The following is "borrowed" from the "All Ford Mustangs" web site at . I would have linked to it directly but stuff like this has a tendency to go "poof" so here it is in all it's splendor. If you wish to learn more about Octane rating other than this directly applicable info then a search for "octane rating" AND article will return a wealth of hits. Some are pretty deep into the lab language.

Following the first article there is another on racing gas additives. It's an eye opener and I heartily DON'T recommend this for home use. YOU'VE BEEN WARNED!


Article by: GTRaptor
Detonation ? Pinging ? Pre-Detonation ? Knocking ? Pre-Ignition ?

The above terms are very commonly used to describe the same thing, BUT not all of those words are interchangeable.

Lots of times you see statements like these:

"My car is pinging, and there's a lot of smoke coming out of the exhaust"
"I can hear lots of detonation"
"My 95 GT pings a lot"
"I hear a knocking sound at high RPMS"
"My car pre-detonates a lot with the A/C on"

Very often what these users hears is DETONATION, other times it is NOT.



This is an intense pressure wave within the cylinder created when a LEAN mix of air and fuel ignites before the spark is delivered.The fuel/air mixture goes off by itself a micro second before the spark plug fires, at that time you have two intense high pressure waves clapping together and that energy wave hitting each other is the sound you hear. The sound you hear is from the actual vibration of the cylinder wall

Under normal circumstances, when the fuel-air mixture in the engine burns, it does so in a controlled manner, releasing the energy that produces the force that drives the pistons. Now, if you think about it, it might seem that the correct time for the spark plug to ignite the mix would be when the piston reaches the top of its stroke. Then, as the mix burns and energy is released, the energy pushes the piston down. The problem is that it takes a certain amount of time for the mix to burn. If you wait until the top of the stroke to ignite it, the bulk of the energy will be applied to the piston only during the latter part of its downward movement.

In order to get the maximum use out of the combustion energy, the mix is ignited while the piston is still moving up. This is called "advance". As the engine speed increases, fuel has less time to burn and advance is increased accordingly. Ignition advance is controlled by the engine management computer. It varies in response to a host of other conditions but the base advance curve is a function of engine speed.

Now we have a new problem. As the mix is ignited earlier, it is not compressed as much and has a tendency to burn at an uncontrolled extremely high rate (it basically explodes). This is called "detonation". When this happens, all the energy in the fuel is released almost instantly. The resulting shock wave slams into the piston which is forced through it by the inertia of the engine and the combustion in other cylinders . The resulting stress on the engine is enormous.

Detonation can be caused by several things not just a lean condition or bad choice of fuel octane: Too much compression and too much timing can also be to blame.

The most common way to describe the sound of detonation is: "like shaking a can full of marbles", but the BEST way to verify the existence of detonation is by reading the sparkplug's. If the plugs are very white, show a very eroded electrode and signs of metal deposits, then you have DETONATION.

The Cure: Less timing and/or higher octane gas and/or colder plugs.

Higher octane gas is a good safety measure. Is the $.10 per gallon worth that extra risk? Another myth about octane is that higher octane produces more power, wrong. Higher octane fuel is less volatile, less explosive, less prone to detonation.

A more 'volatile' fuel actually produces more energy per unit volume.You just have to watch out for detonation more. That's why higher compression engines use higher octane fuel, higher compression ratio (and higher altitude, lower air pressure) increases the tendency for detonation. Run super unleaded, but don't expect to gain power from the more expensive fuel. You CAN get more power, but it's from running more compression and spark advance.

This is premature ignition before the spark plug fires, usually caused by something glowing inside the cylinder such as carbon deposits or too hot a sparkplug tip. Do not confuse this with detonation. It is not the same.

Usually pre-ignition occurs when there are lots of "red hot" carbon deposits and unburned fuel in the combustion chambers.

If you turn of the ignition and your stang wants to keep on running, then you have pre-ignition.

The Cure: Eliminate carbon deposits and/or colder plugs, check fuel system.
Glossary of Terms

Octane is a chemical molecule. When referred to as ISO-octane in testing to find the "Octane" of a sample of gasoline, it is the reference fuel with the value of 100. In the testing of gasoline we use two reference fuels. The other one is N-Heptane which has an Octane value of 0 or zero.

