Determining Octane Rating
The octane number of a fuel is measured in a test engine, and is defined by comparison with the mixture of 2,2,4-trimethylpentane (iso-octane) and heptane which would have the same anti-knocking capacity as the fuel under test: the percentage, by volume, of 2,2,4-trimethylpentane in that mixture is the octane number of the fuel.  For example, gasoline with the same knocking characteristics as a mixture of 90% iso-octane and 10% heptane would have an octane rating of 90.  This does not mean that the gasoline contains just iso-octane and heptane in these proportions, but that it has the same detonation resistance properties.  Because some fuels are more knock-resistant than iso-octane, the definition has been extended to allow for octane numbers higher than 100.
Octane rating does not relate to the energy content of the fuel.  It is only a measure of the fuel's tendency to burn in a controlled manner, rather than exploding in an uncontrolled manner.  Where octane is raised by blending in ethanol, energy content per volume is reduced.
It is possible for a fuel to have a Research Octane Number (RON) greater than 100, because iso-octane is not the most knock-resistant substance available.  Racing fuels, avgas, liquefied petroleum gas (LPG), and alcohol fuels such as methanol or ethanol may have octane ratings of 110 or significantly higher – ethanol's RON is 129 (116-MON, 122-AKI).  Typical "octane booster" gasoline additives include MTBE, ETBE, isotane and toluene.  Lead in the form of tetra-ethyl lead was once a common additive, but since the 1970s, its use in most of the industrialized world has been restricted, and its use is currently limited mostly to aviation gasoline.

Research Octane Number (RON)
The most common type of octane rating worldwide is the Research Octane Number (RON).  RON is determined by running the fuel in a test engine with a variable compression ratio under controlled conditions, and comparing the results with those for mixtures of iso-octane and n-heptane.

Motor Octane Number (MON)
There is another type of octane rating, called Motor Octane Number (MON), or the aviation lean octane rating, which is a better measure of how the fuel behaves when under load, as it is determined at 900 rpm engine speed, instead of the 600 rpm for RON.  MON testing uses a similar test engine to that used in RON testing, but with a preheated fuel mixture, higher engine speed, and variable ignition timing to further stress the fuel's knock resistance.  Depending on the composition of the fuel, the MON of a modern gasoline will be about 8 to 10 points lower than the RON, however there is no direct link between RON and MON.  Normally, fuel specifications require both a minimum RON and a minimum MON.

Anti-Knock Index (AKI)
In most countries, including all of those of Australia and Europe the "headline" octane rating shown on the pump is the RON, but in Canada, the United States and some other countries, like Brazil, the headline number is the average of the RON and the MON, called the Anti-Knock Index (AKI, and often written on pumps as (R+M)/2).  It may also sometimes be called the Road Octane Number (RdON) or Pump Octane Number (PON).

Difference between RON and AKI
Because of the 8 to 10 point difference noted above, the octane rating shown in the United States is 4 to 5 points lower than the rating shown elsewhere in the world for the same fuel.
A fuel with a higher octane rating can be run at a higher compression-ratio without causing detonation.
Many high-performance engines are designed to operate with a high maximum compression, and thus demand high-octane premium gasoline.  A common misconception is that power output or fuel mileage can be improved by burning higher-octane fuel than a particular engine was designed for.  The power output of an engine depends in part on the energy density of its fuel, but similar fuels with different octane ratings have similar density.  Since switching to a higher-octane fuel does not add any more hydrocarbon content or oxygen, the engine cannot produce more power.
Most fuel stations have two storage tanks (even those offering 3 or 4 octane levels), and you are given a mixture of the higher and lower octane fuel for the intermediate grades.  Premium is fuel from the higher-octane tank, and the minimum grade sold is fuel from the lower-octane tank.  Purchasing 91-AKI (where offered) in a place offering 93-AKI as the top grade simply means more fuel from the higher-octane tank than when purchasing 89-AKI.  The detergents in the fuel are often the same.  However, in isolated cases for some producers, the additive package is different between the higher and lower octane rating.
In the United States, especially in the Rocky Mountain (high altitude) states, 85-AKI is the minimum octane, and 91-AKI is the maximum octane available in fuel.  The reason for this is that in higher-altitude areas, a typical naturally aspirated engine draws in less air mass per cycle due to the reduced density of the atmosphere.  This directly translates to less fuel and reduced absolute compression in the cylinder, therefore deterring knock.  It is safe to fill up a carbureted car that normally takes 87-AKI fuel at sea level with 85-AKI fuel in the mountains, but at sea level the fuel may cause damage to the engine.  A disadvantage to this strategy is that most turbocharged vehicles are unable to produce full power, even when using the "premium" 91-AKI fuel.
In some East Coast states, up to 94-AKI is available.  In parts of the Midwest (primarily Minnesota, Iowa, Illinois and Missouri) ethanol-based E-85 fuel with 105-AKI is available.  Often, filling stations near US racing tracks will offer higher-octane levels such as 100-AKI.  California fuel stations will offer 87, 89, and 91-AKI octane fuels, and at some stations, 100-AKI or higher octane, sold as racing fuel.  Until summer 2001 before the phase-out of methyl tert-butyl ether aka MTBE as an octane enhancer additive, 92-AKI was offered in lieu of 91-AKI.  These offerings will continue to change as new regulations and new laws mandate more controls both locally and nationally.

