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High-octane fuel: Will I get better mileage?

By Daniel Gardner, 04 Apr 2017 Car Advice

Octane rating

Low octane fuel may be cheaper but it might not be best for your engine or wallet

With so many different grades of petrol to choose from and virtually no information explaining why 98 RON fuel costs about 20 cents per litre more than 91 RON, why would anyone top up with the most expensive option?

The RON or Research Octane Number indicates the octane rating of the petrol and the fuel’s ability to withstand a combustion anomaly known as detonation. The mechanics of what is happening during detonation do not need explaining for the purposes of this summary any more than to say it is bad. Really bad, so let’s try to avoid it.

 | What is high octane fuel?

The higher the RON rating, the greater its stability in more highly-strung, high-performance engines, but are you risking damage to your engine by filling up with the cheapest option?

The good news is that if your engine has electronic fuel injection, it is likely to be fitted with a system that can detect the dreaded detonation (sometimes referred to as knocking, pinging or pinking) and instantly pulls an ignition trick called retardation.

Retarding the ignition point puts your engine back in the safe zone but, as a general rule, engines are more efficient and powerful with the ignition point as advanced as possible. Modern engines are constantly trying to find the fine line between the earliest ignition point without causing detonation.

By filling up with higher octane fuel you might be giving your engine a helping hand to work more efficiently, but it is not a silver bullet for all motors.

 | petrol vs diesel

 There is certainly no point filling up a stock standard 1980s Ford Falcon with premium fuel unless it has been re-tuned to run on high-quality petrol and even then, it’s likely to return little benefit. However, a highly-strung, turbocharged engine such as the 2.0-litre that powers Mercedes-AMG’s A45 with 280kW will certainly thank its owner with maximum performance when on the gas and improved efficiency when cruising.

Each vehicle model has a recommended minimum octane requirement – often printed somewhere near the fuel filler cap – and owners should not fill up with a fuel grade lower than this, but a higher grade may offer advantages.

The most reliable way to find out if you could be going further on a tank by filling up with 98 or higher is to conduct your own real world test.

Make a note of how far a fill of the cheapest option takes you using your normal driving style and habits then compare the results to a tank of higher grade fuel during the same typical conditions. You might be surprised at the results.

 | How far can I drive with the reserve light on?

Want more?

One of the most effective octane boosters is a fuel additive called Tetraethyl lead (TEL). The chemical is a highly effective detonation suppressant but is also very toxic – for you and the planet – which is why so-called ‘leaded’ fuels were banned in 2002.

Alternatively you could try a similar stunt as employed by Formula 1 teams during the 1980’s turbo era. To get the huge power figures (up to 1000kW from just 1.5-litres) massive boost pressures required serious detonation suppressants and the motors ran on a blend of 86 percent toluene.

But we don’t recommend this 121 RON fuel for your Toyota Corolla due to its nasty health effects. Find out the most economical pump petrol for your car and stick to it.