® Octane

Let's explore the subject of gasoline octane, one of the topics on every user forum's block list. Should be fun. It will definitely be educational.

Octane's origin
The word "octane" is actually a contraction, a word made from two words. It comes from the phrase "iso-octane," a typically scientific (and Greek) description of a test fuel that is "equal to optimum". When determining a fuel's octane, a tester begins by using a test engine and two test fuels (really just chemicals). He already knows how the test engine runs on a piss-poor test fuel called heptane and also how it runs on a really good one called iso-octane. He also already knows how this same engine responds to mixes of the two fuels together in varying percentages. The ratio just before the point that makes the engine run like crap is named after it's iso-octaine percentage. It might be 80/20 iso-octane/heptane, for example. This is a research percentage of 80. So then a production gasoline whose traits mimick those of an 80 percent iso-octane test fuel mix would be labeled 80 octane. (Numbers over 100 are determined in a completely different way, such as with aviation fuel, which actually has two numbers, for low and high supercharger boost.)

But, since it is lab-determined, the octane label is not real-world, and thus it is qualified as "research" octane. That 80 is a white-coat number, in other words. Somewhat ivory tower. The scientists in the field do their own, similar tests, in actual running vehicles having temperatures, loads, wear and who knows what-all mixing things up, and not surprisingly, the lab guys' ratings end up lower than those of the lab guys. The two groups then have to compromise and this is why you see the familiar "R+M/2=P" (Research / Motor = Pump) sticker on every gas pump in the U.S., as well as in most owner's manuals. (By the way, manufacturer's publications sometimes make the mistake of not qualifying whether their octane recommendations are Research or Pump, and sometimes even knowingly list Reseach numbers, so watch out for that. Logically and sensibly, all octane specifications should be the lower and more realistic Pump specifications.)

Octane's job
But what does octane do? To answer that, let's first look at combustion. Combustion is not an explosion. Far from it. The heat of ignition's spark ignites the air/fuel molecules within the plug gap, and this burning kernal of flame grows gradually outward in a concentric fashion until it reaches the ends of the combution chamber. Pressure builds at some 25 psi per crankshaft degree, reaching peak pressure after a period of time that is hundreds times as long as dynamite's. Thus an engine's combustion is not explosion, despite widely used verbiage to the contrary. In fact, so slow is combustion that it needs a pretty significant head start inside the combustion chamber, scores of crankshaft degrees in fact, so that its pressure peak will coincide more or less with the beginning of the piston's downward stroke. One drawback of this head start is that during a part of every upward, compression stroke, the piston compresses an already-burning mixture. This not only wastes power (combustion is trying to push the piston down while inertia leftover from a previous cycle is still pushing it up), it also presents an opportunity for mixture lingering at the edges of the combustion chamber to be mechanically heated in addition to their chemical heat, promoting an early, untimed and instantaneous consumption of these "end gases" before combustion's flame has reached them. And this *is* an explosion. And since it is percussive like a bullet's powder charge instead of a flowing flame like the burners on your stove, it is aptly named "detonation," showing that it is not normal engine combustion. Where normal combustion is a timed, steady, gradual moving flame that takes X amount of time to complete and amoothly pushes against the piston, detonation is an untimed spontaneous burn in 1000th the space of time and which cracks the piston. Normal combustion's pressure builds gradually and is dissipated over some 60 degrees of crankshaft rotation. Detonation's force is presented so much faster the engine has no time to use it. Parts break and there is little energy left to make good engine power. A detonating engine is a slow one.

Octane is nothing more than the measurement of a fuel's resistance to spontaneous combustion, i.e. detonation. In its earliest form octane was achieved by creating a thermal insulating barrier between fuel molecules in thr form of lead that slowed the "contagion," if you will, of thermal excitation resulting from the over-pressurization of a combustion chamber's end gases, those pockets of fuel removed from the flame's access by distance or surface area or both. One of three available remedies to detonation (along with reduced end gases and reduced cylinder compression), octane doesn't fix the problem, only the symptom. The problem is too much end gas, so only the solution that addesses that is realistic. That is, the design of combustion chambers that are shaped so that end gases do not form. Octane is a stop-gap. Better engine design is the actual cure.

