Octane

By: Jason P. Laskowski
CIO Extreme Garage Network

Octane numbers can be very confusing due to several different terms used in discussions.  This article should help clear up some common misunderstandings and define some of the various terms used when discussing octane.

Octane
The octane quality of a gasoline is its ability to resist detonation, a form of abnormal combustion.  Detonation occurs when the air-fuel mixture reaches a temperature and/or pressure at which it can no longer keep from self-igniting. Two types of abnormal combustion are common: the first is detonation as previously mentioned and the other is pre-ignition.

Detonation occurs after the spark plug has ignited the air-fuel mixture and the flame is moving smoothly across the combustion chamber.  If, during this burning process, the unburned air-fuel mixture reaches a temperature and/or pressure at which it is no longer stable, it burns very rapidly causing a new flame which collides with the one that originated at the spark plug.  Excess pressure in the cylinder occurs before the piston reaches top dead center (TDC) and that pressure try's to push the piston down before it’s supposed to.  Piston burning and rod bearing damage are the result.

Pre-ignition is usually started by a hot spot in the combustion chamber, which causes the mixture to ignite before the plug fires.  Under wide-open throttle conditions, pre-ignition will destroy pistons in seconds.

Research Octane Number (RON)
RON is derived from the following testing procedure:  a single cylinder variable compression ratio engine that operates at 600 rpm with a 125-degree inlet air temperature at standard barometric pressure.  Spark advance is fixed at 13 degrees btdc.  In a real world engine, RON is necessary to satisfy partial throttle knock problems.

A good quality racing gasoline has a RON in the range of 110 to 115.  The difference in RON is not very important to racing engines.

Motor Octane Number (MON)
MON is derived from the following testing procedure:  a single cylinder engine similar to the RON testing method with a few changes that make operating conditions more severe and therefore the octane numbers are lower.  The MON engine runs at 900 rpm with a 300-degree mixture temperature.  Spark advance varies with compression ratio.  

In a real world engine, MON is necessary to satisfy octane demands at wide-open throttle.  This is a very important number for racing engines since they spend a high percentage of their lives under high speed and high load conditions.  Racing engines cannot afford to be short on octane quality, since detonation or pre-ignition will quickly reduce an engine to scrap. 

The octane appetite of an engine with 13:1 compression ratio and a four-inch bore varies with operating conditions but is normally around 101.  Quality racing gasolines have MON in the range of 100 to 115.  If your engine requires a 101 MON, there is no point using a gasoline that has a 115 MON.  To cover yourself for extreme conditions, it is wise to have an octane cushion but there is no advantage to using a very high octane quality product if you do not need it.

I’m sure during a fill up you’ve noticed that yellow sticker on the pump that says “Octane Method (R+M)/2”.  Well I’ll explain to you what that means. 

(R+M)/2
This is the average of RON and MON.  It is sometimes referred to as the aki or anti-knock index.  It is required by law, in most states, for this to be posted on all pumps in retail outlets.  It is the most commonly used octane reference today.  It was developed about 20 years ago as a compromise between RON and MON for advertising purposes and also to keep from confusing the consumer with too many different terms.  It has also been referred to as road octane number. 

Observed Road Octane Number (RdON)

This is derived from testing gasolines in real world multi-cylinder engines, normally at wide-open throttle.  It was developed about 70 years ago but is still reliable today.  The original testing was done in cars on the road but as technology developed the testing was moved to chassis dynamometers.  This eliminated a lot of variables and some people have since built very elaborate chassis dynamometers with environmental controls to improve consistency.

[From 76] “As an example, 76's current 108 octane leaded racing gasoline blend is the result of testing over 100 experimental blends. The final blend has a road octane number (RdON) of 110, the same RdON as one of our competitors gasoline that has advertised 116 RON and 116 MON.  Using only RON and MON can lure a person into a false sense of security.  If you want to be certain that your racing gasoline has been thoroughly tested in real world racing engines with the horsepower and road octane number maximized, choose "76" for your engine.  If you want high RON and MON without knowing how it will work in your racing engine, buy from the other guys.  

Are you tempted to buy a high-octane gasoline for your car because you want to improve its performance?

If so, take note the recommended gasoline for most vehicles is regular (87) octane.  In fact, in most cases, using a higher-octane gasoline than your owner’s manual recommends offers absolutely no benefit.  It won’t make your vehicle perform better, go faster, get better mileage or run cleaner. Your best bet is to follow your owner’s manual. 

The only time you might need to switch to a higher-octane level is if your engine knocks when you use the recommended fuel. 

Unless your engine is knocking, buying higher-octane gasoline is a waste of money.  Premium gas costs 15 to 20 cents per gallon more than regular.  That can add up to $100 or more a year in extra costs. Studies indicate that altogether, drivers may be spending hundreds of millions of dollars each year for higher octane gas than they need.

Gasoline FAQ 

What are octane ratings?

Octane ratings measure a gasoline’s ability to resist engine knock, a rattling or pinging sound that results from premature ignition of the compressed fuel-air mixture in one or more cylinders.  Most gas stations offer three octane grades: regular (usually 87 octane), mid-grade (usually 89 octane) and premium (usually 92 or 93). The ratings must be posted on bright yellow stickers on each gasoline pump. 

What’s the right octane level for your vehicle?
Check your owner’s manual to determine the right octane level for your vehicle.  Regular octane is recommended for most.  However, some vehicles with high compression engines, like sports cars and certain luxury cars, need mid-grade or premium gasoline to prevent knock. 

How can you tell if you’re using the right octane level? Listen to your engine.  If it doesn’t knock when you use the recommended octane, you’re using the right grade of gasoline. 

Will higher-octane gasoline clean your engine better?
As a rule, high octane gasoline does not outperform regular octane in preventing engine deposits from forming, in removing them, or in cleaning your car’s engine.  In fact, the U.S. Environmental Protection Agency requires that all octane grades of all brands of gasoline contain engine cleaning detergent additives to protect against the build-up of harmful levels of engine deposits during the expected life of your engine. 

Should you ever switch to a higher octane gasoline?
A few engines may knock or ping — even if you use the recommended octane.  If this happens, try switching to the next highest-octane grade.  In many cases, switching to the mid-grade or premium-grade gasoline will eliminate the knock.  If the knocking or pinging continues after one or two fill-ups, you may need a tune-up or some other repair. After that work is done, go back to the lowest octane grade at which your engine runs without knocking. 

Is knocking harmful?
Occasional light knocking or pinging won’t harm your engine, and doesn’t indicate a need for higher octane. But don’t ignore severe knocking. A heavy or persistent knock can lead to engine damage. 

Is all "premium" or "regular" gasoline the same?
The octane rating of gasoline marked "premium" or "regular" is not consistent across the country.  One state may require a minimum octane rating of 92 for all premium gasoline, while anothers may allow 90 octane to be called premium.  To make sure you know what you’re buying, check the octane rating on the yellow sticker on the gas pump instead of relying on the name "premium" or "regular."  Also, please note that some “premium” fuels contain Ethanol (grain alcohol).  Studies have shown that alcohol is a much cleaner or more efficient burning fuel than standard gasoline, however be very careful if your fuel system uses rubber hoses.  Consistent use of gasolines that contain Ethanol will accelerate the deterioration of rubber fuel lines. 

References: 

76 Fuels 

Encarta

About.com 

Hot Rod Magazine

Street Rodder Magazine