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Understanding fuel octane

Fuel octane rating for petrol (or gasoline) engines is a crucial factor in maximising engine performance and efficiency. This seemingly simple number on the fuel pump is a key indicator of a fuel’s ability to resist knocking or pre-detonation during combustion, which is vital for maintaining engine health and achieving peak performance.

Understanding the octane rating is essential when tuning an engine, as it directly affects how the engine can be calibrated and how it will perform under various conditions.

Now, I’m going to guide you through the basics and the no-so basics of fuel octane ratings, exploring its chemistry, its impact on engine performance, and its critical role in engine tuning.

gas petrol pump station
© Ekaterina Belinskaya

What is octane rating?

One of the main focus points of any discussion about fuel and engine performance is the concept of the octane rating. But what exactly does this term mean? Essentially, an octane rating measures a fuel’s ability to resist “knocking” or “pinging” during combustion.

Knocking is the premature ignition of the fuel-air mixture in the engine cylinder. This knocking can be damaging to the engine over time, or if substantial enough, fatal for the engine, due to detonation.

The history of the octane rating system dates back to the early 20th century when engineers wanted to improve engine efficiency and performance. They discovered that certain fuels were more resistant to knocking, leading to the development of the octane scale. This scale has two primary measurements: the Research Octane Number (RON) and the Motor Octane Number (MON), with the Anti-Knock Index (AKI) commonly used in some countries, like the United States, which is the average of RON and MON.

RON is measured under controlled conditions at a lower engine speed, reflecting the fuel’s performance under mild conditions. MON, on the other hand, assesses the fuel’s resistance to knocking at higher engine speeds and under load, providing insight into its performance under more demanding conditions. The distinction between these numbers is crucial, as it highlights the fuel’s versatility and reliability across a range of operating scenarios.

Understanding these ratings is vital to choosing the right fuel for your vehicle, especially when performance and the reliability of your engine are top priorities.

chemistry liquid close up fuel petrol gasoline
© Pixabay

Pump fuel vs. race fuel chemistry

When it comes to high-performance engines, it is essential to understand the chemical differences between standard pump fuel and race fuel.

Pump fuel, the type most vehicles run on, is designed for everyday use, balancing efficiency, cost, and environmental considerations. Its composition includes a mix of hydrocarbons, ethanol (in some blends), and various additives designed to clean the engine and reduce emissions.

Whereas race fuel is a different recipe altogether. Formulated specifically for high-performance and racing vehicles, it contains a higher concentration of octane-boosting ingredients and often includes additives that ordinary pump fuel does not. These additives can improve combustion efficiency and protect the engine under extreme conditions. Furthermore, race fuels may have a specific blend of hydrocarbons to promote faster, more controlled and consistent combustion.

The difference in chemical composition between pump and race fuels significantly impacts a vehicle’s performance. Race fuels, with their high octane and special additives, allow for more aggressive tuning of the engine, increasing power output without the risk of knocking. This makes them essential in competitive racing and high-performance tuning.

Pump fuel and race fuel are designed for different purposes, and thus, they contain different blends of additives to meet their specific performance requirements. Here’s an overview of the types of additives commonly found in each:

Pump Fuel Additives

Pump fuel, intended for daily driving contains additives that focus on engine cleanliness, fuel stability, and emissions reduction. These include:

  • Detergents: To keep fuel injectors and intake valves clean by preventing deposits.
  • Antioxidants: To prevent fuel oxidation and degradation.
  • Corrosion Inhibitors: To protect the fuel system from rust and corrosion.
  • Ethanol: Acts as an oxygenate to help burn fuel more completely, reducing emissions like carbon monoxide.
  • Octane Boosters: To prevent engine knocking and allow for smoother engine operation.

Race fuel additives

Race fuel is formulated for high performance and often contains higher concentrations of certain additives to enhance power, reduce knock, and withstand the extreme conditions of racing. These additives include:

  • High-Octane Boosters: To significantly raise the fuel’s octane rating, allowing for advanced ignition timing and higher compression ratios without causing detonation.
  • Metal Deactivators: To stabilise fuel by neutralising metals that can accelerate fuel degradation under high temperatures.
  • Oxygenates: Such as MTBE (Methyl Tertiary Butyl Ether) or ethanol, to increase the oxygen content in the fuel for more complete combustion, resulting in more power.
  • Lead Compounds (in some racing fuels): Tetraethyl lead (TEL) is still used in certain types of race fuel (though banned in road fuels) to increase octane levels.
  • Speciality Detergents and Deposit Control Additives: These are formulated to handle high combustion temperatures and prevent deposit build-up under high-load conditions.

Both pump and race fuels use detergents and corrosion inhibitors, but the specific formulations and concentrations of these additives differ significantly.

Race fuels may also contain exclusive additives that are not found in pump fuels, such as lead, due to the specialised performance requirements and the fact that race engines often operate under much more intense conditions than everyday vehicles. Additionally, some additives present in street-legal fuels, such as ethanol, serve dual purposes in race fuels but at different concentrations or in combination with other high-performance additives.

internal combustion engine knock preignition diagram
© General Chemistry: Principles, Patterns, and Applications, 2011

How fuel affects an engine's performance

The importance of the relationship between fuel and engine performance is often overlooked. Our engines rely on consistent and quality fuel for the controlled explosion of the fuel-air mixture to generate power. If this mixture ignites prematurely, it can cause knocking, reducing efficiency and potentially damaging the engine. The octane rating of a fuel is directly linked to its ability to prevent these premature ignitions.

Pre-ignition and knock are critical issues mitigated by the proper use of high-octane fuel.  High-octane fuels resist these untimely ignitions, safeguarding engine integrity and enhancing performance. The diagram above highlights the importance of octane in preventing these negative events, which is crucial for optimal engine tuning and longevity.

