At most gas stations, when you pull up to the pump, you have a choice of three grades of gasoline for your motorcycle: Regular, Plus and Premium.
Each grade of gasoline carries an octane rating, which is expressed as a number. Regular is usually 85 to 87; Plus is usually 88 to 89; and Premium is 90 to 93 – variations are due to location and elevation. But what do those numbers mean? And what is “octane,” anyway?
Octane refers to the chemical makeup of gasoline and its chain of eight (“oct”) carbon atoms. Not to get too deeply into the chemistry, the gasoline we get at our pumps is composed of a mixture of octane and heptane (or a fuel with a similar behavior to heptane). The percentage of octane in the mixture determines the octane number. Therefore, an 87 octane rating or number fuel is made up of 87 percent octane/13 percent heptane. So far, so good.
Fuel can be ignited in at least two ways: with a spark, flame or heat source, or with compression. In a properly tuned motorcycle engine, a spark from the spark plug ignites the fuel. Before the fuel gets ignited, it gets compressed in the cylinder by the rising piston. It’s important that the fuel doesn’t ignite from compression before the spark is applied. This condition is called “pre-ignition,” and is commonly perceived as knocking. Severe knocking can damage or even destroy a motorcycle engine.
Which brings us back to octane. Octane is more resistant to ignition from compression than heptane. So, higher compression engines require fuel with a higher octane rating in order to operate without knocking. Make sense? Good.
Every engine manufacturer specifies a minimum octane rating requirement for fuel. The majority of motorcycle engines, including all current Harley-Davidson engines, require 91 octane or higher (Premium) fuel, thanks to high compression ratios. There’s no performance benefit from using a higher octane rating fuel than your engine requires – octane doesn’t add power; it resists pre-ignition from compression.
If you travel to vacation spots in states with higher elevation, you may discover that the octane numbers on the fuel pumps are lower than those you see at sea level. More chemistry and science reveals that air pressure is lower in the mountains than at the seaside, and the air is less dense up high. As a consequence, compression ratios inside engines are lower, and there is less danger of pre-ignition with lower octane fuels. But I’d still advise you to buy the highest octane fuel you can get at the pump. After all, you’re probably going to descend from the mountains eventually – and you probably won’t drain your gas tank before you descend.
In short, use the fuel that is recommended by your engine manufacturer. If you experience knocking, pinging or pre-ignition, make sure that you have fueled up with the right octane rating. The octane rating of your gas is just one of the possible causes of knocking, but it is one of the easiest to remedy – just replace the low octane gas with higher octane gas. Don’t mess around with octane boosters, which are generally certified as “race-only” formulations. Keep it simple, keep your engine in good tune, and use the right octane fuel, and you’ll be happier in the long run.
And that’s what we’re all in motorcycle travel for, isn’t it? The long run.