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Filling up On Gas Knowledge: Understanding the Difference Between Gasoline Octane Grades

Jul 28, 2016

gasoline octane grades


When getting a new car there is always a stress level as to what type of gasoline you should put in it.  Most people have pretty big misconceptions about the difference between each grade of gasoline based on industry myths but we’re going to clear that up for you.  


Most gas stations offer three choices: Regular, Mid-Grade and Premium.  Some people get really confused.  Car manufacturers always suggest a certain type or grade for your vehicle, and in general, it sort of seems obvious.  The gasoline that has the fancy words - plus, premium or super and that costs more money is the better gasoline right?  People who have luxury cars and sports cars use premium so it must be better right?  If you truly love your car, you’ll buy her top shelf right?  Maybe.  But what’s more expensive isn’t always what’s best for your car.   


Under conditions of extreme heat and pressure, the conditions you find in, well, the combustion chamber, gasoline can…  you guessed it… combust.  Prematurely.  When this happens, it’s called detonation or pre-ignition. Diesel engines rely on this process because they don’t have a spark plug in the traditional sense of the word. Nothing to cause that initial ignition.  However in petrol engines, when this happens (also known as dieseling), and it’s a very bad thing. Engines are designed to have the fuel-air mix burn at a fixed point in the cycle, not explode randomly.  Well, its really not exploding so much as it’s burring up really quickly.  Detonation, dieseling or pre-ignition are all terms for what happens when the fuel-air mix spontaneously explodes rather than burning. Normally this happens when the mixture is all fouled up, and the engine is running hot. The temperature and pressure build up too quickly in the combustion chamber and before the piston can reach the top of its travel, the mixture explodes. This explosion tries to counteract the advancing piston and puts an enormous amount of stress on the piston, the cylinder walls and the connecting rod. From the outside of the engine, you’ll hear it as a knocking or pinging sound.


Octane Rating:


What is an Octane rating?  Octane ratings are the numbers you see next to each grade.  A higher octane rating allows an engine to use a compression ratio of, say, 12-to-one instead of a more usual ten-to-one.  They depict the measure of how evenly a gasoline will burn under different conditions like acceleration.  Basically the fuel’s resistant to causing knocking in your engine.  Something you defintely want to avoid.  Ideally, the vaporized gasoline inside an engine’s cylinder burns by the propagation of a wave of flame, ignited by the cylinder’s spark plug. This allows a smooth transfer of power to the engine’s crankshaft and the car’s wheels. But at higher pressures or temperatures, small pockets of gasoline vapor can prematurely explode, or self-ignite, creating a distinctive “knocking” sound, as well as potentially destructive shock waves.  With high octane gasoline, the greater the compression, the higher the temperature within the combustion chamber. The higher the temperature, the greater the thermal efficiency and power produced. In a nutshell, high-compression engines designed for performance need high-octane petrol.  



Just so you know, the octane number is actually an imprecise measure of the maximum compression ratio at which a particular fuel can be burned in an engine without detonation. There are actually two numbers - RON (Research octane number) and MON (Motor Octane Number). The RON simulates fuel performance under low severity engine operation. The MON simulates more severe operation that might be incurred at high speed or high load and can be as much as 10 points lower than the RON. In Europe, what you'll see on the petrol pumps is the RON. However, in America, what you'll see on the petrol pump is usually the "mean" octane number - notified as (R+M)/2 - the average of both the RON and MON. This is why there is an apparent discrepancy between the octane values of petrol in America versus the rest of the world. Euro95 unleaded in Europe is 95 octane but it’s the equivalent of American (R+M)/2 89 octane.

Believe it or not altitude can also affect combustion rates.  In America, low altitude petrol stations typically sell three grades of petrol with octane ratings of 87, 89 and 91. High altitude stations typically also sell three grades, but with lower values - 85, 87 and 89.

That being said higher-octane fuel creates an advantage in some cars, but not others. It allows performance-oriented engines (specifically, those with higher compression ratios) to burn gasoline at higher pressures and higher temperatures. These conditions at the moment of combustion create better thermodynamic efficiency, so a greater percentage of the gasoline’s heat energy gets converted into motive power, however all gasolines have the potential to produce the exact same amount of power.  The difference lies in how these engines utilize this power into the output of your vehicle’s performance.  

