Which Petrol is Best?

Vortex, V-power, Ultimate, Bio Ethanol – and the numbers: 91, 95 and 98, plus E10: Confusing, isn’t it? Which fuel should you really tip in your car?

There are four different ‘flavours’ of petrol commonly available – under many different retail brand names, just to add to the confusion. If you get it wrong, you’ll either waste money – or potentially even blow up your engine. So here’s how to get it right.

The numbers are what matters – not the names. 91, 95 and 98. That’s the octane rating. Engines are designed to use fuel with a minimum octane rating. That’s how it works. People get very confused about octane rating – it’s got nothing to do with power, and everything to do with the resistance to combustion.

Hang on a sec. Fuel is supposed to burn, right? So, why would they bother making a fuel that’s hard to burn? That’s like wood you can’t light, for the barbecue. Surely resistance to burning is a bad thing.

Not exactly. Why am I talking to myself? Let’s do thermodynamics in the beer garden. Imagine being a molecule of petrol on your way into the engine. One minute you’re just floating along in a conga line with all your mates. Not that there’s anything wrong with that. You can’t stop the music. Obody can stop the music. Next minute you’re tumbling through the air at warp speed. You get sucked into the engine. It’s violent. Brutal. Chaotic. The valves close, the piston slams up, into you. All of a sudden you’re under the most extreme pressure and the temperature rockets up to hundreds of degrees C. You know what happens next, and you’re not going to like it. You’re gunna wake up used and abused, after they’ve turned you into water and carbon dioxide. And spat you out, onto the road. Those bastards.

In these conditions, in the engine – hot, squeezed, pressure cooker not humming along to the Village People any more – it’s very tempting to go off prematurely. Just to start burning uncontrollably. That’s bad, because the combustion event needs to be very precisely timed in relation to the position of piston and the crankshaft – that’s why there’s a spark plug and a computer controlling the spark event. And this happens 25 times every second, at 3000rpm. So, the fuel needs to resist combustion until just the right moment, within milliseconds, and then it needs to go off – and octane rating is an index of that resistance. It’s absolutely vital.

High octane fuels are more resistant to burning uncontrollably. That’s why they’re commonly tipped into high performance engines, where there’s more compression – and therefore more de facto inducement to burn at the wrong time. It’s just a more combustive environment inside a BMW M3 than inside a Toyota Yaris. Go figure.

Petrol approximates a carbon chemical called octane – eight carbon atoms and 18 hydrogens. But in fact it’s a multi-cultural melting pot, full of twisted freaks of the carbon chemistry world, all different shapes, sizes and varieties – and the overall performance is what really matters. In a sense, being petrol in a fuel tank is like living in a housing commission high-rise in Redfern. Or Hell. Or the Redfern in Hell.

In short, 98 Octane delivers 98 per cent of the combustive resistance of ‘real’ octane. And real octane is actually a highly pure lab chemical called 2, 2, 4, tri-methyl pentane. Try saying that on the second bottle of Verve Cliquot. 95 Octane is 95 per cent as resistive, 91-91 per cent, et cetera.

The first vital thing you need to know is: if your car says ’95 minimum’ – don’t use 91 or E10. If it specifies 98, don’t use 95, 91 or E10 – you’ll get that uncontrolled burning thing, which is also called pinging. Or ‘pinking’. (It’s semantically promiscuous. But always bad.) And if that happens at high revs and big throttle openings it can burn the valves and melt the pistons. That’s pretty much the thermodynamic equivalent of dogs and cats living together. Mechanical armageddon, complete with a repair bill you can’t jump over.

If you go the other way – and use a fuel with a rating that’s too high, you can relax a bit: all you’re doing is wasting money. You won’t hurt your engine by using 98 or 95 in an engine designed for 91. And it’s okay to tip 98 into an engine that specifies 95. It might even go a little, tiny bit better if you do that, depending on the technology in the engine control system. So that’s up to you, if you want to waste a little money.

The elephant in the room is ethanol. E10 is a blend of 10 per cent ethanol in petrol. The good points are: the ethanol is renewable, and it’s produced here, so that’s good for our national energy security. Unfortunately, there’s about three per cent less energy in E10 than there is in regular petrol, so fuel consumption goes up. A full tank of E10 has about three per cent less cruising range in it than a full tank of petrol. So if E10 isn’t at least three per cent cheaper, you’re getting ripped off. At the $1.50 a litre price-point, E10 needs to be about five cents cheaper to be fair value.

Even more of a worry, the ethanol is hydroscopic. That means it absorbs water out of the air while it’s being stored in those big, fat, underground tanks at the servo. Especially in summer, when the humidity is through the roof. And when the water gets into your engine it can cause it to run badly – by doing things like hesitating off the mark, running rough at idle, or stalling at the lights. Not fun. If that happens to you, it’s likely you’d hustle off to the mechanic for a tune-up. All you really need to do is run pure petrol for the next few re-fills and see if normal programming is resumed.

And finally, E10’s not really compatible with older cars. You need to be really careful about that. The ethanol can start dissolving the materials in the fuel system, which then flow downstream and block the fuel injectors … kind of like the automotive equivalent of arterial plaque causing a coronary. Once again, a bill you can’t jump over. That’s bad. Check with the manufacturer if your owner’s handbook doesn’t spell out that E10 is OK in your car. Don’t just wing it, and hope for the best.

Basically, stick to the manufacturer’s recommendations across the board, and you won’t get burned – either literally or fiscally.