean up dirty carburetors, fuel injectors, and intake valves even faster than our Regular and Plus grades. This attribute can be important for customers who occasionally purchase a lower quality gasoline, or who own a vehicle that is particularly sensitive to deposits.
  19. '74 UA FJ

    '74 UA FJ

    Yes, petroleum diesel has a higher energy content than biodiesel. This explains the decrease in power and mileage when switching to biodiesel. Petrodiesel is also better for engine parts.
  20. CJF


    From Shell's site:

    Shell Regular and Plus gasolines, which meet the “TOP TIER” standard, also contain more than two times the amount of cleaning agents required by the EPA. Shell V-Power goes even further – it has more than five times the minimum amount of cleaning agents required by government standards and twice the cleaning agents required by the “TOP TIER” standard.

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