This is an anti-knock scale developed in the 1920's to rate the quality of a gasoline's ability to resist knocking or pinging. Samples of any gasoline are placed in a laboratory Knock Engine (this is a small, one cylinder engine with a variable combustion cylinder). While the engine is running, the combustion ratio is increased until the engine begins knocking. Now the gasoline is replaced with N-Heptane with an octane of zero and is mixed with the 100 octane ISO-Octane at various ratios until the motor "knocks." If you end up with 10% N-Heptane and 90% ISO-Octane ratio, your test sample has an octane of 90.

The exact air fuel ratio required to completely combust a fuel to water and carbon dioxide. You get all the energy out of the fuel at this point. This air fuel ratio is almost impossible to achieve. Racing gasoline ideally burns at a 14:1 ratio. (14 parts air, 1 part gas).

Detonation and pinging can destroy your engine.
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And now the racing gas article. This was borrowed from Ray Hall Turbocharging at . It's an eyepopper for us streetriders ......
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WE now come to the use of additives for reasons other than power increase. In this chapter we will deal only with additives that can be of assistance to us in connection with Pre-ignition and the other problem of Detonation.

We again go through our three basic fuels in the same order.


We have three additives in Methanol, Acetone and Benzole (Benzene) and all of them are introduced with the main object of reducing Detonation by increasing in effect the Octane rating of the total fuel. Pre-ignition in general should not present a problem when using as basic fuel petrol.

Methanol in Petrol. This is the best from the point of view of reducing Detonation, followed by Acetone and then Benzole in that order.

Methanol can be added in all proportions up to 100 percent, but as an additive limited to 10 percent will give an Octane increase of about 5 points. For example 98 Octane can be increased to 103, or looking at it another way, cheap fuel of say 91 Octane can, by the use of 10 percent Methanol, or approximately three quarters of a pint per gallon, will produce fuel of 96 Octane.

Acetone in Petrol. Can be used up to 100 percent but with the nominal 10 percent will give an increase of 3 points rather than 5.

The major difference from Methanol being that due to the higher calorific value of Acetone, the consumption does not increase so much, but still provides a higher octane rating.

Benzole (Benzene) in Petrol. Again can be used up to 100 percent but with the 10 percent amount will provide in points a rise of 2.

In many cases this additive is used to counteract detonation since some 10 percent will, in certain cases, provide enough rise in octane rating to do just that.


Now we come to Methanol as the main fuel, and as additives to reduce Preignition and/or Detonation we have two.

Acetone in Methanol. Here we are concerned only with Pre-ignition since Methanol has itself a very high octane rating, and is therefore to be regarded as almost free from detonation problems.

Once again our figure of 10 percent is the most advantageous use of the additive, as over that figure has a declining effect in proportion to the amount used.

The effect of using this additive is to move the auto-ignition point upwards, and this was fully explained as will be remembered.

Water in Methanol. Up to 5 percent or a maximum of 10 percent with the object of increasing the octane rating even higher, to reduce detonation under very high supercharge conditions.


Last of all now we have Nitromethane as our main fuel. Here we have three additives to help with preignition and or detonation.

Methanol in Nitromethane. Since Nitromethane has itself a tendency to pre-ignite and detonate, the sole object of up to 10 percent Methanol as an additive is to reduce this tendency to detonate while having only a minor effect on pre-ignition.

Water in Nitromethane. Up to a maximum of 2.5 percent as this is the maximum amount that will mix without separation taking place. It reduces both preignition and detonation due to the internal cooling effect alone.

In practice a combination of Methanol and Water is the better use of the two additives, the proportions being 2.5 percent water and 7.5 percent Methanol giving a good safe usable blend of Nitromethane, with almost the full power capability of undiluted Nitromethane.

Acetone in Nitromethane. Up to a maximum of 5 percent. This reduces preignition by raising the auto-ignition point and any small decrease in detonation is incidental