Octane Ratings & How They Will Affect Your Dirt Bike Engine’s Performance

What Octane Rating Do You Think You Need?

Not all fuel is created equal, and not all vehicles (dirt bikes included) can run on the same type of fuel.  As a result, octane ratings were established to classify the different qualities of fuel into distinct performance categories.  And it’s this octane rating that ultimately determines what we put into our fuel tanks daily.  Therefore, it’s essential you know the ins and outs of what octane ratings mean, and the effects they can have on your bike.

e10 unleaded Octane rating

What is Octane?

A vast majority of fuels are created from hydrocarbon chains.  These chains are split apart and “rearranged” to form different types of fuel. Methane, butane, propane, and octane are all derivatives of this breaking up and rearranging of hydrocarbons, and each is constructed differently.

You can guess by its name that OCTane fuel consists of eight carbons. But what you may not know is that the fuel we use in our engines also contains heptane (seven carbons).  And it’s the mixture of octane and heptane that starts to define the octane rating of the fuel.

Related: An Introduction To The Two Stroke Power Valve – What It Is & How It Works

What is Octane Rating?

The numbers you see in an octane rating (87, 91, 95, and up) coincide with the percentage ratios of octane-to-heptane (or in some instances, other combination of fuels).  Therefore, an 87 octane rating represents fuel that consists of 87% octane and 13% heptane.  Ergo, 91-octane fuel is made up of 91% octane and 9% heptane.  But what’s the significance of the two?

How Do These Ratings Affect Engines?
Octane can handle compression much better than heptane can.  As a result, the two are combined to control the compression level of the fuel, which is essential to an engine’s performance.

When fuel ignites by compression rather than by the spark created by the spark plug, knocking occurs in the engine.  Knocking inevitably leads to various engine issues, which could ultimately lead to engine replacement.

So, the type of fuel that can be used in your vehicle is determined by the compression ratio of the vehicle’s engine.  Most standard cars and trucks have a lower compression ratio, which means that “regular” 87-octane fuel can be used in them.

On the flip side, high-performance vehicles (such as many of the dirt bikes we ride) have a higher compression ratio, which means a higher compression premium octane rating is needed to maximize its efficiency and power.  The higher the compression ratio on the motor, the higher the horsepower (usually), and thus, the higher the octane rating needed.

Related: Should You Trust The Oil Injection System On Your 2-Stroke Motorcycle?

A Quick Word (of Caution) on Octane and Ethanol
this is what a Blown 2 Stroke engine looks like
Blown 2 Stroke Engine

In recent years, ethanol-blended fuel has become a regular staple at gas stations and is seen in fuel more frequently than ever before.  Ethanol acts as an oxidizer for the fuel allowing it to burn more fully during combustion.  And while most late model vehicles have no problems using the blend, there are still some potential dangers of using it, especially in older vehicles and two-stroke engines.

The problem with ethanol is that it absorbs water.  In the case of long-term storage, ethanol can absorb moisture from the atmosphere, creating a reservoir of water and ethanol at the bottom of your fuel cell.  And when the engine is turned over, the first thing to get sucked up into it is that water/ethanol blend.  And in the engine world, that can mean an early death.

On top of engine damage, prolonged use of ethanol can lead to corrosion in the fuel system and carburetor. Therefore, it should be avoided as often as possible.

So, you now know that octane gas is made up of heptane and octane, that the octane rating reflects the percentages of the ratio of each, that 87 octane is for standard vehicles and 91-octane for high-performance vehicles, and ethanol and octane gas are never a good combination, regardless of the octane rating.

Now, you should be able to choose the type of gasoline you want to use in your bike and vehicles with more confidence. And if by chance you’re still not sure, then you can always check with your dirt bike owner’s manual, and it should tell you which octane rating is best for your engine (most likely not ethanol).

If you have any questions or anything to add, please leave them in the comments or on our FaceBook page!

Keep Reading – Ethanol Damages 2-Stroke Engines

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