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fuel octane question

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  • #46
    Originally posted by Tgresla View Post
    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?
    My 2016 C7 was fine with 87 octane, in normal driving. I found no difference in mpg, even for long trips on the interstate going 75-80 mpg. I suspect 87 is also ok for a C8 in normal street driving, where you never use more than about 1/3 throttle, but so far, I have used premium in mine. For "performance" driving involving full throttle, I'm sure 93 octane, or at least 91 should be used. I make it a point to use Top Tier gas in any car with direct injection, whether a Camry or Corvette This is especially important when using regular, because the Top Tier additives DGI engines need to prevent carbon buildup on the valves are in all grades.

    Regarding BTU/gallon, I've heard that regular gas is slightly higher than premium, but that may not be the case. Of course, E-0 has about 3% higher BTU/gallon than E-10.
    2022 LT2, Elkhart Lake Blue, Natural
    Ordered September 2020, Delivered, October 2021

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    • #47
      Originally posted by Kit Gerhart View Post

      My 2016 C7 was fine with 87 octane, in normal driving. I found no difference in mpg, even for long trips on the interstate going 75-80 mpg. I suspect 87 is also ok for a C8 in normal street driving, where you never use more than about 1/3 throttle, but so far, I have used premium in mine. For "performance" driving involving full throttle, I'm sure 93 octane, or at least 91 should be used. I make it a point to use Top Tier gas in any car with direct injection, whether a Camry or Corvette This is especially important when using regular, because the Top Tier additives DGI engines need to prevent carbon buildup on the valves are in all grades.

      Regarding BTU/gallon, I've heard that regular gas is slightly higher than premium, but that may not be the case. Of course, E-0 has about 3% higher BTU/gallon than E-10.
      As I understand the carbon build up it is usually caused by the PCV system in DI engines. Unlike port injection fuel does not wash the back of the valves keeping them clean. It will be interesting to see what these valves look like after 50 or 100K miles. Then again maybe the PCV system is much better then other DI engines that have carbon build up issues. Time will tell.

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      • #48
        Originally posted by jagtoes View Post

        As I understand the carbon build up it is usually caused by the PCV system in DI engines. Unlike port injection fuel does not wash the back of the valves keeping them clean. It will be interesting to see what these valves look like after 50 or 100K miles. Then again maybe the PCV system is much better then other DI engines that have carbon build up issues. Time will tell.
        We do have that data. C7’s have been direct injected since 2014.
        I haven’t really heard of any carbon build up issues.

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        • #49
          93 Octane almost $4.00 a gallon here in North Texas.

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          • #50
            Originally posted by Buzzsaw View Post
            93 Octane almost $4.00 a gallon here in North Texas.
            Lucky.... it's about $4.50 gal in so cal.
            2021 2LT HTC, front lift, MRCS, GT2 natural Napa 2 tone seats, Perf. exh and all wrapped in Red Mist metallic, lowered w/Paragon collars and riding on BC Forged wheels 😎
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            • #51
              re: 87 octane, a couple of thoughts

              Assuming that because you can't hear pinging everything is OK may not be accurate on modern cars with knock sensors. The ECU will retard the timing when it detects incipient knock before you can hear it. That prevents damage to the engine, but it's definitely not running optimally. In low load, moderate temperature driving 87 might be fine, bnd when you find yourself in a high heat / high load situation in the middle of a tank, the car may not be able to retard timing enough to compensate, and incipient knock turns into real knock, which could cause damage very quickly.

              If premium costs 50 cents a gallon more than 87, you drive 10,000 miles per year, and average 15 mpg, you're going to pay an extra $333 dollars a year for gas. Is the risk, no matter how small, worth saving 300 bucks? (Given that most of us put far less than 10K miles a year on, the savings will be even less.) After spending $60K or more on a car, I'm willing to spend a little extra to feed it what it wants. I wouldn't spend a few thousand dollars on a purebred pup and feed it the cheapest pet food I can find at WalMart just to save a couple of bucks. (Full disclosure: I wouldn't feed a mutt from the pound the cheapest pet food, either.)

              The electronics in the car almost certainly record data from the knock sensors. If you have engine damage, and the black box says the ECU retarded timing as far as it could and the engine was still knocking, GM is probably going to deny any warranty coverage. That could cost a lot more than $300.
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              • #52
                meyerweb , an excellent summation on an important topic.
                Rapid Blue 3LT, HTO Tension Blue, Z51, 5ZZ, E60, ZZ3, ZYC, FE4, J6N, FA5, Q8Q, SHW, VJR, C2Q, R8C

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