All About Octane

Today I’m going to de-mystify fuel octane numbers and what they really mean so that next time you’re at the pump you can make an educated choice for what fuel to put in your car.  When the manufacturer built your car they paid a bunch of test engineers, just like me, to program and tune the engine in your car for a specific octane level of fuel.  They put stickers all over the car too to tell you about which kind to use.  It’s important to pay attention to that and I’ll tell you why.  In the first video below I have 87 octane fuel in my car.  If you listen there is a distinct rasp or rattle which sounds pretty angry between 6750 and 7000 rpm.  In the second video I am using 92 octane fuel and you can’t hear the angry rasp anymore.  What you’re hearing in the first video is engine knock or auto-ignition.

Octane is a measure of how stable the fuel is under extreme pressures and temperature and how resistant it is to spontaneous combustion known as auto-ignition or “knock”.  What is knock? you say.  Knock is another term for the auto-ignition that I talked about in “The Big Bang Theory”, but in this case the ignition is happening before the engine is ready for it.  For gasoline engines knock is a Very Bad Thing.  When the fuel detonates before the engine is ready for it, it actually will increase the pressure inside the engine way beyond what it is designed for and in extreme cases will actually try to force the engine to stop spinning or to spin backwards.  Many times in performance engines which are turbocharged this engine knock will break the pistons inside the engine which will take the smile off your face really quick.

Combatting Poor gasoline Choices

OK so you ignored all of the millions of stickers all over your car that say to use premium gasoline and you used the cheap stuff.  Your car doesn’t make the noise that mine did in the video so you’re OK right?  well, the video is an extreme case and most of the time you can’t hear knock until it’s really bad.  What manufacturers do to handle your uniformed choices is add a knock sensor which can “hear” when knock is happening.  The computer will automatically adjust the ignition/spark timing to lower the heat in the engine and avoid knock and your engine won’t break. 

So what’s the problem then?  Great questions class, I’m glad your paying so much attention. By changing the ignition timing your engine is not operating at the levels of efficiency which it was designed to operate at.  Remember that engines need four things to operate, and that having each in greater quantities will add power and efficiency.  The four things are fuel, air, compression and ignition.  When the engineers who designed your engine were told that they could use 92 octane fuel to tune your car they took as much advantage of that as possible and used a higher compression ratio in your engine.  This allows them to eek the most efficiency out of your engine possible while still maintaining emissions and engine durability.  By using the lower octane fuel you’re making the computer change into a mode which the hardware was not designed for and your fuel economy will suffer. 

In the video below which is trying to sell a tuning tool you can hear what a knock sensor hears while the engine is running.  Note the distinct difference between when the engine is knocking and when it is not.

Why Higher Octane isn’t Always Better

Octane is not a measure of how clean the fuel is or how much energy is in the fuel.  If your car was designed to operate on 87 octane fuel, and the sticker under your gas door says to use unleaded fuel only then there is no reason to use the higher octane stuff.  92 octane fuel does not operate any more efficiently or burn any cleaner in your car than 87 octane will if your car was designed for 87 octane.  Higher octane fuels allow the engineers who made the engine to raise compression and take advantage of the improved fuel stability under heat and pressure, but if the engine compression ratio wasn’t designed for the higher octane then it still ignites at exactly the same time as the 87 octane stuff does.

Where do the Numbers on the Pump Actually Come From?

Octane numbers as presented at the pump are actually “Anti-Knock Index” numbers in North America.  These numbers are an average of two tests which refineries run on fuels before shipping them to gas stations.  That is why the fuel pumps say “Minimum Octane Rating (R+M)/2 Method”.

The “R” stands for the Research Octane Number (RON) method of measuring the octane number in which the fuel is placed into a single cylinder test engine and run at 600 rpm to measure how stable the fuel is.  The stability is measured by changing the compression ratio of the engine until the fuel is auto-ignited by the pressure and heat.  The compression ratio which the fuel ignited at is compared to actual octane fuel (which has a rating of 100 octane, or 100% octane) and n-heptane to determine the fuels octane number.

The “M” stands for the Motor Octane Number (MON) which uses a similar engine as the RON test but runs at 900 rpm and has preheated fuel and variable ignition timing.  The MON result will usually be a bit lower than the RON result in modern fuels.

What is Octane Actually?

Octane is a hydrocarbon with the chemical mixture:


 It is actually only a reference fuel mixture which is used in the RON and MON testing which I described above.    It is possible to have an octane rating which is higher than 100 octane in racing gasoline fuels.

What About Octane Boosters?

Octane boosters are a bottle of fuel additive you can buy at an auto parts store.  Be aware that they are NOT a cost-effective way of boosting your octane.  They will advertise that a bottle will raise your fuel octane level by 5 points, but keep in mind that a point is considered 0.1.  So a 5 point bottle of octane booster will raise your fuel octane from 87 to 87.5.  What a rip-off.

Before emissions regulations made it important to have catalysts on cars you could buy leaded gasoline that used lead as an octane booster.  Lead is a great way to boost octane and it’s also good for the health of your engine because the lead adds a metal coating to all the insides of your engine like an extra protectant layer.  Unfortunately the lead will also coat your exhaust catalyst which makes it no-worky anymore.  Many racing fuels will use lead as an octane booster since most racing divisions do not regulate emissions and require exhaust catalysts.


4 Responses to “All About Octane”

  1. Dad Says:

    Interesting blog. Very knowledgeable. Good work Nate.

  2. Daz Says:

    Many thanks for this enlightenment Nate.

  3. Quora Says:

    Will a driver notice a difference in their performance of his car when using 97 octane petrol compared to his usual 95 octane?…

    In general, unless your car is specifically tuned to use 97 octane petrol, then using 97 octane won’t make a difference. Higher octane allows for the engine designers to design for higher compression ratios, but putting higher octane fuel in a lower o…

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