Which octane level to use?
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2004-2007 Cadillac CTS-V General Discussion Discussion, Which octane level to use? in Cadillac CTS-V Series Forum - 2004 - 2007; The owners manual states that using 91 and higher octane is suggested but it also states in the same paragragh ...
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    iwinagn's Avatar
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    Which octane level to use?

    The owners manual states that using 91 and higher octane is suggested but it also states in the same paragragh that you can use 87 octane. What do you guys suggest? I ran 89 in it today no pinging at all.. I read somewhere though that 91 or higher can help with mpg is that true? Not that i really care about gas milage but every little bit helps right! On the expressway today i was running 70-75 and was getting around 20 mpg is that about right you think? It does only have 217 miles on it i assume it will get better after it breaks in some.. thanks for any future help!

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    Re: Which octane level to use?

    I would stick with premium (91 or above) an only use something else if you cant get premium....

    The fuel maps will change if you use lower octane and you will (I have on other cars) fell a performance difference between 91 and 87... Also if you have a tune, thats a big NO NO

    My opinion.

    Reed

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    Re: Which octane level to use?

    Quote Originally Posted by ctsvett View Post
    I would stick with premium (91 or above) an only use something else if you cant get premium....

    The fuel maps will change if you use lower octane and you will (I have on other cars) fell a performance difference between 91 and 87... Also if you have a tune, thats a big NO NO

    My opinion.

    Reed
    Reed
    thank you I will only use 91 or above from now on then.. I wasnt sure so i thought i had better find out from the guys that have been around these cars. I did search but came up with nothing.. How bout the mpg am i right that once it breaks in some i should see some increase on the eway? Thats where most of my driving is i try to keep it right around 70 mph and i was seeing about 19-20mpg

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    Re: Which octane level to use?

    always run the good stuff...your fuel maps will retard timing if you use the garbage and you wont be getting all the ponies from your V.


    F

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    Re: Which octane level to use?

    Using a lower octane fuel will, under certain circumstances such as higher throttle openings, cause your PCM to retard the timing to avoid knock. If the timing is retarded, your fuel economy and/or performance will be affected detrimentally. I just bite the bullet and run 93 all the time.

    For the purposes of comparison, as an old man, I've averaged 21 MPG overall since I bought the car over two years ago. (Somebody's gonna get a real creampuff from my estate sale someday. ) At a steady-state 70-75 MPH, I get about 23-24 MPG. BTW, I used to get 25-26 at that speed before I installed the headers -- apparently, the stock manifolds scavenge better at typical road-load conditions.
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    Re: Which octane level to use?

    That is very low for 70-75 mph but it is probably because your car is brand new. My mileage barely changed with the headers, if at all! At 75mph I will get 24-25 mpg with the air conditioning on! For the past 3 months my ave. mpg has been 20.4 and driving around town it is horrible to say the least!

    The octane thing is really a no brainer sir! Highest you can buy at normal pumps!

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    Re: Which octane level to use?

    I believe the PCM is actually smart enough not only to retard timing with knock, but also to advance it. I run 93 or 94 octane most of the time - whatever the maximum available is.

    On a dumb engine with no knock sensors and a dumb controller, you want to run the LOWEST octane that does not ping for power ind mileage. On a modern engine like the LSx, I'd go for the highest.

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    Re: Which octane level to use?

    Also, I don't believe that using a higher octane fuel will result in any significant difference in gas mileage

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    Re: Which octane level to use?

    Quote Originally Posted by Luna. View Post
    Also, I don't believe that using a higher octane fuel will result in any significant difference in gas mileage
    Correct!

    Octane is simply a measure of a fuel's resistance to knock; by itself, it has nothing to do with a fuel's energy content.

    The opposite of octane is cetane, a measure of a fuel's propensity to autoignite, which characterizes Diesel fuel.
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    Re: Which octane level to use?

    Higher octane allows more timing advance. Timing advance allows for more power per gallon, OR fewer gallons for a fixed power level.

    So at WOT, higher octane will not get you better mileage. But at a steady speed, the power required is constant, so higher octane *in concert with advanced timing* will actually burn less fuel, since the advanced timing is getting you more out of the fuel you are burning.

    On a car that cannot advance the timing to take advantage of the octane, higher octane actually robs you of a small amount of power and mpg.

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    Re: Which octane level to use?

    On the mpg question, in my Corvette I averaged 30.6 MPG with the cruise set on 85 mph from Rock Springs Wyoming to Salt Lake City using 93 octane. The highest I can get in CA is 91 octane and can only get about 28 mpg at 75. If I could get 93 octane here I would buy it.


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    Re: Which octane level to use?

    Quote Originally Posted by HiTechRV View Post
    Higher octane allows more timing advance. ...
    However, advancing the timing does not always mean more power.

    Ignition for gasoline piston engines is nearly always in terms of BTDC (before top dead center), in other words, before the piston has reached full compression. Advancing the timing too far causes the pressure resulting from the explosion in the cylinder to increase too early thereby reducing power.

    Let me repeat, octane by itself has nothing to do with the available energy in a given quantity of gasoline. (I used to work for Ethyl Corporation as a young engineer testing engines on engine dynomometers doing WOT, LBT, MBT fish hooks ... and we did this stuff all day long every day.)
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    Re: Which octane level to use?

    Rand speaks the truth!

    I recall reading several articles on the subject and they all echoed Rand's sentiment. One analogy I recall was, "Octane is like running shoes. If you wear a size 10, wearing a size 12 running shoe isn't going to make you sprint any faster."

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    Re: Which octane level to use?

    race fuel - daily driver

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    Re: Which octane level to use?

    use the good stuff

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