(91octane + water) Vs. (100octane + water)

[GotH2O]

my understanding is you want a fast burning mixture thats detonation resistant. therefore lower octane fuel, the lower the better + water will be ideal?

[hotrod]

Fuel octane and burn speed have no specific relationship. There are very vast burning 87 octane fuels and very fast burning 120 octane fuels.

Sunoco Maximal 116 octane is designed for large cubic inch High rpm engines, it is sunoco's fastest burn rate fuel.

Octane is a measure of how resistant the fuel is to detonation, not its burn speed.

http://www.ducatimeccanica.com/gas.html

Larry

[GotH2O]

my buddy and i got the same car......

he runs 100octane with 11 degrees/25psi w/o any knocks
i run 91octane and 8 degrees/19psi and i knock very frequently....

it just seem that higher octane fuel = burns slower = allows more timing advance............ :?:

[hotrod]

What your seeing is not a difference in the fuels burn speed but a difference in its tolerance to high cylinder pressures. His higher boost and more ignition advance is creating much higher cylinder pressures. He can use more ignition advance because the fuel will "allow" in not because it "needs it".


In short he's running the proper fuel for what he's doing, your not!

Larry

[GotH2O]

Thanx Larry for the explaination!

With the 2D going on the car, I am hoping to run as much timing/boost on 91 as with 100 octane.

[Art_H]

Why then is a higher octane fuel more resistant to detonation?

[hotrod]

Why then is a higher octane fuel more resistant to detonation?

Keep in mind that detonation is a very very complex process that even after 100 years of investigation is not fully understood. Detonation appears to be caused by uncontrolled combustion of the partially decomposed fuel. Gasoline is a mixture of several hundred different chemicals composed of very long chain hydrocarbon molecules. They burn by breaking up those chains into smaller and smaller pieces (like cracking in an oil refinery), as temperature rises (this is a form of pyrolysis). The actual burning occurs when free oxygen begins to combine with the most accessable hydrogen and carbon atoms of these fuel molecule fragments.

What happens is that at very high temperatures and pressures, fuel begins to decompose even before the main flame front reaches it. This breaks up the fuel molecules into smaller "pieces". Think of this sort of like a piece of paper beginning to char before it bursts into flame, under a magnifying glass.

Also when the main flame front passes through a volume of gas it does not all burn at once. It takes time and the more easily burned "pieces" burn off first, leaving behind some other more difficult to ignite fuel fragments. This is much like what you see in a camp fire where the smaller pieces of wood will burn to ash before the larger pieces are fully ignited. In the case of the larger pieces of wood you will see smoke and vapors cooking out of them and igniting when it reaches free oxygen in the air. The same sort of progressive burning takes place in a gasoline air mixture. If you ground the wood up into a very fine dust and blew it into a fire it would burn just like gasoline.

The effect of these two processes (pre-combustion decomposition) and (post combustion afterburning of partially burned fuel), is what controls detonation. In high speed photographs they have recorded flame fronts sweeping across the combustion chamber several times through the same fuel air mix as these successive combustion processes occur.

Some fuel components tend to break up into fragments that are prone to uncontrolled explosive combustion at high temperatures, and other fuel components tend to break up into molecule fragments that tend to burn smoothly and progressively under high temperature and pressure.

A high octane fuel simply has a higher proportion of the "well behaved" fuel components like Toluene. Toluene is a ring shaped molecule and like the other members of the Aromatic fuels (Benzene, and Xylene) tends to burn in a well behaved way. Straight chain hydrocarbons tend to produce partial combustion products that do not play well at high temperatures and pressures. They tend to decompose into mixtures that burn explosively when they reach certain critical temperature and pressure combinations.

Most octane control additives like TEL (lead) and MMT work by changing the way these decomposition products form, and changing the chemistry of the partially burned fuel.

Water injection appears to control detonation by two different mechanisms. It cools the peak combustion temperatures which changes the type of decompositon the fuel undergoes prior to normal ignition, and it changes peak cylinder pressures. Peak pressures are lower but they tend to persist longer giving more useful mechanical work out of the lower pressures.

Water vapor also has a strong effect on the actual chemistry of combustion as it appears to assist with full combustion of carbon to carbon dioxide. Absolutely dry carbon monoxide mixed with oxygen is very difficult to burn but once water is present it will ignite and burn to CO2 easily.

Larry