Buying fuel for a snowmobile is a perplexing ritual for some snowmobilers. They pull up to a gas pump and see three fuel octane choices that make them wonder, “Which one should I put in my sled? God forbid, I don’t want to sacrifice power or burn down my sled by not having enough octane. But I’d hate to waste money on premium fuel if my sled doesn’t need it.”
Most modern snowmobile engines are designed to run on 87 octane gasoline blends. Many older machines, especially high-performance models, require premium fuel rated at 92 octane. Some new sleds also require premium fuel, but, fortunately, they often employ technology that takes the risk out of selecting the wrong grade of fuel.
How Octane Is Measured
Octane ratings are not indications of how fast the fuel burns or how much power can be liberated during combustion. Instead, octane is a measure of a fuel’s detonation resistance, or knock control. If detonation isn’t kept under control, the engine will suffer damage.
The octane rating system was set up so a fuel’s knock resistance could be predicted. The worst gasoline, when it comes to causing engine knock, is normal heptane, which has an octane rating of zero. A gasoline with very good anti-knock qualities, iso-octane, has a value of 100. The octane value assigned to all other fuels is based upon how they compare to these two reference fuels.
A test agency uses a special four-stroke engine with an adjustable compression ratio. The test fuel is run in the engine while the compression ratio is increased until the engine starts to knock. That compression ratio is recorded and the engine is drained of the test fuel and pure iso-octane is supplied. The engine is re-started and normal heptane is added to the iso-octane until the engine begins to knock with the same intensity and at the same compression ratio as did the sample fuel.
If the knock intensity is equal at a 90 percent iso-octane, 10 percent normal heptane mixture, for example, the fuel would be assigned an octane rating of 90. This test procedure provides the Research Octane Number (RON) of the fuel.
The Motor Octane Number (MON) test is a more severe test on the fuel blend than the RON test. MON is determined by using the same reference fuels and test apparatus but the test procedure differs from the research method; the fuel mixture is preheated, higher engine speeds are used and ignition timing is varied. As a result, MON values are always lower than the RON values.
The octane number you see on a gas pump is an average of the Research Octane and Motor Octane numbers (RON+MON/2) and best indicates the octane response of the fuel in normal use. This pump octane number is also referred to as the Antiknock Index (AKI).
Gasoline went through changes in the early 1970s when new emissions standards were instituted for automobiles and the need for the use of catalytic converters in the exhaust systems emerged. The tetraethyl lead that had been used to boost the octane rating of gasoline would foul the converters. As a result, the lead additive was phased out, the octane rating of gasoline dropped and the search was on to find new additives to boost the octane of gasoline without having to use high levels of expensive gasolines such as iso-octane.
Prior to the phase-out of tetraethyl lead, you could find honest-to-goodness 100 octane, premium gasoline at almost any station in the country. People had been bombarded with advertising that made them believe higher-octane gasoline made their engines perform better. Northern Minnesota had stations that only sold 100-octane premium gasoline. It was Clark, “the premium people.”
As the U.S. Environmental Protection Agency, auto manufacturers and recreational vehicle manufacturers battled with the new gasoline blends, a new standard for unleaded, regular gasoline was set at 87 octane (RON+MON/2). Since then, manufacturers have calibrated their engines to run on this fuel.
All engines are designed to run on one specific grade of fuel, but modern technology has removed some of the risk of engine damage that poor quality fuel presents for some snowmobiles.
Engine management computers adjust ignition timing and fuel/air ratios. Detonation sensors prevent damage that would otherwise be caused by poor-quality fuel; the sensors can also take advantage of high-octane fuel even if a sled is calibrated to run on the cheap stuff. Starting in the 1990s, some machines were equipped with a switch that allowed a rider to set the ignition timing based on the type of fuel.
When fueling up a sled, buy what the owner’s manual suggests. Don’t choose high-octane fuels because you think a machine will run better or faster. Fuel with a higher octane rating will not improve power in an engine that has been calibrated to run on 87-octane fuel.
If you’re forced to fill up with 87-octane gas but your machine requires more octane, run the sled at slower speeds and keep the engine out of the peak power range. That will keep the cylinder pressures down and you should be able to limp back to the truck or cabin without a problem. If you hear the engine knock, let off the throttle or stop and let the engine cool down.
Factors other than fuel cause detonation. Cold air is dense, which means it has more oxygen per cubic foot than warm air. This will lean the fuel mixture and make detonation at wide-open throttle more likely. Just like when using low-octane fuel, slow down when the air is especially frigid or, if possible, switch to a higher-octane gasoline. Higher altitude will richen the air/fuel mixture because atmospheric pressure drops as you go up in elevation. This means the air has less oxygen per cubic foot than air at sea level; the air/fuel ratio will become richer and that will act like a high-octane fuel. So, it’s OK to use lower octane fuel than recommended at high altitudes.
Ethanol, also called ethyl alcohol, was initially used as a fuel extender. Up to 10 percent ethanol was added to produce what was originally called gasohol. Today the corn-based fuel is used as an octane enhancer and oxygenate in almost all blends of gasoline.
Legislation signed into law by President George H. W. Bush in 1990 required the use of oxygenated fuel or reformulated gasoline (RFG) in some areas of the country. The amendments had other provisions that called for the addition of a detergent that keeps carburetors and fuel injectors clean. The legislation also called for the elimination of lead in any automotive gasoline. Many states, such as Minnesota, reacted to the amendment with laws that require use of certain fuel blends in the state.
The Minneapolis/St. Paul metropolitan area had experienced violations of federal carbon monoxide (CO) standards since the 1970s. The Clean Air Act instituted by Bush required a wintertime program of oxygenated fuel beginning in 1992. Since then, all wintertime gasoline sold in the Twin Cities has contained ethanol and there have been no violations of CO standards since then. Some states still offer 92-octane premium, non-oxygenated gasoline for off-highway vehicles, collector cars and older engines with higher compression pressures.