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fuel octane question

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  • #31
    Originally posted by samstone View Post
    Great thread all... I have Ethanol Free 91 Octane where I live... taking an informal poll here... how many of you would opt for that over 93 octane containing ethanol?
    I'll take the 93 octane. Pushing ethanol through a newer car doesn't bother me. Less chance of knock with 93 and therefore less chance of being bumped to the low octane timing table.
    Atomic Orange C6. Plenty of engine and suspension mods.

    C8 HTC with Z51 FE4. Bumped to a 2021.

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    • #32
      Originally posted by samstone View Post
      Great thread all... I have Ethanol Free 91 Octane where I live... taking an informal poll here... how many of you would opt for that over 93 octane containing ethanol?

      In a modern high compression car, I would go with the 93. The pump says "up to 10% ethanol." In practice, the 87 octane gets the highest actual percentage of ethanol. Premium usually get around 6% or less.

      If I had an older car from the pre-ethanol days, like a 1966 Chrysler Hemi, I would go with the 91 and add some octane boost. Those old cars had rubber parts in the fuel system that were never meant to be exposed full time to alcohol.

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      • #33
        I hate to admit it but I am one of those people who puts 87 in all my cars including my old Lexus and current Mercedes that call for high octane fuel. I thought low octane is bad for the engine only because of preignition and as long as I never heard knocking I wasn’t harming anything. I never thought about modern controls changing the timing to make it less efficient. Years ago I ran mileage tests with tanks of premium vs low octane and never noticed a mileage difference. I don’t think there is a BTU/gal difference. Am I missing something here? If the controls adapt to low octane I understand I could be getting less power but are there any other problems/damage using cheap fuel?
        Deposit Aug 2, 2020
        status 1100 Aug 29, 2020
        Sebring Orange HTC, 2LT, Natural Mulan Leather, Performance Exhaust. MRC. Carbon flash nacelles and roof. Battery maintenance package. Splash guards. My first corvette ever.

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        • #34
          Originally posted by Tgresla View Post
          If the controls adapt to low octane I understand I could be getting less power but are there any other problems/damage using cheap fuel?
          I don't have an answer to that specific question but I'm sure there are many folks on the forum that can answer.

          All I need to know is the OEM calls for high octane fuel and thats good enough for me. I tend to follow all OEM recommendations when it comes to my cars. Post #4 contains owner's manual guidance. "If the octane is less than 91, the engine could be damaged and the repairs would not be covered by the vehicle warranty."

          Last edited by Boomer; 09-15-2020, 07:13 AM.

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          • #35
            According to the manual on your New Corvette, Top Tier 93 is recommended for top performance, but 91 is ok but will not give you all the designed performance. Lower Octanes will cause damage to your engine and will void your warranty.
            Rapid Blue 3LT, HTO Tension Blue, Z51, 5ZZ, E60, ZZ3, ZYC, FE4, J6N, FA5, Q8Q, SHW, VJR, C2Q, R8C

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            • #36
              Originally posted by Tgresla View Post
              Am I missing something here? If the controls adapt to low octane I understand I could be getting less power but are there any other problems/damage using cheap fuel?
              Two things: The more minor one is that in addition to less power on 87 octane (in an engine designed for premium), you also get lower mpg. The more major one is that the car doesn't know you put regular in and permanently switch to the low octane settings. It switches to that map when it detects pinging. So under light load it's probably running on the normal map, and then when you push the gas peddle the engine starts to ping, and only then does the ECU trigger the low octane map. So you're constantly triggering engine knock before the map switches. Only minor knock in most situations, most likely, but it still can't be good for the engine.

              Now do the math on cost. Let's say where you live premium costs 50 cents a gallon more than regular, you get 20 mpg, and drive 10,000 miles per year. That means you use 500 gallons of gas a year. At a 50 cent upcharge, you're spending an extra $250 a year on gas. Is saving $250 a year worth taking any risk at all? For most Corvette owners, the actual mileage is going to be a lot lower. If one can't afford that, then one probably shouldn't be driving a Corvette.
              SunKissed, my 2015 2LT, 7MT, Black over Daytona Sunrise Orange Metallic, Stingray convertible (One of about 40)

              Purchased 5/2/2015,
              >36,000 miles

              Proud member of the Old Dominion Corvette Club. Check us out http://www.olddominioncorvetteclub.org

              Never grow up - It's a trap.

