Premium - Regular & the Octane Story Part 1

There is a lot of confusion (particularly on the on-line forums) about grades of fuel and what grade is right for any particular bike or scooter and why.  The following will help to dispel some of the myths about fuel.

FACT 1:

In a few limited cases, using premium grade fuel can improve the performance of your bike or scooter but in most cases it does not.

FACT 2:

In all cases where an engine is designed to run on premium grade fuel, premium fuel should be used as running on lower grade fuel can potentially damage the engine.

FACT 3:

Premium fuels do not necessarily produce more motive power than standard fuels and in some cases when compared in the same engine actually produce less.

FACT 4:

The formulas used in premium fuels are designed to decrease their activation energy so that they ignite less easily and burn in a slower, more controlled manner.

Described variously as Gasoline, Petrol, Benzin, Benzina etc. - depending on where it is sold, motor fuel is made up of organic compounds distilled from crude oil.  The main difference between “Premium” or “Super” grades and what is variously referred to as “Standard”, “Regular” or “Normal” is the octane rating.  Premium fuels have a higher octane rating than standard fuels which means they burn in a more controlled manner and do not as is commonly believed, produce more power.

“Octane” is not (as is commonly believed), some kind of performance additive.  Octanes are in fact a family of hydrocarbons that are present in gasoline and one member of this family, Iso-Octane is used as a measurement benchmark of a fuel’s ability to withstand compression without pre-ignition or detonation which is the cause of the phenomenon known as “engine knock”.  This “detonation” which can usually be is heard as a pinging or knocking sound when the engine is under load is caused by the fuel pre-igniting and exploding in the cylinder rather than burning and expanding in a controlled manner.  Detonation will immediately cause the engine to become inefficient and if allowed to continue will cause major damage, even to the point of melting pistons or breaking connecting rods.

The octane rating of fuel is tested using a specially calibrated laboratory engine to measure the fuel’s performance relative to two of the hydrocarbon compounds, n-heptane which has very low resistance to detonation and iso-octane which has high resistance.  The measurement is calibrated so that an octane rating of zero would be equal to a fuel made up of 100% n-heptane and an octane rating of 100 would equal a fuel made up of 100% iso-octane. 

This does not mean that 100 octane gasoline is pure iso-octane.  It is not.  It is made up of many compounds that when combined, give it that rating and this is why some types of fuel have ratings above 100.

The thing that determines which fuel is right for your bike or scooter’s engine is the actual design of the engine itself and primarily its compression ratio which is the measure of how much the air / fuel mixture is squeezed or “compressed” in the combustion chamber as the mixture ignites.  A range of other factors are also relevant such as the combustion chamber shape, method of fuel delivery (Carburettor or EFI) ignition system design, the type of cooling system and in the case of EFI engines, the software design and level of engine management sophistication.

A more highly tuned engine with a high compression ratio will be more efficient and produce more power, however it will need fuel with a higher octane rating to achieve this without detonation occurring.  While it seems contradictory that some high octane fuels actually have lower energy content by volume and burn less easily than standard gasoline, highly tuned engines need this more stable type fuel to develop maximum power.

So the choice is simple.  Use whatever grade of fuel the manufacturer specifies as this is what the engine is designed for.  Using higher octane fuel will cost more and in almost all cases will not improve performance of your scooter or bike.  Using lower octane fuel than is specified may save you money in the short term but will in all likelihood damage the engine.