A great disseration on fuel ratings

[Spok]

a good write up from the other forum I read....

Technically octane definition is not resistance to burning or detonation, although that is a non-technical way of looking at it. For those who want a detailed explanation of what octane really is, I've got some time and a lot of experience with fuels, so I'll give a short class. Read on if interested.

When crude oil is cracked (distilled), you obtain a lot of different products from it. Methane consists of a single carbon atom (CH4). Ethane has two hydrocarbon atoms chained together (C2H6). Propane has three hydrocarbon atoms (C3H. Butain has four carbon atoms, Pentane has five. Hexane has six. Heptane has seven, and Octane has eight carbon atoms chained together. The larger the number of carbon atoms chained together, the less likely the fuel is to spontaneously combust, because it is harder to break the hydrocarbon chains apart.

Chemical compounds in the range of C5 & C6 are considered "napthas" and are used for things like dry cleaning fluid and paint thinner. C7 through C11are used for blending gasoline and performance fuels. C12 through C15 are your kerosene blends, including Jet fuels. Above C15 are your lubricating oils that can run at progressively higher temperatures without vaporizing or combusting. Standard 10W motor oil has a hydrocarbon chain with 100 carbon atoms, 20W has 200 carbon atoms.

Octane rating is the percentage of octane molecules in the fuel mixture. From a less than technical standpoint, a fuel with an 87 octane rating will consist of 87 percent octane and 13 percent heptane, or some other combination of hydrocarbon chains that has the same performance of the 87/13 combination of octane/heptane. I qualified that as "less than technical" because there are a variety of additives that are thrown into the mix that affect the mixture, but those are beyond the scope of this writing.

Now, if you remember your high school physics class, you will remember that a by product of the compression of a gas is heat. Heat is what breaks down the molecular chain, and causes the combustion. The higher the compression, the more heat is generated. You can cause 87 octane (rating) fuel to spontaneously combust by introducing it into a compression ratio as low as 10:1. You won't even need a spark plug. The fuel will ignite purely from the heat generated from the compression. This is what we experience when the timing is off on the motor and you get "run on" after killing the ignition. "Knocking" is the premature ignition of the fuel prior to the spark due to the timing set wrong so that the compression itself ignites the fuel prior to the spark igniting it. Both of those conditions are very detrimental to the engine and should be avoided at all costs.

If your bike or car knocks, first check your timing. If the timing is in spec, then try a higher octane fuel. Remember, higher octane means less suceptability to premature ignition. As long as your engine is stock and the timing is right, then run whatever the owners manual tells you to run. If you modify your engine (high lift cam, high dome pistons) you will change the compression ratio, and therefore will need to run a higher octane fuel to compensate for it.

Sorry for the long disertation, but I've learned a lot about fuels from my years of racing high compression bikes, and most of my learning has been from the seat of my pants by tearing motors up using the wrong fuels. A general rule of thumb I've developed is that if you're running a compression of 8:1 or less, go with 87 octane, up to 10:1 go with 92, above 10:1 use 93 until you hit about 12:1 and then go with specialty fuels. We run 105 octane rating(a mix of C8 and C12) in the 15:1 fuel bike.

[ispeed77]

is there a cliff notes version ??? hehehehe

[zedx11]

Here is another article on Octane. The only thing that I think he could have added about different fuels is the extra additives that go into the more expensive fuel that cleans the system from the tank to the valves.

What is "Octane"?

Written by Paul Huizenga

There is a lot of confusion about what the "octane number" on the gas pump means, what higher octane numbers really mean, and why some engines need or can take advantage of a higher octane fuel.


In a nutshell, the octane rating of a fuel is a measure of its ability to resist detonation, ping, pre-ignition, or knock. Although purists will quibble, all of these terms can be described as an incorrectly-timed burning of the fuel/air mixture in the combustion chamber. Any motor fuel will spontaneously ignite under the right conditions - too much compression, hot spots from leftover carbon deposits, excessive boost, too much ignition advance, or a number of other causes can provide the source of ignition. When the flame front from this ignition event meets the one created by the spark plug, the result is sky-high cylinder pressures and a destructive shock wave that can shatter spark plug insulators, hammer rod bearings, break rings, or even put a hole in the piston. The octane number is a way to express the resistance of a fuel to this type of unwanted ignition.



