Vernon Motorsports Fuel Info for Motorcycles, ATVS's & Snowmobiles

This tech tip has been issued to help dispel major misconceptions regarding the use of different gasolines across Canada for gasoline powered products.

The Myths

There are two major misconceptions about gasoline by the general public. Firstly, there is the perception that a higher octane rated fuel will provide more power and increased performance. However, once enough octane is supplied to prevent engine knock, there will be little, if any, performance improvement.

The second myth is that using a higher octane rated fuel will result in an increase in fuel economy. In fact, octane is nothing more than a measure of "anti-knock" quality of the fuel blend. Some fuels may contain other components that actually increase the energy content of the blend, but not as a result of the higher octane rating.

Fuel Grades

Gasoline fuel is commonly supplied in 3 different blends and is mostly sold as Regular, Mid-Grade and Premium (or Bronze, Silver and Gold).

The blends differ by having (primarily) varying levels of octane rating. The fuel is usually rated on the "Anti-knock index" rating (AKI).

This AKI is the measure of the fuel's ability to resist engine nock (sometimes referred to as "ping"). The number is an average of Research Octane Number (RON), and Motor Octane Number (MON) and is calculated by (R+ M)/2. This AKI is (usually) the number that appears on the gas pump and so is sometimes referred to as pump octane.

Engine Octane Number Requirement

The engine in any product is designed for specific duties. These duties may include high power output (sportbikes or snowmobiles), high torgue output (cruiser bikes), long-duty cycle (generatorengines) or high load factors (outboard motors). When the engine is designed and built, the subsequent testing will determine what the "Octane Number Requirement" (ONR) is. The owner's manual will advise the owner of the correct octane level for the engine.

Some factors that determine the octane requirement do not change (such as compression ratio, shape of the combustion chamber and with electronic ignition systems, ignition timing).

Other factors can change, such as ambient temperature, humidity, altitude and air/fuel ratios (in carbureted engines) and may require other settings to be modified or temporary use of higher rating of octane to eliminate knocking.

Engines that have been modified for racing use will usually have increased compression ratio as it is one basic first step to increased power. That is why fuel supplied for racing purposes has a much higher AKI (sometimes as high as 105) than regular fuels. A fuel blended for racing will not give more power, but will allow a race-built engine to realize its potential.

Fuel Aditives

The fuel purchased at the gas station is a complicated mix of products to provide the driver with desirable characteristics.

There are detergents and deposit control oils to keep the engine clean. In cooler climatees, anti-icers are added to prevent fuel line freeze up and there are the lead replacement additives to minimize exhaust valve seat recession.

In recent years, some of the "aromatics" (which are the lighter and more volatile components of oil) which occurred naturally have become depleted from many of the oils that are pumped from the earth.

To meet more stringent air quality requirements and retain good start-ability from the fuel that is refined from the oil, many refineries have to add chemicals to improve the combustion process. These chemicals have significantly reduced the storage life of fuel when it is contained in a breathable container such as a motorcycle fuel tank.

It is very important to advise customers of the effects of running "stale" fuel in their machines. These effects can include misfire, reduced power, incomplete burn and excessive deposits building within the caburetor. (Often the caburetor will become contaminated around the float valve because it is exposed to the air and the valve will become stuck in either a closed or open position, the float will be unable to maintain a precise fuel level in the float bowl. As the fuel level has a direct effect on fuel air ratio, the result can be a fuel mix that is too rich which will wash the cylinder wall of oil and resut in excesssive wear or may be too lean which will elevate combustion temperatures and cause other engine damage).

Fuel can become stale in less than one week under the right conditions of temperature and exposure to air.

So, how do we improve the storage characteristics of the fuel? By reducing the exposure to air and keeping the fuel away from sources of heat. This may mean shutting off the fuel petcock (on motorcycles, snowmobiles and ATV's) or, disconnecting the fuel lines (on outboard motors) and keeping the fuel tank more than 75% full. If possible, closing or plugging the tank breather is an excellent way to extend the storage period. This is easily done on ATV's or modern dirt bikes by clamping off the rubber breather tube on the gas cap. Portable tanks on outboards have a breather screw that should be closed when the engine is not running.

By following these suggestions and adding a fuel stabilizer, the storage life of gasoline can extend up to one year.

Marked Fuel

Fuel that is sold in bulk amounts for farm use or some other designated uses is retailed through special distributors with a reduced tax amount and is therefore cheaper. It is not available to most of the public.

This fuel has a dye added to it at the distributor and is known as "marked fuel". The addition of the dye is in such small quantity that it has virtually no effect on the fuel or the engine with the exception that it may discolor some of the carburetor componenets. But one of the biggest problems that are seen in the marketplace is the poor storage of the marked fuel. The containers (which may hold 200+ gallons) are often above ground and have large breather tubes. The fuel is used slowly (compared to the average busy gas station) and quickly becomes stale.

Oxygenated Fuel

With the clean air requirements across North American, gasoline fuels today are blended with oxygenates to improve emissions.

The most common oxygenates are ethanol (ethyl alcohol) which is an alchohol made from fermented grain and can be mixed with gasoline (known as gasohol) up to a 10% level, and MTBE (methyl tertiary butyl ether) which is an ether from the reaction between methanol and isobutylene and can be mixed with gasoline up to a 15% level.

Adding these oxygenates to fuel aids complete combustion and results in a cleaner burn. But the energy value in these blends is lower than non-oxygenated fuel and may result in a slight (2.5%) reduction in fuel economy. A bonus to using ethanol blended fuel is that the grain alcohol absorbs water and so removes any trace amounts of water in the fuel lines and tank. This means much less chance of fuel line freezing in the winter for your snowmobile or ATV.