Moto Mojo: the Octane Problem

Moto Mojo: the Octane Problem

What’s Right For Your Bike

For many motorcyclists, the octane numbers such as 85, 87, 89, and 91 (or 93) on the gas pump might as well read “nope,” “almost there,” and “that’s the stuff.” And it’s a logical assumption that when something costs more, it must be better, right? Not necessarily. The best choice at the pump depends entirely on the specific motorcycle, riding style, and conditions. Incorrect choices can rob riders of performance, damage the engine, and waste money.

Octane is a measure of gasoline’s resistance to self-ignite during the compression stroke, before the spark plug fires. In the U.S., the octane ratings on the pumps represent an average between “Motor” octane and “Research” octane, which are two different methods of testing octane, yielding different results. The octane ratings on European pumps typically are about four to five points higher than the U.S. (for equivalent performance) because they use a different rating system.

In a gasoline engine, if the cylinder pressure is too great as the piston compresses the air-fuel mixture, the fuel will explode prematurely as the piston is still rising. This is called “detonation” and it can blow holes in pistons if allowed to continue. To put it simply, the higher the octane number, the more resistant it is to detonation.

An engine’s octane requirement changes constantly. When coasting or idling, it is very low, and while under a heavy load, the need for octane soars. Some of the factors that determine what octane an engine needs include: ignition timing curves, air-fuel mixture, combustion chamber design, compression ratio, camshaft profiles, port designs, exhaust design, air temperature, barometric pressure, engine temperature, load, and rpm. For example, I use premium during hot summer months, mid-grade when it’s warm out, and regular over the winter or when it’s cold outside for my 2008 Road King.

An engine’s compression ratio is the ratio of the cylinder’s and combustion chamber’s volume at the bottom of the piston’s stroke to its volume when the piston reaches the top. The higher the compression, the more powerful and efficient a given engine is for its size.

As compression ratios increase, the need for higher octane typically follows, but it’s not a direct linear correlation. Air-cooled engines typically run hotter cylinder head temperatures than liquid-cooled engines; therefore, they may need higher octane despite their lower compression ratios. For example, many Harley-Davidsons run around 9:1 compression ratios, but the factory recommends premium fuel. Yet the Honda CBR954RR had 11.5:1 compression and was designed to run on regular gas—thanks to cooler running and an excellent combustion chamber design. Engines with lean air-fuel mixtures also run higher combustion chamber temperatures, which raises the octane requirement. Usually a rider will hear a metallic rattling sound (commonly called pinging) coming from the engine when one places it under load if the octane is too low.

Running fuel with too low of an octane level for operating conditions can seriously damage the engine from detonation, but what if the octane level is too high? The bottom line is that riders can save a little money by using lower octane fuels IF the engine tolerates it, but it’s better to err on the side of caution if unsure, as no damage will occur if one runs more octane than needed.