You may have heard gasoline with a high octane rating will make your car run better or faster, or that the octane number corresponds to the percentage of "good stuff" in your gas. You might just put in whatever your vehicle manufacturer recommends and just know about a price difference. Some people might have even noticed the octane ratings used to go to 93 at most pumps. This article is to clear up the myths and give you readers the facts about octane ratings.
Octane rating has to do with the speed at which fuel burns. Lower octane rating fuel ignites faster. Higher octane fuels take more energy to ignite. The number is the percentage of knock resistance as compared to iso-octane and heptane fuel i.e. 90 octane is comparable to 90% iso-octane, 10% heptane fuel. That is not what is necessary in your gas though. Lead used to be used because it cheaply increased knock resistance, but unleaded gas has alkanes, ethanol, aromatics and other additives in it. Numbers can go higher than 100 because there are chemicals more knock-resistant than iso-octane.
Engine knocking occurs when combustion occurs too early in the piston's stroke.It will result in broken rods, pistons or valves which is why you should only use the recommended octane rating.If you are thinking just use a higher octane fuel because of course you want high knock-resistance, than you are wrong because efficient cars use less energy to ignite fuel and therefore need a lower rating. Racing fuel like 110 octane is used so there is no premature combustion because of the high heat and load on the engine a low octane would blow an engine.
Air and fuel amounts are synchronized so the more air your engine gets the more fuel it will use and vice versa. That is why there are so many aftermarket products to force more air into your engine because your car will add more fuel to compensate and that will produce more combustion and therefore more engine revolutions and ultimately a faster car. It is the whole premise of turbochargers and superchargers.
At high elevations the minimum octane rating is 85 instead of 87 because an engine sucks in less air and therefore wants less fuel. A car that is normally filled with 87 can be filled at 85 at high altitude for better effiiciency but 85 will causes problems a lower elevations so remember to switch at high elevation. It is for this reason turbocharged and vehicles can't reach their full potential at high altitude which is why the Pike's Peak international hill climb is such a challenge.
Next time you pump gas look for " (R+M)/2 "on the yellow sticker with the octane number. In the United States the octane rating is determined by taking an average of the RON and MON numbers. Without getting too complicated the R means it was tested in a 600rpm engine and the M means it was tested in a 900 rpm engine and this number will be about 5 points lower in a countries outside the US and Canada for the exact same fuel. Any detergents added have nothing to do with the octane, all octanes would have the same ones and research has shown that additives advertised to clean your engine have the same effect as their competitors product so just pick your gas by the octane instead of the station.
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