Tech - July 2015

93 or 95? Octane ratings and how they work

I’m sure you are all familiar with the drink “Coca-Cola”. This is probably one of the most popular drinks worldwide and the most widely known. You also get others brands of this product, usually supermarkets own brands with names such as “Cola” in italic text. You see, side-by-side these two drinks are the same colour; they look the same and probably smell pretty similar. But there are two differences, the taste and the price.

Unfortunately here in South Africa, our grade of fuel is like the knock-off Coca-Cola. Why do I say this? Well the most readily available Unleaded fuel here in South Africa has a 95 octane rating, but in the UK for example, 98 octane rated fuel is as easily accessible as water and 99 octane rated fuel is also widely available.

Octane this, Octane that… what am I on about?

Well octane is something that was previously used in fuels to stop pre-detonation in engines. This is when the fuel ignites before the piston has reached the top of cylinder. For example imagine your engine screaming at 8000 rpm, that means your pistons are going up and down around 133 times a second. Every second time the piston reaches the top of the cylinder, it compresses fuel and air, a spark is added and an explosion happens. Forcing the piston back down, creating power.

 Now imagine as the piston is on its way back up the cylinder, compressing the fuel and air mixutre but suddenly, the mixture ignites and explodes before the piston resches anything close to TDC (Top Dead Centre). So now you have a piston moving at an incredible speed in an upwards direction, going against a powerful explosion pushing in the opposite direction,   at a point when it was not supposed to happen. In short, bad things can happen. Octane makes Petrol/unleaded fuel more stable and reduces the risk of the above mentioned situation happening. Octane is not used any more, so fuel companies use additives that do the same sort of thing as octane, hence the octane rating.

How does this affect our engines?

Higher octane rated fuels are more stable, reducing the risk of pre-detonating. In turn, this allows our engines to produce more power, have more efficiency and run cleaner. On an average vehicle, the positive effects may be minimal, you may notice a slightly better fuel economy and a smoother power delivery. If you drive a fire breathing turbocharged monster, the effects will be more prominent as your engine is more likely to pre-detonate, causing the ECU to retard timing and stop your engine blowing itself up, which is great. The problem is that when that happens. you lose power and no-one wants to lose power.

I bring good news. There is a hope for us driving our beautifully high tuned performance cars here in SA. Here are three options that I know of:

1: Octane Booster.

Octane booster is a chemical, which you add to the fuel when filling up, it does as the name says, boosts your octane rating. This stuff is not great for your engine; it can block things like filters and injectors.

2:  AVGAS

Sounds good, doesn’t it? AVGAS is basically aviation fuel. Lean, it has an octane rating of 102.5. Mix it in with standard fuel for decent increase in the octane rating. This is a metal based fuel and isn’t great for your car either.

3:  Unleaded 98 race fuel

This stuff really is the bomb. It is sold by fuel distributors such as BP and Sasol. It’s as clear as the blue ocean and it is rated to 98 octanes.  This is much better than the fuel you get from the pump, but you will pay the price.

Having tried all three, I can really only recommend the third option as it will have no adverse affects on your engine and components. It’s much higher quality than what you buy at the pumps, but like Coca Cola, it has a higher price too. I’m hoping one day South Africa will make the move to higher grade fuels; even just giving us the option would be great.

So next time when the petrol attendant asks you for 93 or 95, in most cars 93 won’t hurt, but 95 is better. If you drive a high-performance vehicle, 95 is always your best bet.