Premium Fuel Vs 91 RON Vs Sulphur Content: You need to get this right

In this report, 91 RON petrol versus premium in your engine, and the questionable ‘advice’ you might get from the dealer’s service department. Also, does sulphur content matter?

Download the PODCAST for this report

---

If you’re receiving so-called ‘advice’ from the kid in your dealership’s service department regarding what fuel to use in your engine, this report aims to demystify some of the rubbish that might be flowing from his or her well-intentioned face.

Putting the wrong octane fuel in your tank is always a scary proposition, but it’s much easier on your blood pressure if you understand the basic principles going on regarding RON (octane rating)

Let’s dive straight in, shall we? Here’s the beginning of an essay question put to me recently:

I don’t even think the hot summer’s day claim stacks up. Octane rating is merely a comparative, standardised index of the fuel’s resistance to auto-ignition, AKA uncontrolled combustion. In other words: Fuel needs to bang on command, then blow, after the perfect amount of sucking and squeezing.

Suck, squeeze, bang and blow - it’s how a four-stroke internal combustion engine operates. Fifty times a second, per cylinder, at 6000rpm. And the timing of those bangs must be perfect. 

Any suggestion there’s more energy, a hotter burn, or colder burn, or greater volatility - anything like that - with high-octane fuel is just untrue. Octane is all about resistance to banging (burning) prematurely.

If the carmaker says the engine will run on 91 (that’s roughly 87 octane in American ‘gasoline’), it will. If they say, use 95 minimum, do that. You can run a higher-octane fuel than the manufacturer recommends, without penalty, but not the other way around. Simple. 

There’s a small performance/economy benefit to using higher octane fuels (but not both at the same time because it’s not magic). Just understand that it’s usually not economically rational to do this. It usually costs more for premium fuel than the amount you save.

Stumpy went on, and unfortunately, the smell of his question got worse.

Yeah, and it’s actually got two turbos, too, and knock sensors and a MAF sensor and oxygen sensors. So what? You might convince yourself that it goes substantially better on 98. But that’s just called ‘confirmation bias’.

USEFUL LINKS FOR PETROL, DIESEL AND HYDROGEN

Petrol price hysteria is BS (fuel is cheap) >>

Petrol Vs Temperature: Is heat ripping you off at the pump?

How long does fuel last in your car before it goes bad? >>

Two simple hacks to make your engine last longer >>

Diesel Buyer's Guide: How to buy, own and maintain a modern diesel engine >>

Top 26 questions on Hydrogen Fuel Cell vehicles (FCEVs) answered >>

---

My AutoExpert AFFORDABLE ROADSIDE ASSISTANCE PACKAGE

If you’re sick of paying through the neck for roadside assistance I’ve teamed up with 24/7 to offer AutoExpert readers nationwide roadside assistance from just $69 annually, plus there’s NO JOINING FEE
Full details here >>

---

The rest of Stumpo’s question regards the specific use of premium fuel and whether it does more or less for your engine.

Ultimately, BP (see what I did there?) and Shell, and United and AMPOL all want you to buy premium fuel, regardless of whether you need it, because every drop you buy, they can charge 20 per cent more for. And they’ll do this while telling you it cleans your engine, which is false.

Here’s Stumpo, still spilling the beans:

So, there’s a lot of misinformation to unpack here. Low-hanging fruit first:

Regarding “More stable and higher flashpoint RON98”, there’s nothing unstable about either 91 or 98. As for flashpoint, it’s a temperature; flashpoint is the lowest temperature at which the fuel will evaporate into a combustible mixture and start burning if you apply an ignition source. That’s it.

And for the record, all petrol is an enthusiastic, low-temperature evaporator. Petrol is a volatile liquid, ready to evaporate and go off, because all petrol has a flashpoint of roughly minus 40 degrees Celsius, which is also coincidentally about minus 40 F, owing to the zero points being offset by 32 degrees F.

The flashpoint doesn’t impact a fuel’s operation in an engine. The way a fuel combusts relates to volatility, auto-ignition temperature, activation energy, density, and latent heat of vaporisation. Flashpoint does not.

Flashpoint is more relevant to safe storage protocols and firefighting than actual combustion dynamics.

It’s nice when the atomised fuel evaporates in the chamber, because it lowers the temperature, and increases the density of the mixture. But 91 and 98 perform roughly the same in this instance. Unless the combustion chamber is colder than minus 40, on either scale, flashpoint is not a factor.