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              • #37
                So, adding some to this great discussion:

                First item - In the OBDII architecture, engine designs using high(er) compression and requiring and optimally tuned for high(er) octane fuels use two complete ignition timing tables. The first (often called the high octane table) has timing optimized to best utilize the prescribed octane (usually 91 or 93 US spec). This table covers the entire RPM range from 0 to past redline,7200, and a spark air mass from .28 to 1.36. Then the next table usually nicknamed the low octane table is optimized for around 85 octane. It too has a similar RPM and spark air mass range, details here not that important. These two tables provide the primary engine timing data; primary BC there are several sensor inputs at any moment that modify the primary timing cell value for any engine running condition.

                How does the EMS use these two tables? This is pretty complex. The fundamental sensor that can change ignition timing is the knock sensor. It can send a signal based on engine knock of varying severity. In fact, based on the severity of the knock the ems converts that to an equal amount of timing retard degrees. So, when the knock sensor activates, the EMS reduces timing in that particular timing cell by an equal amount. Then (and here is where it gets a little complicated) the EMS uses an algorithm that has a "learn factor" and a "time" (or expiration) element. If knock is NOT sensed again at that same timing cell within a given amount of time the ECU forgets the first event and resumes the high octane table value.

                For those who use low octane fuels all the time, tank after tank in a high compression engine, the numerous adjustments the ECU has made over a very short amount of time is now residing on the low octane timing table. So, what happens if the user now fills back up with 91/93 octane? This is a looong journey for the ECU to get back to the high octane table, It will meticulously test each time it arrives at another timing cell by raising timing slightly and NOT seeing any knock until the high octane table is reached again. How long does that take. On average about 50 to as much as 100 miles of driving.

                That's the short story.

                Second item - There are two adverse conditions that occur in IC engines. One is called Detonation, the other is called Pre-ignition. These are two very different things.
                Detonation - Detonation is a condition when the charge is ignited by the spark plug, instead of getting an expected burn that starts at the ignition point and propagates across the charge, it spontaneously ignites instead causing, in affect, a small explosion right at the transition point between compression and power. This is commonly known and sometimes felt as knock or engine knock. It is DETONATION that the EMS is designed to correct.

                Then there is pre-ignition. Pre-ignition is a condition where certain conditions exist in the combustion chamber that causes the charge to ignite before the spark plug can, so in effect, the engine has gone rogue and is jumping ahead of the timing we prescribe and is timing itself, but not in a good way at all. Pre-ignition is a condition that happened a lot in the old days but you hardly hear anymore about it BC something has to be in the combustion chamber that is acting like a spark plug. That is usually severe carbon buildups that, heat up over time and exceed normal cylinder temps on the intake/combustion stroke. Given the top tiers fuels with detergents and additives (such at Chevron's Techron) has all but eliminated pre-ignition.

                Third Item - Ethanol. Such bad press about ethanol, yet to a tuner, it's liquid gold. Ethanol is about 105 octane. Tuners who get really serious tuning big builds often where cylinder pressures and temps are very high and as in FIed engines, will often go to a flex fuel sensor so they can burn E85. The fuels that are found on in the markeplace today that are marked 10% ethanol are making up some of the octane rating with it. So a 93 octane fuel using a percentage of ethanol might be a 91octane fuel and the blend gives the extra two points.
                Last edited by Acta Non Verba; 09-15-2020, 07:59 PM.

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                • #38
                  Originally posted by Tgresla View Post
                  Am I missing something here? If the controls adapt to low octane I understand I could be getting less power but are there any other problems/damage using cheap fuel?
                  If by "cheap fuel" you mean lower octane, the risks have been explained. I don't know why people buy high performance cars and then do things to loose performance, but it is not my problem. If by "cheap fuel" you mean unbranded fuel, you may may not be getting all the additives that help car your injectors clean (and the intake ports and valves on older cars). If you keep the car for just a couple years, it's not really a problem for you; it's the next guy's problem.

                  Atomic Orange C6. Plenty of engine and suspension mods.

                  C8 HTC with Z51 FE4. Bumped to a 2021.

