Engineering Explained: High vs Low Octane Petrol

With fuel prices staying fairly low recently, is it worth it to shell out extra cash for some premium petrol? And what do those numbers mean anyway?

Remind me later
Engineering Explained: High vs Low Octane Petrol - DIY

To understand octane ratings, there’s a bit of a process we need to take:

  1. What does a fuel’s octane rating mean?
  2. Why does it matter?
  3. Will road cars perform better with high octane fuel?
  4. Should you use higher octane fuel?

1. Octane rating - explained

The octane rating of a fuel is a measurement used to indicate its resistance to engine knock. A fuel with a higher octane rating will have more resistance to knock. Another way of thinking about this is how much compression that fuel can handle. A higher octane fuel can be compressed (along with air) more without detonating as a result of the heat from compression.

There are various ways of measuring a fuel’s octane level, and different methods are used in different regions. It’s often thought that European fuels have higher octane ratings than American fuels, when in reality we use different systems for measurement. The following video explains the difference:

Fuels can certainly be rated above 100 octane, and racing fuels often are. This is done by altering the composition of the fuel (using ethanol, methanol, nitromethane, leaded fuel, etc) in order to increase the octane rating and make use of higher pressures.

2. Why does it matter?

By using a high octane fuel, engine designers are able to implement various techniques to improve reliability, efficiency, and power output. First, it helps prevent knock. Engine knock, or pinging, occurs when a separate pocket of air-fuel mixture ignites after the spark has ignited the air-fuel mixture within the combustion chamber. This is obviously undesirable as it can cause serious engine damage, and it can be prevented by using the proper octane rated fuel.

Second, higher octane allows for the use of higher compression ratios. Compression ratio is directly linked with thermal efficiency, so the higher the compression ratio, the more efficient the engine will be (law of diminishing returns certainly applies).

This video explains exactly how much efficiency (and performance) can be improved by increasing the compression ratio:

Third, high octane fuels allow for advanced ignition timing. What this means is the spark is fired well before the piston reaches top dead centre on the compression stroke. The reason this is done is because it takes time for the flame front to expand out and combust the air fuel mixture. If you start this process in advance of the piston reaching top dead center, you can increase the maximum pressure the cylinder will have once the air-fuel mixture is fully compressed (and traveling downward), and this will result in increased power. Obviously you do not want to detonate too early, as you’ll be applying pressure in the wrong direction, but if you ignite too late, you’re losing valuable torque.

This video explains how power can be increased through advanced ignition timing. It also shows a Volkswagen with high octane gas fed to two of the cylinders, while the other two have low octane fuel to see the pressure differential created from altered timing:

3. Will road cars perform better with high octane fuel?

The simple answer is: yes, if they are designed to. If a car recommends regular (or low) octane fuel, it is highly unlikely (read: it won’t help) that there will be any performance gain from using a higher octane fuel. If the car does recommend higher octane fuel, it’s likely for a reason, but keep in mind the engineer’s dilemma: If you create a car that must run on 93 AKI fuel, and sell it in markets where 87 AKI fuel is available, someone (many someones) will put 87 in the tank. You don’t want this, so you have to design the engine to be able to run on 87 fuel as well. You can’t change the compression ratio, and you likely have a desired airflow to reach target power levels, so you’re left with ignition timing to alter.

A smart engine that desires high octane fuel for optimum performance will retard the ignition timing to prevent knock when lower octane fuel is used, and advance the timing once it sees more desirable fuel again. An unintelligent engine which desires high octane fuel likely won’t be able to adjust the timing accordingly, and thus will be able to run on both low or high octane fuel rather equally.

So do cars that recommend high octane fuel perform better? This video looks at test results from five different cars:

4. Should you use higher octane fuel?

My advice is to use what is recommended by the manufacturer. The engineers designed the engine with a specific fuel grade in mind, and it simply makes sense to use the fuel grade they recommend. The only scenario where I would consider using higher octane fuel than recommended is if I had a car with significant combustion chamber deposits. Higher octane fuels almost always have higher concentrations of cleansing additives, so running on a tank of high octane fuel every so often can help in cleaning an engine and preventing future build-up.

What do I do personally? My ’99 Acura Integra recommends 87, so all it ever sees is 87. My ’14 Subaru STI recommends 93, so I fill it up with 92 as it’s the highest readily available fuel in my state. Oh, your car is diesel? You didn’t need to read all this… Cetane number is more up your alley.

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