Octane Myths—What You Don't Know About Gas

Pouring Gas into Polaris RZR.

We all know the guy that refuses to put anything less than 93 octane fuel in his RZR. He might even spout off some good-sounding reasons why 93 octane is the way to go: it’s cleaner, has less ethanol, and his engine performs better. Or maybe it’s simply for peace of mind. Better safe than sorry, right? You know that guy. Maybe you are that guy.

We’re here to clear up any confusion you or your riding friends might have about octane-rating. The truth is that running anything other than the manufacturer recommended octane fuel is a bad idea. It won’t give you better performance, and it might give you worse performance overall. Here’s why.

What is Octane?

Octane itself refers to a hydrocarbon molecule made of eight carbon and 18 hydrogen atoms. It’s a key ingredient in fuel. Ask the next three people you run into what octane-rating you see at the pump is and you’ll hear everything from it being the amount of octane in the fuel to that fuel’s ability to burn better.

Gas Octanes on Fuel Pump Station
Image source: The original uploader was Bobak at English Wikipedia. CC BY-SA 2.5, via Wikimedia Commons
The octane rating you see when you go to the pump refers to that fuel’s ability to resist igniting and the efficiency of that burn as compared to a baseline hydrocarbon fuel mixture. The specific number you see listed—87, 91, 93—is determined through lab tests.

A fuel with an 87-octane rating has 87% of the efficiency of the baseline mixture. A fuel with a 101-octane rating is 1% more efficient than that baseline. It doesn’t necessarily correlate to the amount of octane in fuel at all. Yeah, that’s a weird definition. It’s like describing the definition of a pound. Just go with it.

Combustion

But what does that mean practically in an internal combustion engine like that found in your RZR, Maverick, or any UTV that runs on gasoline? To understand that you need to understand how the pistons in your engine work.

The engine in your UTV is a four-stroke engine which means each piston completes four actions in a cycle to drive the crankshaft. In the first stroke, the piston lowers in the combustion chamber and the chamber fills with a mixture of atomized gasoline and air.

4 Stroke Engine Animation
Image source: Zephyris GFDL, from Wikimedia Commons
In the second stroke, the piston moves up again compressing the gas and air mixture. This compression causes the mixture to heat up so that when the spark plug at the top of the chamber fires, the mixture burns at the perfect time and perfect rate to drive the piston back down in the third stroke and turn the crank shaft. It’s not exactly an explosion, just a fast, expansive burn similar to the way gunpowder drives a bullet out of the barrel. The fourth stroke expels the exhaust and returns the piston to its topmost position so the cycle can start again.

Let’s focus in on the second and third stroke; that’s where your fuel’s octane rating comes into play. The second stroke is all about the piston’s and combustion chamber’s compression ratio. The compression ratio is determined by the difference in volume when the piston is fully down and when it’s fully up. Most UTV’s have a compression ratio between 9:1 and 12:1. What it means is how much the piston squeezes the air fuel mixture before the spark plug ignites it.

The amount the mixture compresses and the moment the spark plug fires is what determines the needed octane rating of the fuel. Lower octane fuels are easier to ignite than high octane fuels. If you use a low 87 octane fuel in a high compression engine, the heat from compression alone will cause it to ignite before the spark plug sparks. That is called detonation or knocking and it’s super bad. When that happens, the piston is driving down early and tries to push the crank in the wrong direction. At best you lose power and at worst you destroy your piston and likely the rest of your engine as well. It’s no good. Most modern machines have knock sensors to counteract this problem, but we’ll talk more about that later.

Using High-Octane Fuel When You Don’t Need It

Now let’s talk about the opposite situation: high octane fuel in a low compression engine. This is what you see a lot more of when you start asking people about what they run in their UTVs, and it’s not nearly as bad. It’s still not great, but you won’t end up doing major damage.

Closeup of UTV Engine
If you use high octane fuel in a low compression engine, you will likely lose power.
When you put 91 or 93 octane fuel in an engine that is built and timed for 87 octane, you end up losing power. Why? Again, it’s all about the second and third strokes—the compression and the combustion strokes. Remember, the compression stroke heats the fuel in the compression chamber and the spark plug ignites it. The burning fuel drives the piston down in the combustion stroke delivering power to the machine.

If you’re using high-octane fuel in a low compression engine, then you run into some issues. The compression stroke will heat the high-octane fuel, but not as much as it needs to. That means that when the spark plug fires, it won’t burn as fast as should. It will still push the piston down but not with as much force as it’s supposed to. At best, it means less power. At worst, you’ll have some extra soot in your cylinder.

What About ECU Tunes?

If you use a SuperATV ECU tune, then you know that some of our tunes require higher octane gas than stock. It’s the same engine, so how do we get more power out of higher-octane fuel?

Hand Holding a SuperATV ECU Tuner
SuperATV's tunes makes the spark fire earlier so that we have peak combustion just as the piston begins the downstroke.
It’s all about timing.

With our tunes we make the spark fire earlier so that we have peak combustion just as the piston begins the downstroke. Using 87 octane fuel would result in detonation and knock in this instance. The more we advance the timing of the spark, the higher the octane rating is required. When we get the spark timing just right with an ECU tune and match it with the right octane rating, you get a major boost in power.

Learn more about how SuperATV makes its ECU tunes and clutch kits.

If you don’t use a SuperATV ECU Tune, you’re missing out on big gains without having to put in much work.

What Fuel do I Need?

RZR Turbos take 91 octane fuel. All other Polaris vehicles take 87. Those are Polaris' fuel recommendations. Turbos take higher octane fuel because that’s the whole point of a turbo. They shove more air and fuel into the combustion chamber, essentially turning your low compression engine into a high compression engine. Don’t skimp out on your turbo unless you want to start blowing pistons.

When it comes Can-Am UTVs, Mavericks are the only vehicles that take 91 octane. Everything else takes 87 octane.

When in doubt, check your manual! UTV manufacturers are kind enough to tell you exactly what you’re supposed to use no matter what make or model you have.

Use the Right Octane!

Hopefully we’ve busted some misconceptions for you and now you can save a few bucks every time you fill up if you’re driving a naturally aspirated machine. Next time you’re out on a ride with your friends and the conversation turns to what octane everybody’s running, don’t stick your nose up in the air and say “well actually…” Instead just challenge the guy who only runs 93 octane in his stock machine to a drag race. They’ll see really quick what their extra $10 in the tank gets them.

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