Octane is one of the most commonly discussed, highly marketed properties of performance fuels. Fuel manufacturers tout their high-octane products. Octane boosters promise added horsepower. And Hollywood promotes “high-octane” thrills for the next installment of their latest street racing franchise. Yet for all the hype, octane remains one of the least understood aspects of gasoline. A fuel’s octane rating, so the marketers would have us believe, is directly related to its power. The higher the octane number, the more powerful the fuel. Simple, right?
As you’ll soon see, this is actually incorrect and it is this misconception that has fueled (sorry) the following myths about octane.
(Please note: The following assumes a basic understanding of how 4 stroke internal combustion engines work. If you need to brush up on your strokes, our upcoming primer — Performance in the Mist: the importance of fuel atomization in combustion engines —will help. Check back soon!)
Octane Myth #1:Octane is a measure of a fuel’s power
Oddly, the answer is no. Octane is not a measure of a fuel’s power. It is a measure of its burning behavior. More specifically, it’s likelihood to combust predictably when pressurized during an engine’s compression stroke. The *higher* the octane rating, the more predictably it will combust under pressure. The lowerthe octane of the fuel, the lesspredictably it will combust. “Wait, what?” I can hear you saying. We want explosive power for our engines, we are not looking just for ‘predictability’?” Yes and no. While we do indeed want explosive power from our fuel, power comes from many different aspects of fuel chemistry. For example: combustion engines need a strong explosion to produce strong power, but that explosion must occur at precisely the right time and uniformly to extract the most energy from it . That “right time” ideally should be when the spark plug fires. Low octane fuels are more likely to combust spontaneously during compression before the spark plug fires.This is called pre-ignitionor detonation and it produces a distinctive audible “knocking” sound from the engine. This is why you may have heard the term “engine knock” or “knock sensors”.
“OK. Fine. High octane is more predictable, but surely it also has to be more powerful than low-octane fuel?” Good question. On to myth #2!
Octane Myth #2:High octane fuels are more energetic than low octane fuels
As discussed in Myth #1, high octane fuels are actually more predictable than low octane fuels. And, dispelling Myth #2, high octane fuels are actually less “powerful” (less chemically energetic) than their lower octane counterparts. The octane rating is actually a measure of the ratio between a fuel’s octane (a compound callediso-octane) and another compound called heptane. So an octane rating of 93 indicates the fuels contains 93% iso-octane and 7% heptane. Heptane is more energetic but tends have less stable combustion under compression. Iso-octane is more stable and, therefor, less energetic.
“OK.” I hear you saying.”Then why does everyone tell us high octane fuels will give your engine more power then??” Myth #3, coming right up!
Octane Myth #3:high octane fuels will make my engine produce more power than low octane fuels
Since high octane fuels are actually lesschemically powerful than low-octane fuels, they do not directlyproduce more power when combusting during the power stroke. But...since they are less susceptible to spontaneously combusting during the compression phase, — and here’s the important part — an engine can take advantage of this and more highly compress the fuel/air mixture without the risk of the fuel detonating before the spark plug fires.It’s the higher compression inside the cylinder that produces more power in the engine, not the fuel itself. So octane doesn’t produce more power, but provides the conditions for an engine to take advantage of more aggressive tuning like higher compression ratios and advanced ignition timing. It is this tuning that produces more engine power. “OK. Fine” I hear you say. “So all I have to do to get more power out of my engine is use high-octane fuel.” Well, not necessarily. Myth #4 please!
Octane Myth #4:All engines will produce more power with higher octane fuel
This myth is where much of the confusion around octane is focused. Octane is marketed by fuel companies as giving all engines more power. But as we learned in Myth #3, only engines that have been tuned to take advantage of high-octane fuel will benefit from it. The good news is that manufacturers will indicate the recommended octane fuel to be used. If 91 octane is recommended, then the engine has been tuned to take advantage of the higher octane. “OK.” I hear you say yet again “So knocking will tell me if I should be using higher octane fuel.” I bet you know what’s coming next right? Yep! Myth #5!
Octane Myth #5:If my engine isn’t knocking on low-octane fuel, it won’t benefit from high octane fuel.
You might think so given what you’ve just read and you are absolutely correct if the engine has been optimized by the manufacturer for low-octane fuel . But this is where it gets a bit tricky, many modern, high-performance engine management systems employ “knock sensors” (see Myth 1). These sensors are used to detect pre-ignition (knock) and will adjust the engine (usually the ignition timing) to accommodate whatever octane fuel is in the tank. So if the manufacturer recommends 91 octane fuel, but you’ve just filled up with 87 octane it’s likely that the engine management system will retard the ignition timing down to where the engine won’t knock. These systems can also work the other way — advancing the ignition timing to take advantage of higher octane fuel - thus boosting performance. It’s important to note, however, that the performance gains from higher spark advance is not infinite. With all other parameters being equal, an engine that can adapt to various octane levels will probably find performance gains switching from 93 Octane to 100 Octane, but not from say 100 Octane to 110 Octane.
Hopefully, this helps clarify some of the confusion around octane. And while octane is an important element to reduce detonation in high performance fuel, it is not the only element.
There are many more properties that contribute to a fuel’s resistance to detonation—as well as overall performance for racing—such as vaporization speed, combustion speed, and cooling effect - many of which we’ll be covering in detail in later blog posts. Because at ETS Racing Fuels, we believe an educated customer is our best customer. Happy racing!