Why higher octane gas is not usually better

Discussion in 'SN95 V6 Mustang Tech' started by ChrisYellowV6, Oct 28, 2004.

  1. I posted this in a reply to premium vs regular, but it seems nobody is viewing that thread so I thought i'd give it it's own.

    Here is a basic clarification of where octane comes from and how it is rated, and exactly why it is that there is no advantage to using a higher octane gasoline.

    Firstly, keep in mind that a gasoline’s octane rating is simply a measurement of the fuel’s ability to resist engine knocking. It does not refer to a substance or to the quantity of energy or power in the fuel. More correctly, an octane rating is often called an “anti-knock index”.

    The octane rating of gasoline tells you how much the fuel can be compressed before it spontaneously ignites. When gas spontaneously ignites, it causes knocking in the engine. Knocking can damage an engine, so it is not something you want to have happening. Lower-octane gas (like "regular" 87-octane gasoline) can handle the least amount of compression before igniting.

    To understand octane, you have to have a basic understanding of how fuels are refined. It is essentially a long string of hydrocarbons; every different fuel has a different number of hydrocarbons in the string. For example, methane is just a single hydrocarbon, propane has three, butane has four, pentane has five, hexane has six, heptane has seven, and octane has eight hydrocarbons in a chain.

    One thing the octane number tells you is what pressure it has to have to spontaneously combust, or explode without an ignition source…just pressure. The combustion due to the pressure and due to the spark plug is timed to be at the exact same split second, which makes a single even explosion in the chamber. Heptane explodes at a lot lower pressure than octane, and therefore it is mixed with octane to make gasoline that we use in our cars. The (R+M)/2 method is used to derive the octane number, however 87 octane also refers to a product that is approximately 87% octane and 13% heptane, or a mix that will produce the same reaction. Higher octane needs higher compression to explode it.

    The (R+M)/2 method is how it’s generally derived. The percentages of octane and heptane just happen to be how it’s mixed. The formula is as follows; RON (R), or Research Octane Number, over MON (M), or Motor Octane Number, divided by two. The RON is measured under fairly easy test conditions, while the RON is measured at higher engine speed and temperatures.

    Tetraethyl lead is a product discovered back around the WWII era. This was added to octane and hexane mixtures to decrease price. However lead is fatal to all forms of carbon based life (everything on the planet earth). This is still used in some airplanes that use a piston stroke engine (jet engines typically are burning kerosene).

    Now that you understand where the octane rating comes from, here’s something on engine knocking. Up until the 1985 mustang, carburetors were used to let air into the chamber, and they were very easily knocked out of alignment. So if air wasn’t getting in enough, this would cause a knocking with lower octane gas. Now that we have gone to fuel injectors, that’s where there air comes from…and it’s something extremely accurate that doesn’t need to be adjusted. So there is no need for a higher octane gas.

    However, with the lower compression engine, you have no need to use higher octane gas. You will not get a better mileage, because it is still compressing at the same rate, and it is not actually burning all the fuel. That’s why you may notice some perfectly fine unused gasoline dripping out of your exhaust pipes...however most of it turns into a gas state due to the heat.

    Hopefully that cleared a lot up.
  2. nice sig,

    ACLU=legal solutions to problems that do not exsist, I say live and let live. Whereas they live by the creedo I do not like it so nobody can do it.

    if Bush first tried attacking him, what were those missions that Clinton ordered in 98, damn I must have been pretending to be in Diego Garcia during those imaginary missions. Zarqawi has stated that he would like to see eff'n K elected.

    and while we are at it I believe that OBL is under a mountain in N. AF. (B-52 fodder)
  3. Nice post on octane stuff, at least I think. Too much to read for this old boy.

    Since Sleeper responded, I suppose I will too. The ACLU also = a bunch of Christian hating hypocrites who try to blame religion for all the worlds troubles. Frankly I am tired of being told that my being a Christian is wrong and thats exactly what's its turned into anymore.

    Quote from ACLU website...
    "The Ten Commandments advocate believing in God, observing the Sabbath and not worshipping idols," Friedman added, noting that different faiths have different versions of the document. "Those are religious beliefs that should be left to each individual. Especially in a courthouse, people should not be made to feel like outsiders in their own community because they may not share the prevailing religious view."

    Give me a break. What exactly does "One Nation Under God" mean then??? If you don't want to subscribe to the 10 commandments and Christianity, then don't. Its not that hard. These cry babies make it sound like someone walks up to them and shoves the bible down their throats every time they leave their homes. I am not forcing my Christian views on anyone, but don't try taking all of the symbols of MY religious freedom away from me either.
  4. You Got that right. Since when does the minority rule in a democratic society. I guess it's just who screams the loudest.
  5. oh i dont particularly agree with about 95% of the ACLU's foundation. I just hate bush. What most people don't like to admit is that in the year 2000, we first lost a constitutional right...the right to vote. George W. Bush was not elected the President, he was appointed by the supreme court. And he got appointed due only to the fact of the judges that his father appointed, one female justice (can't recall her name at the moment) even admitted that since his father got her her seat, she owed it to him.
  6. That's why I pray that whoever wins, whether its Bush, Kerry or Nader for that matter, that it is by a substantial margin; one that can't be disputed or left up to the courts to decide.
  7. sorry but you are wrong, bush won florida by 537 votes, even after gore had 1500+ military votes dissmissed. Hate bush all you want but do not spread your crap about losing rights. the electoral college was established to give a reason for people in states like wyoming and new hampshire a reason to vote.

    this is not the first time that the popular vote has not concurred with the presidential selection it was the 3rd.

    popular misconception is that you are voting for the president, you are not!!! you are voting to decide which way your delegates will vote.