Go high, you'll fly, go low, it'll blow.
It's a question I hear occasionally: Is it a bad idea to use gasoline with a higher octane rating than the manufacturer's recommendation?
The short answer to this question is a simple “No.” Modern motorcycle engines are designed to run just fine using gasoline with a higher octane than recommended. If your owners’ manual calls for a minimum of 89 octane, you’ll be OK using 91 octane gasoline.
Digging deeper, this question often arises over a misconception over what an octane rating really is. I’ve had this discussion with people who think higher-octane gas “has more energy” or “is cleaner.”
Some petroleum companies do sell high-octane gasoline with additives that supposedly help your engine burn more cleanly, but this has nothing to do with octane rating—it's just an upselling technique.
An octane number is a rating of how much compression the gasoline can handle before igniting. Higher octane ratings mean the fuel is less likely to pre-ignite under high pressure.
Here’s what that means for a motorcyclist. Take an average motorcycle, like my Suzuki DR650SE. In standard form, it’s a fairly low-compression engine. It’s designed that way so it will work with a wide variety of fuels, and because reliability was more important to the designers than performance. Low-octane gasoline was fine for this stock engine.
A couple of years ago, I installed a high-compression piston, which raises the pressure of the combustion chamber. Now, under certain circumstances, low-octane fuel will pre-ignite (some motorcyclists call this “detonation”). The engine’s compression causes low-octane fuel to spontaneously combust before the spark plug ignites the fuel. This results in a “pinging” sound, and if I were to ride around like this for a while, I could damage my engine. The pre-ignition means my engine is now firing out-of-time.
High-octane gasoline is formulated to resist pre-ignition, thanks to chemical additives and other voodoo that takes place at an oil refinery. When I run 93 octane gasoline in my DR650, I have no pre-ignition. When I run 89 octane, I might have a problem.
Motorcycle engines with high compression from the factory will typically require high-octane fuel. For instance, late-model BMW adventure bikes require higher-octane gasoline, unless you’ve had them re-programmed by a dealer. Some owners do this, because high-octane gasoline might not be available while traveling in remote areas. Running low-octane gasoline could cause pre-ignition, and be a problem in these bikes. Running gasoline that’s higher-octane than manufacturer spec will be OK.
If your motorcycle’s manufacturer does not recommend high-octane fuel, there’s no guaranteed advantage to running it. Some owners may claim there’s a difference in fuel economy or horsepower, but these claims are usually based on seat-of-the-pants observations, not any sort of scientific comparison.