What the Hell is Octane?!
Mar 16 '00 (Updated Mar 17 '00)
What the hell is an octane number (rating) you ask? And why does Premium gasoline have a higher number than Regular gasoline? And why the hell should I pay $.20 a gallon more for Premium?
The issue of Premium vs. Regular gasoline is one of those mythical, anecdotal, ethereal subjects that refuses to come out of the dark and into the light so that all may know and understand...
Let there be light:
Octane n. Chemistry Any of 18 isomeric saturated hydrocarbons having the formula C8H18, some of which are obtained in the distillation of petroleum.
Octane Number (of gasoline) A designation of anti-knock quality, numerically equal to the percentage of isooctane by volume...blah, blah, blah, blah and more blah.
The Random House College Dictionary - Revised Edition
What the above stuff means is that an octane number (or octane rating) is the measure of gasoline's resistance to "knock" at higher temperatures. Knock is when part of the gasoline and air mixture compressed in the cylinders of your car's engine ignites before it should (before the spark plug fires) because it is hot enough to self-ignite. You may have experienced knock as the "pinging" or "knocking" sound you heard coming from your engine when operating under a very heavy load (very hard acceleration) on a hot day. Knock can lead to preignition and even detonation in extremely severe circumstances. These three terms (knock, preignition, detonation) describe a progressively worse condition that may ultimately result in severe engine damage. In cars, knock is about as far as the problem progresses before the owner takes some corrective action. Premium has a higher octane number (e.g. 92) and is more resistant to knock than Regular with a lower octane number (e.g. 87).
Auto manufacturers design the engines in their cars to operate best with a specific octane number gasoline. Most are designed to run best with Regular. You should use Regular if the manufacturer recommends it for your car. In fact, if you use Premium instead of the manufacturer recommended Regular in a modern car, you may actually reduce your car engine performance. The reason is the car engine computer fuel-mapping and spark-advance-curves are designed for Regular, and using Premium causes it to adjust fuel-injection and spark-timing out of their normal operating range. If you have a newer car that is well maintained, it should work best with exactly what the manufacturer recommends.
So, why do some older cars that used to work well with Regular now work better with Premium? As cars age, and depending upon how well they are maintained, their engines start to run hotter because of various factors like deposits and fouling in the engine cooling system, build up of deposits and corrosion in the engine air intake system and on the exterior of the engine, and carbon deposits and buildup on the exhaust valves and exhaust system, etc. All of these combined cause the engine to run hotter than when it was new. And because it now runs hotter, the gasoline and air mixture compressed in the engine cylinders heats up greater allowing knock to occur. One solution is to clean the air intake system, clean the exhaust system, clean the exterior of the engine (carefully), flush out/chemically clean and refill the coolant system, and give it a tune-up. Or, you could switch to Premium from Regular because as you recall from above, Premium resists knock better than Regular.
If you have an older car that was designed to run on Regular (87 octane) gasoline and now pings and knocks, you should clean it, give it a tune up, and then first try the Medium grade of gasoline before trying Premium. Keep in mind that what was once called "Regular" gasoline 20 or 30 years ago is quite different from what is Regular today. Back then "Regular" gasoline had lead added to it and probably had a higher octane number than Regular does today. So, your 1975 car that ran on "Regular" in the 70s and 80s may need Premium today.
So, why do most high-performance cars and luxury-cars require Premium? High-performance cars typically have engines with higher compression ratios that need Premium to prevent knock. Manufacturers of these cars are trying to get more horsepower and performance from their engines so they design them with higher compression ratios. Higher compression ratios result in compressing the gasoline and air mixture more, heating it up further leading to knock unless Premium is used. Also, manufacturers know the owners of very expensive cars will think that Premium is "better" and therefore tend to put Premium in the car. So, they design it to use what most luxury car owners would use anyway. Isn't marketing and the human psyche amazing...
The extreme end of the high-performance engine spectrum is of course racing. Racing engines have very high compression ratios, or are turbo-charged or supercharged and require very high octane (e.g. 104 to 116) racing gasoline. A racing engine designed for very high octane racing gasoline would experience knock then preignition and finally detonation if it were to run on Regular or even Premium. This would most likely destroy the racing engine in as little as a few minutes! Of course they would hear the knock and experience a dramatic loss of power and stop the engine before preignition and detonation occurred. A normal car designed for Regular would have a difficult time running on racing gasoline, if it ran at all.
What about additives and detergents that are supposed to be in Premium? The additives and detergents package that are added to gasoline are the same in all grades of gasoline (Regular, Medium and Premium) from most reputable gasoline manufacturers. However, in some areas there may be some manufacturers who try to save some money by putting in less additives and detergents in all of their gasoline or just their lower grades. The only way to know is to test the gasoline. You will have to check with the various state regulatory or private watchdog agencies who audit this. I have not checked, and simply rely on using gasoline from any of the major brands. In California there are only 6 gasoline manufactures (not the number of refineries, but the number of companies that have refineries). They all tend to just keep up with each other. The rest is marketing hype...
Bottom Line:
Buy Regular or Premium as the car manufacturer recommends.
If your car is older and pings, give it a thorough tune up, then try Medium before trying Premium.
Happy motoring! ;-)
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