What type of petrol should you use?

What type of petrol should you use?

One common question we tend to get a lot from newer car owners is which type of fuel should I pump my car with? With the rising cost of fuel these days, it certainly pays to know what’s worth or not worth spending on.

So the common types of petrol available here are 92 octane, 95 octane and 98 octane, each increasing with price as the octane increases. Now the most straightforward answer to this question is to simply pump the octane recommended by the manufacturer of your car. Pumping anything higher than recommended is simply a waste of money and could even lead to engine inefficiencies. So why do we always tend to hear that higher octane fuels are better?

Compression ratio illustration
Compression ratio illustration

First we need to understand what exactly octane is. Contrary to popular belief, octane is not the amount of “power” contained within the fuel. Octane is simply the fuel’s level of resistance to detonation. So basically, a 92 octane petrol will ignite under compression earlier than a 98 octane petrol would. Now why is this important? Well high performance engines typically have a higher compression ratio. This means that the piston in a high performance engine compresses the air-fuel mixture more than your average engine would. If you were to use too low an octane in such an engine, the higher pressures would cause the fuel to detonate earlier than it should thus resulting in what is called knocking, and can cause serious internal damage to the engine. What we really want is a smooth and controlled combustion of the air-fuel mixture hence the need to use a higher octane fuel in high compression engines in order to prevent knocking.

So what would happen if we were to use a high octane fuel in a low compression engine? Well there’s really no benefit to it. In fact, if the compression ratio of the engine is too low, it could even lead to incomplete burning of the air-fuel mixture and result in inefficiencies. One argument for using high octane petrol on standard engines is that they contain more performance enhancing additives. While this is entirely debatable, it is also largely unfounded as there are regulations for the amount of additives that are added to fuel here. In our completely subjective opinion, we do not feel there is any significant advantage to justify the higher price.

So getting back to the question, our answer is to use whatever is recommended by the car manufacturer as you will be hard pressed to find any performance gains from purchasing a more expensive fuel than required. 

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