Octane and Power:

It’s a common misconception amongst car enthusiasts that higher octane = more power. False. Its not black and white. The myth arose because of sportier vehicles requiring higher octane fuels.  This is just because their engines are specifically designed to get the most out of them. Without understanding why, a certain section of the car subculture decided that this was because higher octane petrol meant higher power.  But look at it this way - double A batteries and triple A batteries do the same exact thing, send power to an electronic device, each device is simply optimized to use one or the other. 

Additionally power is limited by the maximum amount of fuel-air mixture that can be jammed into the combustion chamber. Because high performance engines operate with high compression ratios they are more likely to suffer from detonation and so to compensate, they need a higher octane fuel to control the burn. So yes, sports cars do need high octane fuel, but it's not because the octane rating is somehow giving more power. It’s because it's required because the engine develops more power because of its design.


There is a direct correlation between the compression ratio of an engine and its fuel octane requirements. The following table is a rough guide to octane values per engine compression ratio for a carburettor engine without engine management. For modern fuel-injected cars with advanced engine management systems, these values are lowered by about 5 to 7 points.  Find out your car's compression ratio, and you'll have an idea what gas is best for your engine.  


Compression ratio and octane

Octane and Gas Mileage: 


Can octane affect gas mileage. YES! Yes it can, but not for the reasons you might think. The octane value of a fuel itself has nothing to do with how much potential energy the fuel has, or how cleanly or efficiently it burns. All it does is control the burn. However, if you're running with a petrol that isn't the octane rating recommended for your car, you could lose gas mileage. Why? Lets say your manufacturers handbook recommends that you run 87 octane fuel in your car but you fill it with 85 instead, trying to save some money on filling up. Your car will still work just fine because the engine management system will be detecting knock and retarding the ignition timing to compensate. And that's the key. By changing the ignition timing, you could be losing efficiency in the engine, which could translate into worse gas mileage. Again as a practical example, my little tale above about our trip to Vegas on low octane gas. (Whether you want to believe some bloke on the internet or not is up to you). On the low octane gas on the trip down, we could barely get 23.5mpg out of the Subaru. Once I was able to fill it up again with premium at the recommended octane rating, we got 27.9mpg on the way back. A difference of 4.4mpg over 450 miles of driving.

Doing the math, you can figure out that buying cheaper gas may save money, but it costs in the long term, if your car requires higher octane, because you’re going to be filling up more often to do the same mileage. 


So basically we can sum it up in these facts: 


  • In regard to the effectiveness of regular gas, the Federal Trade Commission says there are no advantages to using premium gas in cars that don’t require it. The FTC states that using a higher octane on cars that don’t require it will not lead to better gas mileage or to the car running cleaner or faster.
  • Some luxury brands, such as BMW and Mercedes, have high-compression engines and require the use of premium gas to prevent the engine from knocking. In cases where the manufacturer recommends premium gas, auto experts say using regular gas is typically fine, unless the manufacturer requires premium - and not using premium in cars that require it can result in the engine knocking, which can eventually decrease the engine’s efficiency. The FTC reports that light knocking typically won’t cause any engine damage, although heavier pinging can.
  • Some people think that the lower grade gasolines are “dirty” and using the premium gasoline will keep the engine clean.  Honestly, all gasolines have good additives but it has nothing to do with really keeping the engine clean. It’s all about the higher octane vs the engine you are putting it in. Higher-compression engines need the higher octane so that the fuel doesn’t pre-ignite in the combustion process and damage the motor. Octane slows and controls the detonation of the fuel during the compression and detonation of the fuel.


Lets make it even easier: 


  • Do what the manufacture suggests.  He doesn’t make any money off of selling gasoline, but he does make money off how well your car performs and that your engine is at it’s best.  
  • If your car does not have the higher compression ratio for higher octane, putting premium gasoline then recommended in your engine will not help your performance.
  • If your engine is knocking or pinging, upgrading your octane level may help.  
  • If your car has a higher combustion rating and engine that is optimized for performance, using a lower grade will not hurt necessarily general functionality of your car, but you will loose performance and gas mileage as your engine adjusts to the lower octane levels.  
  • Its not the octane rating in the gasoline that necessarily makes it better, its the ability/efficiency of each specific engine to get the best performance out of it.  Performance cars are designed to get the most out of the higher octane fuel.  

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