Just so we’re clear on the differences between Pre-ignition and knock, let me explain the differences:


This occurs when the fuel-air mixture ignites prematurely before the spark from the spark plug, often due to hot spots within the combustion chamber. This early ignition can cause a significant increase in cylinder pressure ahead of the engine's normal combustion phase, leading to potential engine damage.


Engine knock or detonation occurs when the fuel-air mixture ignites prematurely due to extreme pressure and heat during the engine's compression stroke, but after some initial proper ignition. The flame front from the spark-ignited combustion collides with the flame front from the auto-ignition, causing a knocking noise. Knock is harmful to engine components as it creates a sharp increase in pressure and can lead to severe engine damage.

Engines with high compression ratios or those equipped with turbochargers or superchargers require fuels with a higher octane rating. The increased pressure and temperature within these engines’ cylinders make them more susceptible to knocking. By using a higher-octane fuel, the risk of damaging your engine is significantly reduced, allowing the engine to operate more smoothly and efficiently.

Tuning for optimal performance

In the context of tuning and modifications, the choice of fuel becomes even more critical. This is not only about choosing premium over regular; it’s about understanding the implications of using the wrong fuel and the benefits of the right one. Tuners adjust engine parameters to squeeze out maximum performance, often increasing the stress on the engine. The right fuel can make the difference between achieving a new level of performance and causing serious engine damage. High-octane fuels allow tuners to push the engine closer to its limits by advancing ignition timing and increasing boost pressure in turbocharged engines without the fear of knocking or detonation.

When you get your engine tuned, that is the configuration file adaptations will be made to suit. Filling your tank full of fuel with an appropriate octane rating before tuning is crucial. Using a fuel with a lower octane will only limit the potential of the engine. If you decide to use premium fuel after tuning, it will be an unnecessary expense without tangible performance benefits.

Don’t go and do the opposite either. Don’t use premium fuel prior to tuning and then go back to a lower octane. You won’t fool the ECU; it will pull ignition timing, limiting the performance of your engine and you will do damage if you push it under high load. Before you get your car tuned, don’t just fill up with the highest grade possible, ensure you fill up with a grade that is readily available in your area, not a fuel that appears once in a blue moon or is from a pail or drum, or in other words, race fuel.

If you ever find you are running low on fuel and need to fill up, but the fuel station doesn’t have the grade you need, use as little of the lower-grade fuel as possible until you can get a refill somewhere else. This is when it helps to keep an octane booster in your car!

If you’ve had your car tuned and have any questions about fuel, consult with your engine tuner.

volkswagen vw mk5 golf r32 blue rolling road dyno
© Simon Cousins

Can you gain more power just from using a higher-octane fuel?

Simply using a higher-octane fuel does not automatically result in an increase in power for all engines. Remember, the octane rating of fuel is a measure of its resistance to knocking not its energy content. High octane fuel is beneficial in preventing knock in high-compression engines, turbocharged engines, or engines that are designed or tuned for high performance. These engines can take advantage of the higher-octane fuel to run more aggressive ignition timing or higher boost levels, which can lead to an increase in power.

For standard engines that are not designed to run on high-octane fuel, using it won’t necessarily improve performance. These engines are typically optimised to run on regular octane fuel, and they will not benefit from higher octane fuels because their ignition timing and fuel mapping are not designed to utilise the anti-knock properties of high-octane fuel.

In some modern vehicles, the engine management systems can adjust to take advantage of higher-octane fuel to a certain extent, leading to slight improvements in performance or efficiency. However, the gains might not always justify the extra cost of the fuel, unless you’re specifically tuning the engine to take advantage of the higher-octane rating.

The more sophisticated engine management systems that can take advantage of higher-octane fuels typically feature sophisticated engine management systems with knock sensors, flex-fuel sensors and adaptive learning capabilities. These systems can detect the type of fuel used and fine-tune the engine’s timing and air-to-fuel ratio to match.

While flex-fuel vehicles are designed to run on ethanol blends up to E85 (85% ethanol), they aren’t the only ones that can benefit from high octane levels. Ethanol has a higher octane rating than petrol, and vehicles equipped with a flex-fuel sensor can adjust engine operation to utilise this. However, even non-flex-fuel vehicles with advanced engine control systems can adapt to different octane ratings to a certain extent, but will not necessarily operate at their best.

Nevertheless, the extent of performance enhancement can vary significantly by vehicle, and some might see no benefit, especially if not designed with these adaptive technologies. It is always best to adhere to the manufacturer’s recommendations for the ideal octane rating for your specific vehicle unless it is tuned to suit.

The bottom of the tank

Understanding fuel octane ratings and engine tuning can seem daunting at first. However, even with a basic understanding of how fuel chemistry can affect engine performance, you can make informed decisions, ensuring your car not only performs at its best but also avoid causing damage from using the wrong fuel. The octane rating is more than just a number on the fuel pump, it’s a direct correlation to your engine running at its full potential, making every drop of fuel count towards achieving performance.

Key points to take away:

  • If your car is stock, stick to the manufacturer-recommended fuel octane rating for your particular car.
  • When you are considering which fuel to use when getting your car tuned, use a high-octane fuel grade that is readily available in your area.
  • Once your car is tuned to a specific fuel, try to stick to using that same fuel, unless you have flex-fuel capabilities.
  • When you cannot get your fuel grade and need to refill, don’t get a full tank, do your best to refill with as little as possible of the lower octane fuel and be gentle with your right foot.
  • Keep an octane booster in your car for when you can’t get your desired fuel grade.

Remember, choosing the right fuel for your vehicle is an investment in its performance, longevity, and your driving enjoyment. So, take the time to understand your vehicle’s needs, explore the options available, and fuel your passion for driving with every tank.

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