The truth about Regular, Plus and Super gasoline.

zmyride

High Octane Myths – What Does That Number at the Pump Really Mean?

The truth about Regular, Plus and Super gasoline.

What are octane ratings? Simply put, when you pull into the gas station and are presented with the array of different fuel grades that you can pump into your car, the numbers written above each button or nozzle indicate that particular grade’s octane rating. Typically, the range will run from 87 octane on up to 93 octane.

In the world of automobiles, it is tempting to associate higher numbers with higher quality. After all, more horsepower and more miles per gallon are a better thing, which feeds into our perception that more impressive octane ratings also equal higher performance. Gasoline companies understand this mentality and do nothing to dissuade drivers from it, labeling 87 octane gas ‘Regular,’ and 91 or 93 octane ‘Super’ in an attempt to reinforce this way of thinking. After all, since ‘Super’ is priced higher than ‘Regular,’ it’s in their best interest to convince as many people as they can that paying a bit extra means getting better quality gas.


Unfortunately, this is where the myth of what gasoline’s octane rating really means creeps into the picture. Time and again, people will be told that high octane fuel burns cleaner or more completely, and that it will give them extra power and better fuel mileage than Regular octane gasoline because it contains more ‘energy.’ These blanket statements are simply not true. In fact, the octane rating for gasoline has nothing to do with the amount of power locked inside of it – it actually relates to just how much a fuel can be compressed before igniting. The higher the number, the less likely it is to ignite under pressure.

Why would anyone want to produce a fuel that was actually harder to ignite once inside an engine? To understand the role that octane ratings and ignition pressures play in a motor, it helps to be familiar with the term ‘knock.’ Essentially, when gasoline is sprayed into a cylinder by a fuel injector and mixed with oxygen, engine designers expect it to remain there in vapor form until it is time for the sparkplug to light it up, causing the explosion that drives the piston down to generate horsepower. The timing of this explosion is critical, as gasoline that ignites too early causes ‘knock,’ which reduces engine output and efficiency and which, in worst-case scenarios, can actually physically damage an engine.

In most engines, knock is rarely an issue because the compression ratio – that is, the pressure that the air/fuel mixture is put under in the cylinder – is low enough that Regular gasoline’s octane rating is sufficient. There is absolutely no benefit to running Premium fuel in a standard motor, since it will never be able to take advantage of that gasoline’s higher knock resistance. However, more aggressive engine management schemes, especially those found in turbocharged or supercharged vehicles, can turn up the compression to a high level, requiring much higher octane gasoline to avoid knock.

How can you know whether your car actually requires Super gasoline or whether it can get by on Regular? Almost every vehicle will list the octane rating necessary to run it safely inside the owner’s manual. Some luxury or sportscars might even place a small reminder on the gas gauge itself stating ‘Premium Unleaded fuel only’ to make sure you don’t forget.

To wrap things up: no, you won’t see a power or fuel efficiency increase by running high octane fuel in an engine that has been tuned and designed for Regular gas, nor will doing so perform any extra ‘cleaning’ inside the motor. On the flip side, running low octane fuel in an engine built for Premium can increase the risk of engine damage and will certainly impact that unit’s overall performance. The next time someone tries to pass these myths off on you, you can try explaining to them how octane ratings really work – or you can just smile and nod, pay for your gas and move on.

G35guy84

^^Excellent write-up!

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kannibul

Old news.

j.arnaldo

Is it true that the ECU will advance (or the viceversa, if appropriate), when the octane rating varies, to compensate for the lack of power in the combustion process? Thanks, zmyride! Finally, would downgrading to 87 octanes hurt my 3.5L engine--not that I'm planning on doing it, but...

Modshack

Yes, it is triggered by the knock sensors. If you use a lower than optimal octane the detected knock will retard the engine timing (and consequent performance.)

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Vegitto-kun

well my 370Z demand 98 octane

GT-R is even more insane its all "ONLY UBER DUBER 100 octane plz"

gumpy

I don't know about over there but in Aus, there are cleaning additives added to the higher octane fuel, similar to the techron stuff but in lower doses and i do believe it "cleans" the engine.

We have a car that doesn't detect or need higher octane fuels but when we go for a long drive (4-6 hr drive) in it, we do put the higher octane fuel in to give the system a bit of a clean.

Daishi

Ive heard from many different people even the owner of a gas station that often times all different octane gases are mixed in the tanks anyway so it doesnt matter. for example a truck comes in with 87 octane fills the 87 octane tank and has gas left over.. they will actualy put the remaining 87 octane in the 93 octane tank as to not waste gas. How true is this statement.

theDreamer

That I do not think is true.
What is true though is that many trucks deliver to a number of gas stations, and then they put in the additives of that stations (be it Shell/Exxon/etc.). Very few stations today control from port to station, and even more so it is around 75-80% of stations are independently owned (no control from the brand on the billboard).

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IDZRVIT

Good explanation but the myth of the gas mixture "exploding" is just that. The gas mixture actually goes through a controlled burn expanding the gas from the combustion process through heat which pushes the piston downward on the power stroke.

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kannibul

ECU will not advance for more octane. Only retard for low octane to (hopefully) safeguard the engine from detonation.

kannibul

False.

The only "mixing" that occurs is when you get midgrade. Fuel Trucks deliver 2 grades of fuel, regular and premium. The actual octane depends on the place where they get the fuel from and how well mixed the fuel is (or if it's settled out a bit)

The tag on the pump will tell you if it's rated higher or lower when it was last tested - that said, gas stations know in advance when they are going to be tested, so they can spend a bit "more" on ensuring they get a tank load of 93 stuff in there to dillute the 91 (or worse) gas, and dump some 91 in the 87 tank to bump it up a notch, so they don't fail the test.

kannibul

If the octane isn't high enough for a given compression scenario, it does explode. This is called detonation, and it's heard as "knock"

vReath

i remember watching the local news a few months back on something similar to this. the news channel investigated on what grade is actually going into the underground tanks. they didn't mix the leftover 87 with 93 but just filled the 93 tank with 87. i don't remember how many gallons but there were a quite a few spots in Houston doing this.

Daishi

ok i did speak with someone who used to work at several gas stations and he said that the gas stations are constantly being tested so if the octane is off they can get a huge fine.. now what he said is also true no matter what theres always be some water at the bottom of the fuel station tanks. which is interesting.