Pre-Detonation (Auto-Ignition)
There are basically two causes of pre-detonation.  Abnormal combustion occurs in an internal combustion engine, igniting the fuel, due to excessive heating of the internal engine components, i.e., piston crown and combustion chamber or from high pressure due to compression.
Engine design can cause hot spots due to the shape of the combustion chamber and/or the location of the spark plug.  Dome shaped cylinder heads with the plug placed in the center create an even burn that spreads out from the ignition point applying an even pressure of expansion.  Wedge shaped cylinder heads with the spark plug offset to one side can create a situation where fuel ignites at the narrow side prior to the spark plug firing.  These two events collide creating that familiar knocking or pinging sound.  Hot spots can also occur in engines where the plug is placed in the center causing the same pre-ignition when conditions like carbon buildup occur but are less prone to the condition

Nailhead Buicks have an unusual head configuration that creates a cylinder top shape allowing the spark plug to be located in the center of the combustion chamber.  Igniting the fuel from this position aids in creating an even burn.

Engines with wedge shaped combustion chambers like this small-block Chevrolet have the spark plug offset to one side and when the fuel/air mixture is ignited it burns across the combustion chamber.  The increased tendency to pre-detonation from this design is very small but still can be a factor.

Hemispherical shaped combustion chambers like the one on this Chrysler Hemi also place the spark plug in the center but achieve this by spreading the intake and exhaust valve locations to allow it.  They claim that the shape of the combustion chamber aids smooth and complete burning of the fuel/air mixture.

The cost common cause of pre-ignition is the low-octane rating of the fuel.  The higher the compression ratio of the engine, the higher the pressure that is created in the cylinder.  Using a higher octane rated fuel raises the ignition point of the fuel.  That allows the fuel to be compressed more without pre-detonation before the spark is introduced.  However, this only works up to a point.  Then some resort to decreasing the timing advance; but this results in a corresponding reduction in power.

Dealing With The Problem
Modern engines are designed utilizing a lower compression ratio, plus ignition systems and induction systems that allow the use of lower octane fuels.  However, if you are going to use one of these early high-compression engines you will have to take into consideration the engines basic design and the tuning of various systems.
Fuel can vary from one brand to another because of the blending done of different grades and additives present in a particular fuel.  One brand may include more MON or RON in the blend when achieving the desired AKI octane rating.  This means that one brand with an 87-AKI octane rating may cause pre-detonation in an engine while another brand of the same 87-AKI octane rating may run just fine.  So the simplest solution is to try different grades and brands of gasoline until you find one that works under a full range of operating conditions.  While there is no perfect solution, simply buying the highest-octane fuel available is not the answer.

Another Possible Solution
There is another possible solution to the problem of pre-detonation.  More and more folks are looking at Electronic Fuel Injection.  It must be noted at the beginning that this may not be practical, particularly in areas with tight emission ("smogging") inspections.  In the meantime, let's look at the basic principle of EFI.

EFI Benefits
The purpose of EFI is to match the amount of fuel burned to the amount of oxygen present under all operating conditions and reduce emissions.  Maximum power is generated with an air/fuel ratio by weight of 12.5 to 1.  While during normal cruising the ratio should be 15 or 16 to 1 to achieve maximum fuel economy and for the engine to run neither too rich nor too lean.
The carburetor will work fine when adjustments are set for operation at a particular atmospheric pressure.  The problem with the carburetor is that it cannot be easily adjusted for different elevations.  A carburetor calibrated for operation at sea level will not permit the engine to run as efficiently as possible at higher elevations.
Unfortunately no carburetor or EFI system can run as efficiently at altitude as it can at sea level.  If for example, you drive around at 6400' above sea level where atmospheric pressure is only 11.6 psi then the best that can be achieved is 79% efficiency.  If you drive over passes like Donner Summit at about 8400' the efficiency drops to 74% due to reduced combustion pressure.  High passes in Colorado such as Loveland or Monarch Pass at 12,000' are even worse where efficiency drops to 65%.
Running lean also creates problems with knocking and overheating plus potential damage to the pistons, valves and exhaust manifolds.  In addition to less efficient operation with either a carburetor or EFI at altitude, the carbureted engine suffers a further loss of efficiency by not burning all the fuel.  Mileage suffers accordingly and pollution increases.
The EFI system provides the compensation required for operation at different altitudes as well as reducing carbon monoxide (CO) and hydrocarbon (HC) emissions.  This can help with the problem of pre-detonation but to solve the problem you will still have to experiment with various grades and brands of gasoline until you find the right combination.

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