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PKRWUD · 08-21-2002, 01:16 PM

I am not aware of any specific formula, probably because there are so many variables. For example, all other things being equal, an engine with aluminum heads can use a lower octane fuel than one with iron heads.

This might help you understand things a little better. It's from a reply I left for someone else who was asking about octane:

Unfortunately, many people assume that higher octane fuel is better for all engines, no matter what. This, as I'm sure you know, is absolutely false. Ultimately, the best octane for any vehicle is the lowest one you can run without detonation (pinging) occuring.

Running too high of an octane will ultimately make it necessary to continue using that octane.

Just to clarify, it's not the fuel that increases the performance, it's the fact that the timing has been advanced. The higher octane permits this to happen. Basically, the way it works is something like this:

Octane represents a fuel's stability. When you compress an air fuel mixture, it becomes quite unstable, and explosive. In fact, this is why diesel engines don't require spark plugs. They commonly have compression ratios of 22:1 (as opposed to a stock Mustang with 9:1), which makes the air/fuel mixture so explosively unstable that it ignites itself when the piston reaches TDC.

Octane, for all intents and purposes, is used to slow down, or control the burn rate. Modifications that increase performance, such as increased compression, or advanced ignition timing, will cause the mixture to become too unstable, and pre-ignition (aka: detonation, pinging) will occur. This is when the mixture fires on it's own, at the wrong time. Severe damage will occur if left untreated. Anyway, the only way to perform the mods that increase the power is to further slow down and control the burn rate of the air/fuel mixture, and the method used to do this is increased octane.

People often think of high octane fuel as being more flammable, and easier to explode, whereas that is actually the opposite of the truth. 110 octane race gas is much tougher to light than low grade 87 octane. BUT, because of that fact, it is the use of 110 octane fuel that permits the compression to be increased to 12:1 without melting the pistons.

On a side note, nitrous oxide works on a very similar principal. With nitrous, the power comes from the added oxygen. The problem has always been trying to find a way to add oxygen safely. As I'm sure you know, oxygen is EXTREMELY flammable, and if you were to try and add it straight to your engine, you would likely die, and not even be able to explain to people why it was you died. It was discovered that the nitrogen in the nitrous oxide was just enough to keep the oxygen from burning uncontrollably, thus permitting it to be used in a gasoline engine. In other words, the nitrogen is to nitrous oxide what the octane is to gasoline, except for the fact that octane is a calculation, and nitrogen is an actual element, but you get the idea.

Okay, back to the story...

Use of a fuel that has a higher octane than is required by the engine and the way it has been tuned will result in unburned deposits being created and left behind in the combustion chamber, and on top of the piston. Often referred to as carbon deposits, these cause the combustion chamber to become very unstable, for several reasons, all of which will ultimately require you to use a higher octane fuel to restore stability. Among the reasons, are increased compression (the deposits take up space, and actually increase the compression enough to make it unstable), retained heat (deposits retain heat at times when those surfaces shouldn't, increasing the likelyhood of pre-ignition), and the development of sharp edges (like sand under the waves, carbon deposits can be "shaped" by their environment, and can develop sharp edges, which act to the compressing mixture like a hat pin would to a balloon.

Take care,
~Chris

08-21-2002, 01:16 PM	#3
PKRWUD Junior Member Join Date: Mar 1999 Location: Ventura, California Posts: 8,981	I am not aware of any specific formula, probably because there are so many variables. For example, all other things being equal, an engine with aluminum heads can use a lower octane fuel than one with iron heads. This might help you understand things a little better. It's from a reply I left for someone else who was asking about octane: Unfortunately, many people assume that higher octane fuel is better for all engines, no matter what. This, as I'm sure you know, is absolutely false. Ultimately, the best octane for any vehicle is the lowest one you can run without detonation (pinging) occuring. Running too high of an octane will ultimately make it necessary to continue using that octane. Just to clarify, it's not the fuel that increases the performance, it's the fact that the timing has been advanced. The higher octane permits this to happen. Basically, the way it works is something like this: Octane represents a fuel's stability. When you compress an air fuel mixture, it becomes quite unstable, and explosive. In fact, this is why diesel engines don't require spark plugs. They commonly have compression ratios of 22:1 (as opposed to a stock Mustang with 9:1), which makes the air/fuel mixture so explosively unstable that it ignites itself when the piston reaches TDC. Octane, for all intents and purposes, is used to slow down, or control the burn rate. Modifications that increase performance, such as increased compression, or advanced ignition timing, will cause the mixture to become too unstable, and pre-ignition (aka: detonation, pinging) will occur. This is when the mixture fires on it's own, at the wrong time. Severe damage will occur if left untreated. Anyway, the only way to perform the mods that increase the power is to further slow down and control the burn rate of the air/fuel mixture, and the method used to do this is increased octane. People often think of high octane fuel as being more flammable, and easier to explode, whereas that is actually the opposite of the truth. 110 octane race gas is much tougher to light than low grade 87 octane. BUT, because of that fact, it is the use of 110 octane fuel that permits the compression to be increased to 12:1 without melting the pistons. On a side note, nitrous oxide works on a very similar principal. With nitrous, the power comes from the added oxygen. The problem has always been trying to find a way to add oxygen safely. As I'm sure you know, oxygen is EXTREMELY flammable, and if you were to try and add it straight to your engine, you would likely die, and not even be able to explain to people why it was you died. It was discovered that the nitrogen in the nitrous oxide was just enough to keep the oxygen from burning uncontrollably, thus permitting it to be used in a gasoline engine. In other words, the nitrogen is to nitrous oxide what the octane is to gasoline, except for the fact that octane is a calculation, and nitrogen is an actual element, but you get the idea. Okay, back to the story... Use of a fuel that has a higher octane than is required by the engine and the way it has been tuned will result in unburned deposits being created and left behind in the combustion chamber, and on top of the piston. Often referred to as carbon deposits, these cause the combustion chamber to become very unstable, for several reasons, all of which will ultimately require you to use a higher octane fuel to restore stability. Among the reasons, are increased compression (the deposits take up space, and actually increase the compression enough to make it unstable), retained heat (deposits retain heat at times when those surfaces shouldn't, increasing the likelyhood of pre-ignition), and the development of sharp edges (like sand under the waves, carbon deposits can be "shaped" by their environment, and can develop sharp edges, which act to the compressing mixture like a hat pin would to a balloon. Take care, ~Chris __________________ Webmaster: Rice Haters Club Jim Porter Racing Peckerwoods Pit Stop Support Your Local RED & WHITE!