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      11-29-2012, 07:10 AM   #17
Kozality
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Drives: 2013 335i M Sport w/xDrive
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Without breaking out the wiki,

Gasoline is a distilled fuel from hydrocarbons. There are a number of different compounds in the family of hydrocarbons. Any of these fuels will ignite with the right pressure/heat. High-octane fuel contains a higher percentage of these that are resistant to combustion at higher pressures. This will become important in a moment.

Remember that the fuel needs to be able to withstand the increasing pressure in the chamber as the piston travels upward and compresses the mixture. If it detonates early, then you don't get the full power stroke, because the piston is stil on the upstroke when the ignition occurs.

For most car engines, this high grade of fuel isn't necessary. The compression ratio is relativally mild, and so there's no danger of premature detonation; the piston will travel the full distance and the fuel will only ignite with the spark plug. In a high performance engine like the N55, the compression ratio is high, and the engine compresses the mixture higher to extract more energy. Because higher compression ratios are used, the grade of fuel needs to be high to allow for the engine to compress it further, and only detonate with sparked...drawing maximium power out of the stroke for which the engine was designed. Using lower grade fuel results in the scenario as stated above. Modern engines will compensate with their sensors and computer control, and so adjust the timing such that the sparks occur earlier, resulting in the power loss as others described earlier.

So, to recap: Higher octane fuel means it can withstand higher pressures. Same energy amount. For economy engines, it has no effect. On a performance car, it determines whether or not you obtain all of the power for which the engine was designed.

In any case, without going back to the book, I believe that's the rough idea. Hope this helps; my apoligies for reiterating some facts others posted before me. =)

Last edited by Kozality; 11-29-2012 at 07:12 AM.. Reason: Removed pkim1079's quote, I misread his post.
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