The mythical, magic number of motor fuel: Octane. What does it mean for your ride?
Over the years, I’ve heard a lot of conflicting views about which fuels to use in motorcycles, and whether higher-octane gas has more “power” than lower octane gas.
This includes some views that say running high-octane gas is virtually the same as adding horsepower to your bike right at the gas pump.
I’ve even heard of riders who go out to a local airport to buy 110 octane aviation fuel in the belief that they are getting extra horsepower by doing so. They aren’t getting that stuff cheap, either.
The truth of the matter is that 110 octane gas has no more “power” or thermal efficiency than 87 octane pump gas, though it may be faster-burning, suggesting ignition timing changes could benefit performance, at least in bikes with big-bore, high-compression motors.
Fuels with lower specific gravity—higher octane—tend to be those with faster burn and more rapid flame-front in the combustion chamber. Stated another way, high octane fuels may allow engine characteristics that increase thermal efficiency—increased compression ratio, for example. But, by itself, just pouring some high-test in your tank won’t add horsepower.
What high-octane does have over 87 octane pump gas is thermal stability and octane stability. That is, it resists so-called engine knock caused by detonation (the air/fuel mixture igniting spontaneously due to high pressure and heat in the combustion chamber as in a high-compression racing engine) or pre-ignition where the air/fuel mixture ignites prematurely due to superheated engine components or glowing-hot carbon build-up before the spark plug fires.
Our friends at Sunoco Racing Fuels explain octane ratings so clearly, even I can understand it:
“At fueling stations in the United States, octane is typically displayed as a number, 87 through 93. This number is known as the Anti-Knock Index (AKI). AKI is the average of Research octane number (RON) and Motor octane number (MON).” Or, for those of you who recall your algebra, as this formula: R+M/2.
“Octane numbers are measured using single-cylinder engines that look more like something out of a Model T than any modern engine. These so-called ‘knock’ engines are operated by trained technicians in labs under controlled conditions. Two tests are used – one for Research Octane Number (RON) and another for Motor Octane Number (MON). The RON test results in a higher-octane value than the MON test.”
“Essentially, these numbers provide a scale to measure how much heat and pressure can be put on the fuel before it spontaneously combusts. Spontaneous combustion is a source of engine knock which can quickly damage an engine. Octane ratings are important because different engines expose fuel to different amounts of pressure and heat. Engines must use the proper octane fuel to avoid knock and provide reliable operation.”
So higher octane ratings are important for engine preservation and operation in bikes that have high compression ratios and, depending on boost pressures anticipated, turbocharging. Another thing that higher octane fuels do possess over lower octane fuels is resistance to breaking down into fuel system-fouling deposits and varnish.
Sunoco explains why:
“87 octane fuels tend to be less refined and contain more unstable hydrocarbons. As the months pass during storage these unstable components react to form gums, varnishes and lower octane hydrocarbons. As a result, the octane can decrease within months for 87 octane fuels, especially when stored under less than ideal conditions.
“Ninety-three octane fuels are more refined and contain more stable hydrocarbons. These stable hydrocarbons can last two, three times longer than 87 octane fuel. Even in proper storage, 87 octane gas can start to degrade in three months, 93 octane fuel should last closer to nine months before degradation is noticeable. Keep in mind that 93 octane fuels are still susceptible to octane loss and vapor pressure decreases due to butane evaporation.”
What about pump gas containing ethanol (E10)? Sunoco offers the following information aimed at the person who may use a vehicle as both a daily runner and weekend warrior:
“Regular pump gas (E10) is great for commuting but won’t offer added protection at the track. After a few passes, regular grade might not be enough to prevent detonation so it is a smart choice to use premium at the track even if your car [or bike] is tuned on regular. Regular gas is designed to be used up quickly and shouldn’t be left in a vehicle that is being stored for more than a couple of months.”
This would explain why some bike and car collectors “pickle” their fuel tanks and fuel systems by keeping them full of 110 octane gas for long-term stability in storage. It is highly refined–even more than 93 octane fuel–doesn’t contain ethanol, and is more stable over long periods of time.
That’s why I keep 91 octane non-ethanol gas in my 1976 Honda CJ360T and my other older motorcycles that may not see regular use—not in the belief that it might boost horsepower, but to help keep the fuel system from getting varnished up by lower octane gas as it degrades during storage.
The same goes for my snowblower for over the summer and lawnmower for over the winter and chain saw all the time—they don’t really need 93 octane to run right, but they each sit for months, so fuel stability is a benefit.
Sunoco also offers some pointers on recreational gas and ethanol:
“Recreation gas is a common name for premium without ethanol. Usually 2-3 octane numbers lower than the E10 counterpart. Rec gas is a great choice for a wide variety of applications that struggle with ethanol. Ethanol blended fuels can have compatibility issues with small engines or older fuels systems designed before ethanol was commonly used.
“Another scenario where ethanol blends fail is when the fuel is stored in an open vent system. Ethanol is able to absorb humidity from air and draw water right into the gas tank. Boats, lawn equipment, and other rarely used equipment fall victim to water in fuel issues and can be fixed by using 100% gasoline. It is important to know that using rec gas in something setup on E10 can make the engine run slightly rich because of air/fuel ratio differences.”
While I’m sure the legend (and misunderstanding) of “high octane gas makes more power” will live on, the bottom line is, unless other things are done such as timing adjustment and possibly fuel system adjustments (whether normally aspirated, fuel-injected, turbocharged or supercharged), simply pouring high-test gas in your tank won’t result in any noticeable outpouring of additional power at your crankshaft.
To be sure, there are some benefits to be had with using higher octane fuels, such as prevention of engine-crippling detonation or pre-ignition for high-compression or high-boost engines, greater fuel stability in long-term storage, less potential for varnishing older fuel systems and so on. Moreover, there doesn’t appear to be a downside to filling the tank with higher octane fuels, other than the added cost.
In the majority of circumstances and bikes, using whatever grade of fuel the manufacturer recommends is probably the easiest way to get the level of engine performance you expect, without the added costs you don’t. And, all other things being equal, keeping up maintenance on that machine of yours is vital to the performance of all systems.
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- Zachary Santner, Technical Specialist at Sunoco Race Fuels provides more details on this subject here: www.sunocoracefuels.com/tech-article/octane-stability-high-octane-vs-low-octane-fuels
- See also: www.sunocoracefuels.com/tech-article/beyond-octane