Octane Ratings – What do they mean and what should you use in your car?

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93 or 95, we explain what the numbers mean and which you should use in your car.
93 or 95, we explain what the numbers mean and which you should use in your car.

At the beginning of the month, just after pay-day, you’ve probably filled up your car with a healthy dose of 95 octane petrol, and as the month wears on and your wallet gets thinner, you’ve migrated to 93; after all it’s cheaper, it saves you a couple of Rand, so why not?  But what exactly do those numbers mean?  How does 93 octane petrol differ from 95 octane?  Which is more beneficial and which should you be putting in your car?

What is an ‘Octane Rating’?

An octane rating is defined as the standard measure of the performance of a motor fuel.  The higher the octane rating, the more pressure the fuel can withstand within the compression stroke of an engine prior to igniting.  As such, fuels with a higher octane rating (95, 98 etc.) are higher performance fuels than lower rated fuels (93).

Simply put: The octane rating is the guide to how much compression a particular fuel can withstand within an engine’s compression stroke.

How does 93 differ from 95?

One of the strokes of typical internal combustion petrol engines (4-stroke engine), is compression.  During this stroke, air and fuel are compressed within the cylinder prior to being ignited by the spark plug.  The volume of uncompressed air in relation to its compressed volume within a cylinder is known as the compression ratio.  Fuel can only withstand a certain amount of pressure before it self-ignites, without there being a spark present from the spark plugs.

93 octane fuel is a low-compression fuel, and as such can only withstand lower compression thresholds before it self-ignites.  95 and 98 octane fuel can withstand higher amounts of compression respectively without combusting.  93 is therefore ideal for lower compression ratio engines (generally low performance engines with a *ratio below 8.5:1), whereas 95 and 98 (*Ratios between 8.5:1 and 10:1) are more stable and suitable for high compression engines (often higher performance engines).

*Compression ratios given are guideline figures; always refer to vehicle ownership manuals for correct fuel requirements.

What fuel should you put in your car?

The simple, and most correct, answer to this is to refer to your manufacturer’s recommendation in the owner’s manual – which you receive when buying the car.

However, general guidelines would be that if you car has a high compression ratio, above 8.5:1 you should use 95, and if your car has a compression ratio below 8.5:1 you should use 93.  But, you might not know what the compression ratio of your car is.  If this is the case, and your owner’s manual is nowhere to be found, the general rule of thumb is that performance engines require higher octane fuel.

Some manufacturers will recommend multiple octane ratings for different altitude locations (Usually on naturally aspirated engines as they suffer from reduced air densities).  Generally the recommendation will be for a higher octane fuel at sea level, and a lower octane rated fuel above altitudes of 1200m.

Turbocharged engines should always use at least 95 octane rated fuels, as these engines are subjected to higher stresses than a naturally aspirated engine.  This is as a result of the high starting pressure within the cylinders caused by the turbocharged setup.  For example, the Mercedes-Benz A45 AMG may only have a compression ratio of only 8.6:1, yet the manufacturer recommends high octane fuel (98) due to the starting pressure within the cylinders.

In South Africa, where 98 octane is seldom found, 95 is acceptable, however the engine doesn’t run at its full potential, and in local conditions cars such as the A45 AMG and other high performance vehicles will seldom match the figures achieved by their foreign counterparts.

What are the possible effects of using the incorrect fuel for your car?

Using lower octane fuel than recommended – In a regular engine, the air-fuel mixture is heated by the compression stroke of the combustion cycle.  When the spark plug activates, it triggers a ‘slow’ (The term slow is used relatively as the process takes fractions of a second) burning process of the air-fuel mixture.  If the fuel used has a lower octane level than required, the heat generated by compression may cause the air-fuel mixture to either explode or self-ignite before the ignition system sparks.  The pressures generated by this are much higher than the engine components are designed to withstand, and cause a phenomenon known as ‘knocking’ or ‘pinging’.  Engine knocking, if severe or prolonged, can cause major engine damage.

Using higher octane fuel than recommended – Using a higher octane fuel than recommended will generally have no adverse or favourable effects on your car that result from the octane rating alone.  It is widely believed that using higher octane fuels will give you greater performance benefits, and there are cases where 95 might result in slightly better fuel consumption figures, however, this is not directly linked to the octane rating so much as it’s related to the density of the fuel itself and its potential energy to be released once ignited.  However, at an extra cost, there is little advantage to using premium fuel over the lower grade recommended.

Conclusion:

Next time you stop at a petrol station to fill up, make sure you ask for the correct fuel for your vehicle.  It’ll either save you a bit of cash at every fill-up, or it might just save your engine and unlock a world of performance potential you never knew you were missing out on.

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