The Octane Number (sometimes erroneously called Octane Content) is a measure of the antiknock quality of petrol - the ability of a petrol to resist knocking upon combustion in an engine. Knocking is a description of the sound that an engine makes when it runs on a too low octane fuel, and it causes a drop in fuel efficiency.

In 1927, Graham Edgar suggested a test method to quantify the knocking behavior of a fuel. Edgar proposed the use of two fuels: n- heptane and 2,2,4-trimethyl pentane (iso-octane); fuels with a low and a high antiknocking value respectively. Any commercial fuel that was available on the market at that time could be compared in knocking behavior to blends of these two compounds. The introduction of reference fuels lead to the design of various test engines and test conditions. Nowadays, the most commonly used test methods are the Research Octane Number (RON), and the Motor Octane Number (MON).

The fuel property the octane ratings measure is the ability of the unburnt end gases to spontaneously ignite under the specified test conditions. Ideally, a fuel should resist decomposition before arrival of the flame-front in the piston. This is a dependent on the hydrocarbon composition of the fuel, but also on the presence of additives, such as oxygenates.

The Research Octane Number settings represent typical mild driving, without consistent heavy loads on the engine.

Test Engine conditions               Research Octane
======================       ==================================
Test Method                         ASTM D2699-92 105
Engine                       Cooperative Fuels Research (CFR)
Engine RPM                               600 RPM
Intake air temperature       Varies with barometric pressure
                           ( eg 88kPa = 19.4C, 101.6kPa = 52.2C )
Intake air humidity           3.56 - 7.12 g H2O / kg dry air
Intake mixture temperature            Not specified 
Coolant temperature                      100 C
Oil Temperature                           57 C
Ignition Advance - fixed            13 degrees BTDC 
Carburettor Venturi           Set according to engine altitude          
                           ( eg 0-500m=14.3mm, 500-1000m=15.1mm ) 

The conditions of the Motor Octane Number method represent severe, sustained high speed, high load driving. For most hydrocarbon fuels, including those with either lead or oxygenates, the Motor Octane Number will be lower than the Research Octane Number.

Test Engine conditions                Motor Octane 
======================       ==================================
Test Method                         ASTM D2700-92 104
Engine                       Cooperative Fuels Research (CFR)
Engine RPM                               900 RPM
Intake air temperature                    38 C
Intake air humidity           3.56 - 7.12 g H2O / kg dry air
Intake mixture temperature               149 C
Coolant temperature                      100 C
Oil Temperature                           57 C
Ignition Advance - variable     Varies with compression ratio
                                 ( eg 14 - 26 degrees BTDC ) 
Carburettor Venturi                       14.3 mm

In general, a combination of the RON and MON is used; e.g. in the US a combination called the Antiknock Index (AKI) is defined as:

AKI = (RON + MON) / 2

This is usually specified on the pump; next time you pull up at the gas station, you will see :-) Another important factor to quantify fuels is Sensitivity, defined as:

Sensitivity = RON - MON

A fuel can have a high Antiknock Index value, but behave poorly under varying operating conditions. Most modern fuels have a Sensitivity around 10.


information & data from: http://www.faqs.org/faqs/autos/gasoline-faq/
The octane number you see at the gas pump is an indicator of the gasoline's ability to resist "knock", which is the engine pinging caused by premature ignition.

To determine the octane of a gasoline, two test fuels (heptane and isooctane) are mixed and compared against it. Heptane has an octane number of zero, while isooctane has a value of 100. The subject gasoline is compared against various mixes of heptane and isooctane until an equivalent knock is found. The percentage of isooctane in the mixture used for that test is determined to be the octane rating of the tested gasoline.

For example, if a gasoline is found to have a knock similiar a mixture of 90% isooctane and 10% heptane, that gasoline is marked as having an octane rating of 90.

Contrary to what seems to be popular belief, using higher octane fuel in your car will NOT necessarily increase its performance. In fact, it will often decrease it and hurt your engine in the long run.

Here's why.

After fuel is injected into the intake tract and sucked into the cylinder, the intake valve closes and the piston moves up in the cylinder and compresses this charge of fuel and air. Here's where the octane rating comes in to play. If the engine compresses the fuel/air charge a lot (as in a sports car, which is typically 10:1 or more), the temperature will rise quite a bit. If you're using a low octane fuel, the chances that this temperature/pressure increase will spontaneously ignite the fuel charge are much greater. Since the mixture is supposed to be ignited by the spark plug, which does not fire until the piston reaches its highest position, the resulting explosion will be pushing AGAINST the piston. This phenomenon, called preignition or detonation, is what causes the "knock" or "ping" that people refer to.

The key difference between different fuels is that higher octane fuels are harder to ignite, and they generally burn cooler. That, in turn, lowers cylinder temperature and decreases the likelihood of preignition.

If you're talking about a high compression sports car engine, that's the reason for higher octane ratings. But if the engine in question is a typical sedan or van (or whatever else) engine, chances are it has relatively low compression. And if you're using high octane fuel in a low compression engine, not only will it be harder to ignite, but the chances are good that it won't burn as fully. Incomplete combustion will leave nasty carbon deposits in your engine. The thing about carbon deposits is that they retain heat much better than metal does and they decrease the combustion chamber area, thus gradually increasing the compression ratio. That means you've got a higher compression, higher temperature cylinder. And this, of course, will require higher octane fuel in order to prevent detonation.

So that's the deal. If you use high octane fuel in an low compression engine, you'll be ultimately increasing your octane requirement, and possibly causing carbon fouling of spark plugs, valves, and other important stuff.

All things considered, you'd be wise to just go with the recommendations in your owner's manual. If it says use "87 octane or higher", then use 87! This is one time that it pays to be cheap. If it says "91 or higher", then don't hesitate to use 91 or higher. Usually only 92 or 93 are for sale, though-- the available octane ratings vary depending where you live-- higher altitude areas have lower octane fuel due to the lower oxygen content of the air.

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