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                  • #39
                    As AAA has noted, Top Tier fuels results in better economy, so while sometimes more expensive it might even be worth it financially. Of course in addition to the good info/benefits Ragtop99 posted as shown here:

                    Click image for larger version

Name:	E4BEE31C-5E67-4D5A-B2D2-560C084C18DF.jpeg
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ID:	190688
                    GBA Black; HTO Twilight/Tension interior; Z51 & Mag Ride; E60 lift; 5VM visible carbon fiber package; 5ZZ high wing; FA5 interior vis CF; ZZ3 engine appearance; 3LT; Q8T Spectra Gray Tridents; J6N Edge Red Calipers; SNG Edge Red Hashmarks; VQK Splash Guards; RCC Edge Red engine cover; VJR illuminated sill plates. Lifetime, annual contributors, and 22 year members of National Corvette Museum. Home is the beautiful Pacific Northwest.

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                    • #40
                      Per Consumers Reports:

                      Originally posted by Consumer Reports
                      Gas Test Findings


                      For its test, the lab operated an engine continuously for 100 hours on a cycle to represent 4,000 real-miles of use. The engine was then disassembled, photographed, and its key components weighed and measured to determine the thickness of carbon deposits. Six fuels were used, randomly selected and split among three basic gasoline sources and three Top Tier.

                      The results showed that on average, Top Tier gasoline had 19 times fewer carbon deposits on injectors, intake valves, and in the combustion chamber when compared to regular gasoline.

                      AAA also found Top Tier gasoline can have a cleansing effect, reducing intake valve deposits by 45 to 72 percent when used over a 5,000-mile interval. Variation in the results is attributed to the detergents used by different brands.

                      Further, analyzing gas prices over a 12-month period found just a three cent price difference between non-Top Tier and Top Tier gasoline.

                      Bottom line: For the nominal investment, this study shows that motorists would benefit from using Top Tier gasoline as their primary fuel.
                      https://www.consumerreports.org/car-...h-extra-price/
                      GBA Black; HTO Twilight/Tension interior; Z51 & Mag Ride; E60 lift; 5VM visible carbon fiber package; 5ZZ high wing; FA5 interior vis CF; ZZ3 engine appearance; 3LT; Q8T Spectra Gray Tridents; J6N Edge Red Calipers; SNG Edge Red Hashmarks; VQK Splash Guards; RCC Edge Red engine cover; VJR illuminated sill plates. Lifetime, annual contributors, and 22 year members of National Corvette Museum. Home is the beautiful Pacific Northwest.

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                      • #41
                        Originally posted by Acta Non Verba View Post
                        So, adding some to this great discussion
                        Thank you for all that information. I learned something.
                        SunKissed, my 2015 2LT, 7MT, Black over Daytona Sunrise Orange Metallic, Stingray convertible (One of about 40)

                        Purchased 5/2/2015,
                        >36,000 miles

                        Proud member of the Old Dominion Corvette Club. Check us out http://www.olddominioncorvetteclub.org

                        Never grow up - It's a trap.

                        Comment


                        • #42
                          Originally posted by John View Post
                          As AAA has noted, Top Tier fuels results in better economy, so while sometimes more expensive it might even be worth it financially. Of course in addition to the good info/benefits Ragtop99 posted as shown here:

                          Click image for larger version

Name:	E4BEE31C-5E67-4D5A-B2D2-560C084C18DF.jpeg
Views:	67
Size:	29.6 KB
ID:	190688
                          Unfortunately, in a Direct Injection engine the fuel doesn't wash the backs of the valves, so there's no benefit to Top Tier in terms of deposits on valves. There ARE benefits in terms of build up on the tops of the pistons and probably the fuel injectors.
                          SunKissed, my 2015 2LT, 7MT, Black over Daytona Sunrise Orange Metallic, Stingray convertible (One of about 40)

                          Purchased 5/2/2015,
                          >36,000 miles

                          Proud member of the Old Dominion Corvette Club. Check us out http://www.olddominioncorvetteclub.org

                          Never grow up - It's a trap.

                          Comment


                          • #43
                            Instead of trying to save money by buying low octane fuel, I do four things. First, I keep an awareness of which stations have lower prices, with help from apps such as Gas Buddy and Gas Guru. When near the cheap stations, I fill up.

                            Second, I use a credit card with a rebate on gas purchases. Many cards have a 3% rebate.

                            Third, I participate in the gas rewards programs. I use the Shell, ExxonMobil and Irving programs, often earning extra bonuses on premium fuel.

                            And last, I look out for deals on gasoline gift cards. Last December, Irving, a competitively priced gas supplier in New England, had a special on gift cards, selling $100 gift cards for $90. I bought five of them for $450, and just used up the entire balance this month.

                            My late friend, the real Milliwatt Rob, gave me a mind set to squeeze the dollar. It can be done without sacrificing quality.

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