Let's explore the subject of gasoline octane, one of the topics that should be 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 (thus 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 would be called 80 percent. 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. 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 field 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. In fact, Honda mistakenly used Research numbers for a very long time. 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 a very brief peak pressure of over 1000 psi. 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 the last part of every 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. This actually is an explosion. And since it is percussive like a bullet's powder charge instead of an advancing flame, it is aptly named "detonation," showing that it is not normal engine combustion. Where normal combustion is a timed, steadily rolling flame that takes X amount of time to complete and amoothly pushes against the piston over some 60 degrees of crankshaft rotation, detonation is an untimed spontaneous burn in a mere fraction of the time and which often cracks the piston. And since the engine can't use the force normally, the 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 rubbing against each other and transfering their heat. The combustion chamber's end-gases, those pockets of fuel removed from the flame's access by distance or surface area or both, didn't pop off on theur own. But octane doesn't fix the problem, only the symptom. The problem is too much end-gas, so only the solution that addresses that is realistic. The solution is a combustion chamber that so shaped that end-gases do not form. Octane is a stop-gap. Better engine design is the actual cure.
And that cure came eventually. 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. Because 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 some harm to a the motor vehicle industry, but the good thing it did was spur engine design that had been dormant for some time and would likely have stayed dormant for a lot longer.
Power?
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. But one thing is being confused with another. Race gas can indeed improve engine performance because it's formulated to a completely different ethic, a different goal. It is made to most efficiently convert the fuel's thermal energy (BTUs) into 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. 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.
Summary
Octane is the measurement of resistance to detonation, and a high octane fuel is simply one that compensates for imperfect combustion chambers. It allows such combustion chambers to burn better. 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.
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