Does the octane matter when filling a tank

Does the octane matter when filling a tank

Posted by Ben Baker on Jan 4th 2016

Bike manufacturers recommend specific octane grades of gas. Why? Does it really matter what octane you put in the tank?

The answer is yes, but maybe not for the reasons you may be thinking. First, let’s look at octane.


Bell Performance says “The octane rating of gasoline essentially tells you how much the air-fuel mixture can be compressed before it will spontaneously ignite.” In other words, octane is a measure of how explosive the fuel is when under pressure.

This matters because most motorcycles produced today are fuel injected. The engine relies on compression to ignite the air-fuel mixture. The lower the octane rating, the less compression that is needed to make the mix ignite. Here’s a look at the difference between carburetors and fuel injection systems.

This is why high-pressure engines require high octane fuel. Airplanes use fuel that ranges from 100 to 130 octane. Diesel is generally not rated in octanes.

In short, low octane ignites and burns faster than high octane. You might not be able to tell the difference, but your engine can. Since the motor “turns” at thousands of revolutions per minute, these tiny differences are important.


Octane is measured differently in different countries. A US octane rating of 90 is equal to a United Kingdom “research” rating of 95. Manufacturers list the recommended octane for the country where the ride is sold. If you import one from another country, google the bike’s listed octane rating as compared to how your country sets octane. Alternately, find a factory-certified mechanic and ask what octane to use.


Octane is important in reducing engine knock or ping, but it’s not the only thing that influence engine knock. How Stuff Works Auto takes a long look at engine knock.

Low quality and low-octane gasoline “can cause a whole cluster of problems, such as increased combustion chamber temperatures and higher cylinder pressures,” Cherise LaPine wrote.

“The knocking sound is caused by two exploding "flame fronts" - one explosion from the pre-ignition of the fuel-air mix caused by compression and the other from the rest of the fuel-air being ignited at a slightly different time by the spark plug. The two flame front explode and send shock waves through the air of the cylinder, which meet in the combustion chamber and give you that annoying knock effect,” Bell Performance says.

In other words, the wrong octane will lead to engine damage long term.


Bearing all this in mind, the company that built and tested the motorcycle engine you run did so with a specific octane in mind.

Using lower-than-recommended octane means the mixture can ignite too soon.

Using higher-than-recommended octane means the mixture can fire later than expected.

Bill Whisenant tested this and wrote about it for Motorcycle Performance “An engineering fact: THE MOST HORSEPOWER IS MADE AT THE THRESHOLD OF DETONATION. We have often gained horsepower on the dyno and felt improved starting and driveability going from premium grade gas to regular,” he wrote.

His article explains more about why regular can be better than premium, depending the bike. But as he says, maximum performance is when the fuel-air mixtures combusts at exactly the moment the engine maker had in mind.

Too soon or too early and you get reduced mileage, power and premature engine wear.