Next, all those variables like variable valve timing, variable injector timing, variable ignition timing - there’s hardly any moving parts - so it’s inconceivable that 91 is going to have a reliability/endurance impact.

Variable valve timing is related exclusively to RPM, with more valve overlap at high revs, and less overlap at low revs - it’s independent of the fuel. Combustion signals don’t control variable valve timing; revs do.

Injector timing, well, injectors do their fuel-spurting job once every two revs, per cylinder. They’re tiny, high speed, electrically controlled taps. Most of their job is just holding fuel back. There’s no real difference to an injector running on 91 or 98 - it just opens and closes, letting fuel squirt into the chamber when the computer tells it to. The injector doesn’t give a shit what fuel it’s delivering.

Variable ignition timing (also known as ‘advance’), means spark plugs fire in advance of the piston getting to top-dead-centre on the compression stroke. It’s the ‘Squeeze’ in between ‘suck’ and ‘bang’, and is distanced from ‘blow’. On high octane fuel, the spark occurs further in advance of top dead centre. If the advance gets too big, the engine knocks (also known as pinging). That’s bad. 

This happens far too often if you use a fuel with an octane rating below the manufacturer’s minimum recommendation - so don’t do that. But there’s a whole control system listening for knock and dialling advance back into the safe zone, and all you need to do is use the right minimum octane fuel.

Importantly, there are no moving parts in this ignition timing process - ignition timing is completely computer controlled. There’s no greater demand on any of these systems if you use 91.

Put simply, these systems that adapt inside your engine don’t work any harder on 98 or 91. There’s no operational impact there at all. Just for clarification, we stopped using centrifugal weights in a distributor to manage ignition advance decades ago. For a service mechanic to sell this idea to punters is irresponsible and uninformed, from over the counter in the service department, however well-intentioned it may be.

How does sulphur affect your engine?

Now, let’s talk about sulphur in fuel, and debunk that bullshit as well. As for it being better for your engine, long term, that’s agricultural grade manure.

Australia has disgracefully high sulphur content in 91 RON petrol, because the Federal Government is married to the fuel industry. This is, allegedly, to keep local Australian refineries open, despite the fact they keep closing anyway.

We are a third-world nation on fuel quality. This is objectively true. In fact, some third-world nations might be better.

This non-policy leads to disgracefully high exhaust pollution levels, compared with, say, Europe, which should concern you if you live in an Australian city, because exhaust pollution kills more people prematurely in Australia than car crashes. And certainly a lot more than COVID.

The sulphur makes it hard to control the catalytic conversion of toxic exhaust chemicals. Essentially it limits catalytic converter processes and efficiency. It’s also bad in its own right when released into the air after combustion. If you ever smell that ‘rotten egg gas’ when a car saunters past, that’s a tangible aspect of the sulphur content.

This is just another example of the Federal Government being controlled by industry lobby groups, and putting people like you and me, in the electorate, in hospital. Business first, people last.

But in your engine, operationally, sulphur in the fuel is quite OK. It’s actually a pretty good lubricant for the high-pressure common rail fuel pump. And, yeah, it does contaminate oil somewhat, but if you service your car on time and use the right grade of oil, there’s no discernible reliability impact.

Pro Tip: Nobody’s advocating for lower sulphur to improve engine reliability. They’re doing it for emissions. There is no engine reliability dimension to the sulphur issue. 

The differential diagnosis-type proof of that statement is: there’s no pandemic of 91-RON sulphur induced engine failures. It’s just not happening. If there were, we’d see clapped-out engines everywhere blowing copious blue smoke past their worn out rings.

So, sulphur isn't bad inside your engine. It’s bad inside you.

In closing this one out I’d suggest: Don’t elicit technical advice from your average service advisor in a dealership. If you’re lucky, he (or she) might be a former service technician. And if that’s the case, they can certainly give you great advice on the hands-on stuff, like using the jack, tightening the wheel nuts, checking the fluids, servicing frequency, assessing the condition of your brakes. That vibration and ‘What could it be?’, is all fine.

But they never went to engineering school - and that’s not an insult, it’s just a fact. These people are essential for doing the hands-on work. It’s just the technical nature of combustion is not their forte. Unfortunately, they’re likely to be more opinionated, but just as uninformed as you, on things of that nature.

Have your say