Octane is not what most people think. “Gimme the good stuff.” “I’ll take the high-test.” Remember when people said that? Octane has nearly become entrenched in western folklore: “Ethyl” was the name of a company before it became slang for premium gas, and “high octane” is almost a euphemism for anything having a significant level of effect—even coffee! But the reality is, octane is pretty unexciting and mundane. It is not something that delivers an increased effect—it actually has a minimizing effect on gasoline. It is nothing more than the rating of a fuel’s resistance to self-ignition. And while that is important, it is not very sensational, not very exciting. It doesn’t warrant a sticker on your toolbox.

Pockets of fuel/air mixture (“end-gases”) lingering at the edges of the combustion chamber are mechanically heated by the rising piston and can instantaneously pop off on their own, before ignition’s spark gets to them. When they do we call this detonation, and it can be very destructive because it focuses all the fuel’s thermal energy, energy that is supposed to be spread out over many crankshaft degrees, into a highly concentrated moment. High octane is in reality merely a detonation stop-gap. Detonation-resistance was achieved by adding an insulation of chemical lead between the fuel’s molecules that prevented them from transfering heat. This discouraged the gases from self-igniting. The unfortunate formation of end-gases was once considered an unavoidable given, so either compression ratios were kept low, or high octane gas was used. Superior combustion by means of flatter, less convolute combustion chamber shapes would eventually result in less octane requirement because they discouraged end-gas formation by having fewer nooks and crannies that harbored it. Octane never actually fixed the problem, it only pushed back its solving by addressing the symptom not the problem. The problem was combustion chambers that were not highly developed, and with no incentive to improve them, the spectre of detonation remained for quite a while until engine design became a priority again, which started to happen when lead was outlawed in the late 1960s.

When determining a fuel's octane, a gasoline formulator begins by using a test engine and two test fuels. He already knows how the test engine runs on a low grade test fuel called “heptane” that induces detonation right off the bat, and how it runs on a really good one called “iso-octane” that thoroughly resists detonation. He also has experienced how this same engine responds to mixes of the two fuels together in varying percentages. The ratio that just barely, with a given compression and ignition timing, staves off detonation, is named after it's iso-octane percentage. It might be a 80/20 mix for example. A production, consumer gasoline that mimics the characteristics of this test fuel formula would then be called an 80-octane gas. However, since at this point it is yet not field-tested, the gasoline is provisionally qualified as a "research" octane. Then the gasoline is tested in actual running vehicles subject to variable temperatures, loads, and wear. Not surprisingly, the result of these field tests result in octane ratings that are lower than the first ones. Therefore, before the gas gets to the dispensing station there is a compromise in the octane rating. The final number is a lower, “pump” octane designation. This is why you see the familiar "R+M/2=P" (Research / Motor = Pump) sticker on the gas pump. The pump octane rating is what we go by, though Honda and other OEMs have inexplicably published the reseach number more than once in their service literature. That’s what happens when you let engineers into the copy room, I guess.

There is no consumer-level labeling of a gasoline’s power potential. Gasoline’s performance comes from its thermal potential (BTU energy), not from octane, though it is true engines can run better on higher octane gas. This anomaly happens in a few different ways. First and most obvious, if the engine is detonating—and thereby is down on power—then higher octane gas can improve the engine’s performance. Another way is when using race gas, but not in the way you might imagine. There is an attribute of race gas that is more important than its octane, and that is its lack of politics. It is formulated with no concern for emissions or volatility or evaporative tendancy or anything else but will it burn good and do it consistently. Thus it is not unusual for an engine to run better on race gas, completely irrespective of the octane issue. Another thing about race gas that many don’t know is that much of it achieves its necessary octane from ethanol. Yes, there is both leaded race gas and ethanol race gas. Moreover, race gas lasts many times longer than does pump gas. But this is not because it is free from ethanol—remember, much of it contains ethanol. Race gas lasts longer, again back to the politics, because it contains chemicals that were once common in all gasoline but were removed for reasons of health in some cases, and also for reasons of evaporative emissions regulations. These chemicals are toluene, xylene, and benzene, the latter being a significant health hazard and the impetus for the closing of many waterways to two-stroke personal watercraft many years ago.

A high octane fuel is simply one that takes up the slack in less than perfect combustion chambers. It allows older-design chambers to resist detonation as well as newer ones. It is not a measurement of a fuel's power potential, not a gauge of how good a fuel is, and not even the primary consideration in the making of a high performance or racing gasoline. It is simply a detonation buffer.

Being in the vintage Honda service business, I frequently encounter a surprising misunderstanding about premium gasoline. Premium gas, completely aside from its octane or anything else, contains solvents and detergents for the purpose of working best in old cast iron V8 car engines that are probably terribly sludged up. It’s a survivor fuel, in other words. As such it has no business in your Honda, unless it’s a CX500 or CX650 Turbo, a four-stroke motocross bike, one of the high-output hyperbikes that unlike most Hondas was designed to require the most octane. Using premium in a Honda not designed for it will result in excessive carbon buildup, to the point that valves will sweal intermittently and performance will suffer.