Detonation plagued engine designers for a very long time, and continues to be a hurdle to engine modifiers. Because combustion chambers were so badly shaped in earlier days, before studies showed they were a problem, end gas formation was a given, and thus compression ratios had to be kept low. End gases remember are pockets of fuel not only farthest from the spark plug but also in recesses and nooks and crannies that resist ready access by the rolling flame front, giving ample opportunity to self-heat before they are consumed. Bad news. Even worse in my mind, fuel developed faster than did combustion chambers, and as tetra-ethel lead was cheaper than engineering time, things stayed this way until the late 1970s when fuels began to be scrutizined and controlled for emissions reasons. Only then did combustion chamber development really start. The start of the emissions era did a lot of harm to a lot of parts of our society, but one very good thing it did (besides save peoples' health, obviously) was it spurred engine design that had been dormant for some time and would likely have stayed dormant for a lot longer. High octane fuel was too easy a bandaid. Superior combustion, by means of flatter, less convolute combustion chamber shapes, results in less octane requirement. Fewer nooks and crannies means the flame front gets to the chamber's edges before the fuel there can self-ignite, and there is also less fuel there at the edges anyway.

Octane is not a source of power. Many argue against this, citing examples of the use of say, race gas, and the resulting improvement in performance. Here again one thing is being confused with another thing. Race gas can indeed improve engine performance (not all race gas is made to increase power -- its prime attributes are consistent, predictable metering and long storage life), but this is not due to any octane increase, though there usually is that. Rather it is due to the fact that race gas is formulated to a completely different ethic; a completely different goal, than is pump gas. In short, race gas is made with a passionately devoted eye toward most efficiently converting the fuel's thermal energy (BTUs) into physical piston pressure, whereas pump gas increasingly has its priorities shifted away from that to meeting other goals such as economy, reduced oil dependancy and emissions and as such is less a fuel each time it is reformulated. Specifically, race gas has most of the aromatics refined into the fuel, whereas pump gas has them taken out due to evaporative emissions standards. Remember too that the origin of the word octane is the test fuel iso-octane, which itself means the same as "optimum." There is an optimum octane for each engine. This is the point at which the engine with a given cooling efficiency, load, ignition timing, compression ratio, and all the rest, is free from detonation. Going over that optimum by adding octane, adds nothing, as there is no more detonation to eliminate. Now, on the other hand, if you go to a higher octane pump gas and experience a power boost, this wil be due to another thing altogether, and that is, your engine was suffering at least a little detonation, whether it was apparent or not. Remember, detonaton robs power because it steals energy that should be taking place over a longish time, and instead pinpoints it into a very brief, destructive period. So this is where perceived power improvement can happen when comparing lower and higher octane pump gas. You might also experience this if the gas in your bike is stale or has absorbed water.

Detonation vs. preignition
Beware of confusing detonation with preignition. While they can occur in combination, they are not at all the same thing. You couldn't tell that from most of what is said about them on the Internet. However, first-semester technical school students are taught that three things distinguish preignition from detonation. Detonation is spontaneous, that is, the fuel's own heat ignites it. Preignition on the other hand is caused by an outside heat source, usually a hot spot in the combustion chamber (typically a glowing piece of carbon). Detonation occurs after the spark plug's spark, preigniton before. And lastly, the results of detonation are a hammering of the piston (resulting in broken ring lands, typically), while preignition more often melts the piston crown. There are more differences, but these are the rules of thumb.

Octane is the measurement of resistance to detonation, and a high octane fuel is simply one that takes up the slack in less than perfect combustion chambers; allows imperfect combustion chambers to burn perfectly. It is not a measurement of a fuel's power potential, not a gauge of how good a fuel is ("...give me the good stuff!"), and not even the primary ingredient in high performance gasoline.

Further reading:
Fuel economy
Motor oil
Oxygenated fuels
Last updated February 2022
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