Premium gasoline must be premium for a reason. After all, one of that adjective's definitions is "a high value or a value in excess of that normally or usually expected," according to Merriam-Webster's Collegiate Dictionary. Therefore, premium gasoline must be better, otherwise why would it be called premium? The answer to that question lies in the dynamics of the typical internal combustion engine, the process of refining gasoline from oil, and another definition of "premium"—this one from its noun form: "a sum over and above a regular price paid chiefly as an inducement or incentive."

First and foremost, premium gas really is a better fuel in terms of the power it provides in the right engine. All gasoline is a heady brew of many different hydrocarbon molecules, ranging from heptane (seven carbon atoms and 16 hydrogens) [see endnote] to decane (10 carbons and 22 hydrogens) and beyond. The hydrocarbon clearly identified on the pump is octane (eight carbon atoms and 18 hydrogens). This number, however, is not a measure of the percentage of octane actually in the gas itself. Rather, it is a measure of how that gasoline compares with a pure mixture of octane and heptane. At special laboratories across the globe, chemists concoct such reference fuels and then use them in comparison with refined gasoline following the dictates of standardized measures. "The American Society of Testing and Materials has this thick document on how you determine octane rating with this specialized one-cylinder engine," explains Joseph Shepherd, a mechanical engineer at the California Institute of Technology. "The higher the number the harder it is to have knock."

"Knock"—an unregulated explosion in a chamber designed for highly regulated combustion—is the bane of an internal combustion engine. During the four-stroke cycle of a typical car motor, the piston drops in the cylinder, allowing it to fill with a mixture of gasoline and air. The piston then moves up again, compressing the fuel mix and, when it reaches the top, the spark plug ignites the explosive vapor, driving the piston down again. As the piston returns to the top of the cylinder it expels what remains of the spent fuel out through the exhaust valves and the whole process starts again. Knock occurs when the compression of the fuel and air mixture alone, and not the spark plug, sets off an explosion. This results in a very loud noise and a lot of vibrations in the engine itself; "it's very bad for engines mechanically," Shepherd notes, driving the piston down before it has reached the top of its cycle. Each hydrocarbon molecule behaves differently under pressure, but octane resists the temptation to explode better than its volatile cousin heptane. "You rate the gasoline about how it knocks compared to this reference mixture," explains William Green, a chemist at the Massachusetts Institute of Technology. "One's that don't knock very much are the premium." That is, they behave in an engine as if they have a high proportion of octane, even if they don't.

Most modern cars, however, are designed to employ a specific compression ratio, a measure of how much room is available to the fuel when the piston is at the bottom and the top of the cylinder. This compression ratio—somewhere in the neighborhood of eight to one—tolerates lower octane fuels (such as regular gasoline, good old 87 octane) without knocking. "The compression ratio is fixed by the designer of the engine," Green says. "The regular fuel will burn properly and the premium fuel will burn properly and therefore there is no reason you should pay the extra money." High-performance engines, such as those in some sports cars or older, heavier automobiles, often boast much higher compression ratios. These cars—for example, Shepherd's Subaru WRX—require premium gasoline and will definitely knock without it. "I have to put the 92 octane in," he says. "It has a turbocharger."

6 Comments

1. 1. Billstecko 05:59 PM 6/5/08

   Since engine timing is governed by the Erasablble prom, where can I get a prom re-programmed to utilize regular gas on my 2001 3.6 Outback?

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2. 2. Saltyal in reply to Billstecko 07:42 PM 5/18/09

   If your engine has high compression ratio, requiring higher octane (slower burning fuel) then changing your ignition timing & fuel delivery with a different ECM & regular gas probably won't produce any benefit.
   
   this message uses 100% recycled electrons

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3. 3. ecstatist 10:37 AM 9/19/09

   OVERSIMPLE clarifications (rule of thumb)
   "premium" = self esteem felt with purchase of x.
   true value = miles (kilometers) per buck.
   -
   the denser the fuel (approx = average molecule size) the further you go per unit volume (gallon) = not greater than 0.5-1% per "octane" (compare consequent price increase)
   therefore lowest octane without knock = best value.
   alcohol = relatively lower density (but raises octane) "do the math"
   -
   combustion: best = predictable "slow" and complete burn allowing maximum useful energy to be extracted
   knock = explosion = high speed combustion due to burn caused by fast "(super?)sonic" high pressure wave front progressively igniting mixture.
   advertising marketing = extracting maximum bucks for minimum value! while making sucker feel good.
   
   

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4. 4. JonBirge 03:01 PM 5/14/10

   The definition of knocking is incorrect. I believe knocking is when the combustion front doesn't progress evenly from the point of ignition, such that pockets of fuel/air burn in advance of it, causing higher than normal peak pressures in the cylinder and essentially messing with the timing. This causes the "pinging" sound and rough running, but won't destroy an engine unless is happens for a long time. What the article describes--ignition due to compression alone--is actually called detonation, and it will destroy and engine very quickly. You won't get a pinging sound, but more of a loud diesel sound followed by the sound of your engine falling apart, I'm guessing.

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5. 5. arejay 11:58 AM 6/28/10

   I believe that a service station has been selling regular gas in place of premium and of course increasing their profit. How do I discriminate between premium and regular or have fuel tested to prove my suspicion. Ron

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6. 6. jimfromcanada 11:33 AM 3/7/11

   No, pinging IS preignition or detonation, and is destructive to engines.
   Modern engines generate a lot of power from small capacity by the use of the fine tuning that electronic ignition and fuel injection allows. So they are always operating on the edge of detonation when being pushed as in high speed highway driving, heavy acceleration, or hauling large loads.
   Having destroyed a couple of engines by using low octane fuels in such situations I always use at least mid grade when highway driving or towing a trailer. It is much cheaper than rebuilding an engine.

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