Octane Rating at the Pump

For many motorcyclists, the octane number such as 91 and 95, or 85, 89, 93 and even 98 if you travel outside Thailand, on the gas pump might as well read 'who knows what I need'. 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 or gasohol's resistance to self-ignite during the compression stroke, before the spark plug fires. In all countries around Thailand the octane measurement is specified in RON (Research Octane Number) which is a different standard then used in the US, some people I spoke to had downloaded US/Canadian owners manuals for there motorcycle and got confusing advised octane levels. In America and Canada the octane ratings are specified with another standard called MON (Motor Octane Number) which is a different method of testing octane levels.

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 the fuel 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 for a modified track bike RON 98 (what is basically standard RON 95 with some additional octane to make it 98) on the average hot day in Thailand, when in December the temperature drops a bit the bike will run fine with standard RON 95.

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-Davidson motorcycles run around 9:1 compression ratios, but the factory recommends RON 91. Yet the Honda CBR954RR had 11.5: compression and was designed to run with the same RON 91 fuel – 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 money by using lower octane fuels if the engine tolerates it, but it's better to be on the side of caution if unsure, as no damage will occur if one runs a bit more octane than needed. Of course the perfect fuel economics can only be had with an engine that gets the right octane it needs.Tag: Octane Fuel RON Rating Pump Gasoline Gasohol MON RON91 RON95
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