What does 'Octane' mean?

Firstly, keep in mind that a gasoline’s octane rating is simply a measurement of the fuel’s ability to resist engine knocking. It does not refer to a substance or to the quantity of energy or power in the fuel. More correctly, an octane rating is often called an “anti-knock index”.

When unburned gasoline vapours spontaneously explode in the cylinder before the expanding flame in the combustion cylinder reaches them, it actually causes two simultaneous explosions (the other is from the spark plug). This results in a knocking or pinging sound, and when an engine knocks the result is a reduction in the power it delivers.

The higher a fuel's octane number, the higher its resistance to engine knock.

Technically there are three different "octane numbers" associated with every gasoline. The Research Octane Number, or RON, is measured under fairly easy test conditions. The Motor Octane Number, or MON, is a tougher test measured at higher engine speed and temperature.

The value that relates most closely to actual driving conditions is the average of these two values: Road Octane Number = (RON + MON)/2. This Road Octane value is the one referred to in Shell stations: Shell Bronze gasoline has an octane rating of 87, Shell Silver is 89 and Shell Optimax Gold is 91.

Occasionally, less scrupulous Canadian gasoline outlets will use the confusion of these different octane measurements to exaggerate their octane rating claims, by advertising their fuel's Research Octane Number - which will be higher than the Road Octane Number. It is also a common practice in many European countries to advertise the Research Octane Number on their pumps, so you may see unexpectedly high octane values when travelling abroad. In Canada, motorists should always be sure that the octane number a vendor advertises is its Road Octane value, not its RON.

Engines in vehicles built for sale in North America are designed to a specified octane requirement to make sure they don't knock or ping (engine knocking reduces the amount of power it can deliver to turn the wheels). Once that octane level has been met, in normal instances your car will not experience more power or better mileage if you use a higher octane fuel.

In other words, if your Canadian vehicle owner's manual specifies an octane rating of 87, running on gasoline with an octane rating of 89 or 91 or more will normally not make a difference.

However, there are some exceptions to this, and it must be noted that if the owner’s manual specifies using gasoline with an octane rating of 89 or 91, then that’s what you should use because that’s what the engine is designed and tuned for.

One - and a very important - exception is that when a vehicle gets older, the normal build-up of fuel- and lubricant-related deposits in the engine can increase the fuel octane number a car requires to prevent engine knock. For this reason, if a car more than a couple of years old experiences engine knocking, the problem may be solved simply by moving to the gasoline with the next-higher anti-knock index.

You will also benefit from a higher octane rating if:

* the engine is knocking on the gasoline you are using, or
* you have knock sensor ignition that can take advantage of higher octane values at times of peak demand, or
* high summer temperatures, mountain driving, pulling heavy loads (including loaded roof racks) has temporarily increased your engine’s anti-knock index requirements.

While fuel with a higher octane rating cannot of itself deliver more power, in the above circumstances it can give an engine a greater ability to resist knocking, thereby helping it to deliver the full measure of power it is designed and tuned to provide.

In these cases, where drivers require their vehicles to continue providing peak performance despite challenging driving conditions, for improved responsiveness we recommend using a mid-grade fuel like Shell Silver (octane rating 89) or a premium fuel like Shell Optimax Gold (octane rating 91) gasolines.

One other exception is found in high-performance vehicles. The use of more complex computer algorithms in their engine control systems to control spark timing using one or more knock sensors, enables better performance on higher-octane fuel. Owner’s Manuals for most of these vehicles will specify use of 91 octane fuel.