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.