Re: 87 octane ??
Just to add my 2c.
All gasoline is a MIXTURE of various length short and long chain hydrocarbons. This means the backbone of the chain is Carbon atoms linked together like a pearl necklace and they are surrounded by Hydrogen atoms. Shorter chain hydrocarbons are usually more volatile than longer chain molecules. This means they vaporize easier and quicker. However, volatility and flash point varies with each specific molecule and is not necessarily linear.
The refinery tailors the mix between shorter chain and longer chain molecules depending upon the areas of the country and the weather where it will be used. So, for example, here on the Northeast coast, there are more short chain hydrocarbons in the mix during the winter for easier starting and smoother warm-up. The refinery also tailors the mix so the Octane rating is constant. Shorter chain hydrocarbons have less carbon and produce less BTU - British Thermal Units - per pound, so would produce slightly less power, only, because the air is colder and denser, it is offset and you don't notice any power loss--indeed, the colder air charge may produce slightly more power. (near sea level)
Ever notice how your car seems to run better during a Summer rainstorm? The rain cools the air, the increased humidity smooths and slows the burn, and the extra humidity generates more steam for a higher mean pressure. Net effect? Slightly more horsepower and smoother operation. But only if you are an astute driver who can recognize the subtle difference.
Now--way back when God was deciding where to put the dirt, the SAE --Society Of Automotive Engineers-- arbitrarily decided to set the octane rating of N OCTANE (Normal Octane--oct= 8 carbon atoms --and 18 hydrogen atoms in a molecule) to a round figure--100
--Methane, Ethane, Butane, Propane, Pentane, Hexane, Heptane, Octane, etc, on up to 100s of carbons. I believe that they start becoming liquids with Pentane.
Back then, they didn't have computers or sophisticated equipment, so they used a variable compression engine to measure the point at which a given fuel started to pre-ignite. Comparing the compression ratio to the compression ratio of Octane gave an octane rating. So, the octane rating is a measure of how well a given fuel compares to Octane in regards to pre-ignition or resistance to "knock" only. Simplisticly speaking, it has nothing to do with power.
87 octane gasoline has 87% of the resistance to "knock" as Octane and will run quite well in an engine of 9 to 1 compression ratio. Above 10 to 1 higher octane fuel is required. In new engines the computer regulates fuel injectors, timing, and sometimes valve opening , so slightly higher compression ratios can be tolerated with 87 octane fuel.
High octane fuels tend to have a slower flame front propagation and pressure rise within the cylinder is slower. Peak pressures are not quite as high so temperature is not quite as high and will not "diesel" or auto ignite. High octane rated fuels are more difficult and expensive to refine though and you get less of them from a barrel of crude oil.
Now, even though the flame front is slower and pressure not quite as high, an engine REQUIRING high octane rated fuel (automotive, not aircraft) will have a higher compression ratio. This shifts the BMEP (Base Mean Effective Pressure) upward on the horsepower graph and the engine produces more horsepower.
Some piston engine aircraft require high octane because after take-off, a supercharger is engaged, and of course, supercharging increases the volume packed into the cylinder and effectively raises the compression ratio. Coupled with the fact that a nine inch diameter piston having two opposing spark plugs will have two flame fronts colliding, aviation fuel must be premium quality. This is one reason why jet engines came to dominate commercial aviation--fuel is much cheaper. --and jets are WAY more efficient at high altitudes--but that's another story.
