What is Octane Rating

Have you ever wonder what is the Octane Rating of Gasoline?

Every time we go to a gas pump to fill our car’s tank, we always have the choice of what octane rating we wish to put in. We can also see a sticker on the gas pump with the different ratings available in that station and if living on the USA, will also see the rating calculation method used.

In USA, those ratings vary among 87, 89, 91 or 93 octane. Gas with higher numbers than those, is called “Racing Gas” and (1) it is not commonly seen on standard gas stations and (2) price is considerably higher than standard octane rating gas. But, what about it???

Octane rating is a number to define the property of gasoline to resist detonation by high pressure and high temperature conditions. It does NOT mean that 93 octane gas is way more powerful than 87 octane, like Dynamite is way more powerful than gun powder. Resistance to detonation means; how hard can we push or compress that gasoline in our engines (taking in account temperature) before it detonates without the spark plug firing. When gasoline ignites and goes off without the spark, it is called a detonation. It is also known as “pre-ignition”, “knocking” or “pinging”. The last one is called “pinging” because of the metallic noise it makes which sounds like a ping. The same goes to the term “knocking”.

Your car’s engine is an internal combustion engine and the “combustion” we want is a controlled and gradual explosion or deflagration, not a detonation, which is way faster and more powerful. Detonations at low engine speeds and power, will sound like pinging and may cause no important damage at short, but in the long run, it will cause the engine to fail. At high engine speeds and power, detonations may blow a piston or the engine itself. Detonations provide no usable power to the engine.

The higher the octane rating, the more it will resist detonations from happening. Racing and muscle cars, often produce a lot of compression and elevated temperatures inside their engines and that is why it is normal to use very high octane rating gas on such vehicles.

Why Detonations Occur?

It is not convenient at all that gasoline fires up before it is meant to. It may cause physical damage and a lot of trouble to the performance of the engine. Just imagine the gas/air mixture detonating at the compression stoke, several degrees before the “Top Dead Center” or TDC, which is where the piston stops going up to start going down. That will be an unwanted situation where the piston motion pushes upward and it is still compressing, while the detonation tries to push it downwards at the same time. What happened was that the gas/air mixture gave up to the pressure and temperature put into it and could not wait for the spark to start ignition. At that very moment, the air/gas mixture, instead of gradually expand by combustion to gradually push the piston downwards, all of it went off at the same time, hitting the piston like a hammer instead of pushing it. That is where the ping sound comes from. This situation produces zero power, as the detonation won’t succeed pushing the piston down, primarily because of its short duration, remember, it is like a hammer blow, not a push, and because the piston was not at the position where it starts moving down. It was still going up instead, so more likely the detonation force will try to stop the piston in a period of time of milliseconds. If this is a common situation in a combustion engine where everything else is good and correctly set up, then a higher octane gas is needed.

Modern cars are equipped with knock sensors, programmed to “hear” and distinguish these detonations sounds among all other engine noise. When the sensor detects a knock, the engine controller module has a routine programmed to retard ignition timing several degrees in an attempt to stop detonations, as too much of ignition timing advance is an usual cause of detonations.

Not more power then?

Octane is not a direct measure of gasoline power. All gasoline will have similar combustion reactions (explosive power) depending on its refining quality, additives and cleanliness, regardless of what octane it is rated (87, 89, 91, 93, 104, etc.). The difference will be that lower rating gas (87 and 89 octane) will go to combustion before higher rating gas (91, 93 and higher octane) under the same temperature, pressure and volumetric conditions, without spark, which will cause loss of power in the affected engine.

Octane Rating Calculation Methods

There are various calculation methods, RON, MON, and PON or AKI.

RON: (Research Octane Number). Most common fuel used in Europe, Australia and some other Countries. It is determined with measurements of fuel behavior in a variable compression engine. Results are compared with other iso-octane fuels, wish means, compared with other fuel types with equal number of similar octane molecules present, but not of the same organic compound.

MON: (Motor Octane Number). Determined also with a variable compression engine, but with the fuel already preheated, variable ignition timing and higher RPMs than the RON method. This method is more precise on determining fuel behavior on an loaded engine.

PON or AKI: (Pump Octane Rating or Anti-Knock Index): This is the method used in USA and Canada, expressed as [R+M]/2, wish means RON number plus MON number, divided by 2. In other words, it is the average between both methods above.


Due to the fact that RON is always from 4 to 5 points higher of its equivalent to PON or AKI, the number conversion between USA and Europe octane ratings will approximately be as follows:

USA (PON) –> Europe (RON)

87 –> 91
89 –> 93
91 –> 95
93 –> 98

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3 Responses

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  3. Cordelia says:

    Engineering and technical terminology. For the term octane rating may also exist other definitions and meanings, the meaning and definition indicated above are indicative not be used for medical and legal or special purposes.

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