Octane Myth #1 - Higher octane fuel is "more powerful" and "burns hotter"

The difference in total energy between high octane and regular gas is insignificant. In fact, race gas often has slightly less energy by weight than pump gas! High octane fuels are generally harder to ignite, and burn more slowly than ones with low octane. In some cases, an engine optimized to run on 87 octane gas will produce less power when fed racing gasoline due to the slower ignition and combustion characteristics of the high octane fuel.

The number we most often associate with octane is the "Anti-Knock Index", or the "Pump Octane" number. This rating is an average of two different measurements; the Motor Octane Number, and the Research Octane Number. Both of these measurements are taken using a special single cylinder test engine that has a variable compression ratio. The RON measures the knock resistance of a fuel during low RPM, light load conditions, while the MON is representative of high-speed, high load operation. As a result the MON will always be lower than the RON, but for our purposes the Motor Octane Number is more significant because it represents the way racing engines are operated.



Octane Myth #2 - Putting premium and/or race gas in my car will give me better performance

Engines are designed to run on gas with a certain AKI. Putting premium in a car that was designed to run on regular gas and doesn't knock is just throwing money away. If an engine doesn't knock on 87 octane gas, there is no advantage to using gasoline with a higher octane number. However, many engines with electronic controls have a knock sensor as part of their computer system. This sensor actually listens for knock, and can change the engine timing or even cut boost in a factory turbocharged car when it detects knock. Some anti-knock systems are sensitive enough that it's possible for them to reduce the performance of the engine without you ever hearing audible detonation. The bottom line - Read your owner's manual to determine the correct octane rating for your car and see if it can take advantage of premium fuel.

Obviously, an engine's octane requirement can change. Increasing the compression ratio, adding a turbo or supercharger, changing the air/fuel ratio, or advancing the ignition timing can all cause knock. Even without modifications, over time combustion chamber deposits build up and can lead to a need for higher octane fuel. The best way to determine what your engine needs is to listen to what it's telling you - Audible knock is a sign that damage to the engine is imminent! Better still are scan tools that can read the engine control computer's built-in knock sensor, or a stand-alone system that detects detonation. There's also the old fashioned but most accurate method is reading your plugs for signs of detonation. Whatever the case, it's worth the time to determine what your engine's real octane needs are. Too little, and you flirt with engine damage. Too much, and you're wasting money and quite possibly reducing performance.



Octane Myth #3 - My brother went to high school with this guy who had a Chevelle, and he went to the airport and filled it up with jet fuel. That gas was so high octane, it blew up the motor!

While aviation gasoline is indeed higher in octane than gas destined for street use, jet fuel is actually kerosene, which has an extremely low octane number. Jet engines have absolutely no use for high octane fuel, since their combustion process is totally different from that of gasoline engines. Putting jet fuel in an internal combustion engine is a sure way to destroy it immediately - not because the jet fuel is "too hot", but because it has no knock-suppressing capability. Incidentally, aviation gas also has a lot of tetraethyl lead in it. Despite the fact that the pump is labeled "100 Octane LL (low lead)" it still has more lead in it than the worst 1960's automotive gas. Running this through a car with an oxygen sensor or catalytic converter is sure death for those parts.

So what is the correct octane rating for your car? There's a delicate balance between many factors that determines how much knock suppression you need. Simply adding higher octane gasoline can keep your car from knocking, but it may mask other problems like an incorrect fuel/air ratio, ignition system trouble, or too much boost. Ideally, the car's state of tune should work in conjunction with the proper fuel, rather than using high octane gas as a band-aid to cover up some other problem. It's often cheaper in the long run to determine the root cause of knock instead of switching to a more expensive high-octane fuel. On the other hand, tuning an engine for maximum power will often demand the added knock resistance of higher octane gas. Only by understanding how and why knock occurs, and the interaction between ignition, compression, fuel delivery, and the fuel itself can you reach the state of optimum performance.


http://www.techweasel.com/articles/octane.htm

[Tod]

Good info, thanks.

[CowboyBob]

Very good info, and it just goes to show....the best choice is what MOM (Manuacturers Owners Manual) says to do. And not to open up another can of worms, this also goes for engine oils also. The engineers who designed this stuff, utilized certain paramaters to come to the octane and oil viscosity. It is always the safer bet. Unless you are deviating due to some extreme conditions....such as racing in a closed course competition, or heavily modifying your motor by doing head work, raising compression or high lift cams, what MOM recommends will work. If your spending extra on some other product that tells you it will do xy or z better, I got a big chunk of ocean front property in Arizona and for a large $um of cash it can be yours!!!