Can High Octane fuel will make a car/motorcycle faster?

‘High Octane’ is a phrase that conjures up images of fast cars. It’s also a phrase that is bandied around by nearly all of the petrol companies in advertising slogans, along with products described as ‘ultimate’, ‘racing’ and ‘extreme’.

So, high-octane petrol must be the fuel of choice if I want my car to go faster… After all, petrol companies refer to ‘improved performance and acceleration’ and racecars use it — it must have a performance advantage!

Not necessarily…

Let’s have a look at what an octane rating is and what it does.

In Australia petrol uses an octane rating called RON (Research Octane Number). The RON number is achieved from the results of lab tests in which the fuel is run through a test engine which varies its compression ratio under controlled conditions.

The results of the test are compared to results for a mixture of iso-octane and n-heptane. For example, if a fuel is found to perform in a similar way to a mixture of 95 per cent iso-octane and 5 per cent n-heptane, it is given the rating of 95 RON.

When a fuel mixture is compressed there is a point at which the extreme heat and pressure will make the fuel explode, or spontaneously combust. This is the process that diesel engines rely on. However, in a petrol engine the point of combustion is controlled and the fuel mixture is ignited by the sparkplug.

Basically a high RON fuel will have a higher resistance to pre-ignition (also called knocking or pinking or pinging). This is because the higher octane rating means that the fuel has a high activation energy. In other words the fuel needs to be subjected to more extreme conditions before an explosion will occur, either by high levels of compression or high engine temperatures.

So will high octane fuel give a car better performance?

Let’s look at the case of a normal road car. Most road cars have a recommended octane number of the fuel which should be used. The compression that takes place in the combustion chamber and the internal engine temperatures while running are suited to work efficiently with the recommended fuel rating.

If a lower octane fuel is used then it’s possible that some problems may occur. Pre-ignition (also known by the terms noted above) can damage the engine internals in the worst cases.

If a higher octane fuel is used what happens? Well, nothing. The engine is already able to perform at its highest efficiency on the recommended fuel. The extra octane rating has no effect.

So why do high performance cars or racing cars use high octane fuels?

This is because their engines are designed to take advantage of the properties of high octane fuels. High-performance road cars tend to make their extra power from generating higher pressures in the combustion chambers of the engine, either by having high compression ratios or by using turbo or superchargers.

To avoid pre-ignition due to the high pressures, high octane fuels are used and, in the case of road cars, recommended by the manufacturers.

What clouds the issue is some of the ‘smarter’ engine management systems used by car manufacturers today. As well as being able to measure when pre-ignition occurs and alter the ignition timing, boost (in turbocharged cars) and other factors to stop it, these engines can also ‘tune’ themselves up to take advantage of the extra octane rating and potentially make more power.

One example of this is Saab’s BioPower turbo fours. By taking advantage of the higher octane number of ethanol rich fuels such as E85 (85 per cent ethanol, 15 per cent petrol), the turbocharged 2.0-litre engine offered in the 9-3 Sport Sedan increases its turbo-boost and tweaks ignition timing automatically to increase its claimed output from 129kW/265Nm on normal fuel to 147kW/300Nm on E85.

Okay, but if the octane rating typically doesn’t improve performance, why do premium fuels claim better power and acceleration? Well, such improvements are possible, not because of the octane rating, but because the chemical composition of the fuel may be capable of producing more power.

It’s a matter of how much ‘bang’ each drop of fuel contains. Looking at the chemistry of the fuels in question, Carbon-to-Carbon bonds actually contain more energy than Carbon-to-Hydrogen bonds. Therefore a fuel which contains more Carbon-to-Carbon bonds will give more power, regardless of the octane rating.

It’s more likely that a premium brand fuel will contain more Carbon-to-Carbon bonds and boast a high octane rating.

So yes… Premium fuel with an octane rating higher than recommended by the car manufacturer may well be able to improve the performance of your car, but it’s not necessarily because of the high octane rating…

The myth ‘High Octane fuel will make a car faster’ is therefore — Busted.