Wednesday, March 01, 2006


High-octane fuel: What is it? When do you need it?

Article #1 in the Tips for New Riders series which appears at

By Tim Kreitz

One Saturday back in the summer of 1999, a friend of mine and I rode to the annual motorcycle rally at the state park in Andrews, Texas. After hanging out and looking at bikes all afternoon, we eventually met a group of guys who were riding mostly sportbikes. They seemed pretty cool, so we accepted an invitation to ride back to Midland with them.

As one of the guys started his bike and let it idle, I could hear the top end of his engine making loud, very pronounced pinging sounds. I could tell immediately that the fuel-air mixture in his cylinders was pre-igniting. For those of you who may not know, pre-ignition is the characteristic of a given fuel-air mixture to combust before it’s supposed to (without a spark from the spark plugs), and is sometimes caused by running the wrong kind of fuel in a high-compression engine.

I didn’t initially say anything about it to him, but when we stopped for fuel on our way out of town, I noticed that he was putting 87 octane gasoline in his tank. When I asked him why he was using that particular grade of fuel, his response was something along the lines of,”I don’t care what the manual says, my dad told me buying high-octane fuel is just a waste of money.” He went on to explain that his manual called for a minimum octane rating of something like 91, but that his bike ran “just perfect” on 87. When I explained that the reason his engine was pinging could likely be blamed on his choice of fuel, he looked at me like I was out of my mind. “What do you mean, ‘pinging’?” he replied. I pretty much dropped the conversation at that point.

I never saw that guy again, because he probably ruined his bike’s engine and never rode again.

So Tim, why are higher octane fuels less prone to pre-ignition?
In short, because they are less volatile than lower octane fuels. I know that sounds backwards, but it's true. For the sake of simplicity, you can think of octane rating as a measure of the gasoline’s chemical stability: The higher the octane rating, the more stable the fuel actually is. For some sportbikes, this is important because engine compression ratios are so high as to cause more volatile, low-octane fuel mixtures to ignite without spark -- as aforementioned -- which will eventually damage the engine.

Does that mean all sportbikes need high-octane fuel?
Not necessarily. Aside from manufacturer recommendations, temperature, altitude, oxygen density, and climate issues all play a part in how your motorcycle converts fuel and air into power. If your bike runs properly on lower grades of gasoline, don’t worry about it. You’re fine. But if your engine is pinging, you should go to a higher grade.

Final thoughts
If you prefer running higher octane fuels than necessary, that’s okay too, but be aware that in some cases, variants of the octane molecules can cause carbon to build up on your valves and pistons. Pulling your carbs or throttle bodies during regular valve clearance checks and shining a flashlight into your intake ports will usually show some minor backside buildup if this is occurring.

I'm fortunate to live in a geographical area that has a dry climate and about 3,000 to 4,000 feet of climatic altitude value most of the year. We make decent horsepower out here and our bikes run cleanly if kept in a proper state of tune. Selecting the proper fuel grade as part of that equation is another tool we have at our disposal for going fast while prolonging the lives of our bikes at the same time.

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