octanes(gas question)

[JKWGA400EX]

whats 90 octane,is 93 like preimunum(sp?),or what dp the numbers stand for?

[GPracer2500]

The octane of a fuel is a rating. It's a measure of the fuels ability to resist combusting abnormally (which is bad for your engine). The compression of an engine determines how high of an octane rating its fuel must have. The higher the compression the higher the octane rating needed.

"Premium" is a term used for the highest octane fuel available at the pump. That can range anywhere from 90 to 94 depending of where you live (usually it's either 91 or 93).

You should use premium fuel in your 450r (I'm assuming your piston is stock).

[400exrider707]

The higher the number the higher the octan rating. The higher the octane rating, the higher temperature the fuel burns at. This is why high compression and advanced timing require higher octane, they create more heat, and if the fuel burns before the plug ignites you get detonation. Preventing detonation is done by running these higher octanes, as they burn at a higher temperature. Hope this helps?

[JKWGA400EX]

thanx,that helps.


-jared

[GPracer2500]

Originally posted by 400exrider707
The higher the number the higher the octan rating. The higher the octane rating, the higher temperature the fuel burns at. This is why high compression and advanced timing require higher octane, they create more heat, and if the fuel burns before the plug ignites you get detonation. Preventing detonation is done by running these higher octanes, as they burn at a higher temperature. Hope this helps?

This is sorta close but not quite right.

The temperature a fuel burns at is not determined by octane. Octane rating is a measure of a fuels ability to resist detonation and that's it--nothing more nothing less. Also, your confusing pre-ignition with detonation. They are both forms of abnormal combustion but are distinctly different phenomenon. Detonation ALWAYS occurs after the spark plug fires and pre-ignition ALWAYS occurs before the spark plug fires.

Detonation is when the fuel mixture at the edges of the combustion chamber explode before the flame front of the normal burn (started by the spark plug) has a chance to get there. In understanding abnormal combustion it's important to realize there is a difference between an exploding mixture and a burning mixture. Burning is good, exploding is bad. Pre-ignition is when a hot spot inside the combustion chamber ignites the fuel before the spark plug even fires. Detonation can lead to pre-ignition and vice versa but they are different things.

That's an somewhat simplified explination. I can dig up some old posts where I describe abnormal combustion in more detail if you want to know more....

[TMSmx47]

So basically what your all saying is that a higher octane burns slower.. example: "using higher octane if you have advanced timing or high comp to prevent detonation.. or EARLY burn."

I thought and heard in theory you make more power when you can get the fuel to burn and exit faster. hense using regular like 87 octane. Easier for the 04/05s using a stock piston. 91 (Plus) for 06/07s cuz of 12.1:1. How do you all feel about this?

[400exrider707]

Originally posted by GPracer2500
This is sorta close but not quite right.

The temperature a fuel burns at is not determined by octane. Octane rating is a measure of a fuels ability to resist detonation and that's it--nothing more nothing less. Also, your confusing pre-ignition with detonation. They are both forms of abnormal combustion but are distinctly different phenomenon. Detonation ALWAYS occurs after the spark plug fires and pre-ignition ALWAYS occurs before the spark plug fires.

Detonation is when the fuel mixture at the edges of the combustion chamber explode before the flame front of the normal burn (started by the spark plug) has a chance to get there. In understanding abnormal combustion it's important to realize there is a difference between an exploding mixture and a burning mixture. Burning is good, exploding is bad. Pre-ignition is when a hot spot inside the combustion chamber ignites the fuel before the spark plug even fires. Detonation can lead to pre-ignition and vice versa but they are different things.

That's an somewhat simplified explination. I can dig up some old posts where I describe abnormal combustion in more detail if you want to know more....

yeah thanks for clearing that up, makes sense.

TMSmx47, thats kind of right. The theory agrees with it. As far as making more power Im not sure, but I do know that running too high of an octane will cause starting problems as it requires more heat to combust, so in theory you would make more power with the correct lowest octane, that doesn't detonate. GP that sound somewhat right?

[GPracer2500]

Originally posted by 400exrider707
yeah thanks for clearing that up, makes sense.

TMSmx47, thats kind of right. The theory agrees with it. As far as making more power Im not sure, but I do know that running too high of an octane will cause starting problems as it requires more heat to combust, so in theory you would make more power with the correct lowest octane, that doesn't detonate. GP that sound somewhat right?

Somewhat. ;)

Cold staring performance is not directly related to octane rating. The fuel specification that IS directly related to cold starting is a fuel's distillation curve. The distillation curve is a separate and mostly independent specification. Octane ratings and distillation curves often correlate just like you suggest--many high octane racing fuels don't have especially good cold start performance. But it's because of their distillation curves not because of their octane rating. Some high octane racing fuels have great cold start performance even though they have high octanes.

Here's the deal with ignition advance, engine compression, and why having lots of either one (or both) needs high octane to stave off deto:

First, three terms-

Deflagration: This is a type of combustion. This is the good kind and it's the kind that happens inside the combustion chamber under normal conditions. It's when something combusts because of thermal conductivity. In other words, hot burning material (air/fuel mixture) heats up the material right next to it until that layer reaches its ignition temperature. That burning layer of material heats up the next layer to its ignition point and so on until everything is consumed. Deflagration happens at subsonic speeds. In the context of engines and combustion chambers, deflagration = the burning of air/fuel mixture.

Detonation: This is the other type of combustion. This is a bad kind that can destroy an engine. Detonation is when a material DOES NOT combust in the organized and steady manner of thermal conductivity, but instead combusts violently, at supersonic speeds, from many points simultaniously. In the context of engines and combustion chambers, detonation = the irregular explosion of air/fuel mixture.

End gas: The air/fuel mixture that is the farthest away from the spark plug (i.e. located around the edges of the combustion chamber) is the last part of the mixture to combust. These parts of the mixture are commonly called "end gases".

Still with me? Ok, good.

So what causes fuel to detonate? Answer: When a fuel is compressed and heated to extremes it begins to decompose the end gas into new chemicals through processes called preflame reactions. The products of these reactions are VERY unstable and have a tendency explode (detonate) all by themselves.

When the spark plug fires the burn begins at the spark plug and spreads outward from that central point. As the burn takes place the temperatures and pressures in the combustion chamber raise dramatically. Now, remember that the spark plug doesn't actually fire exactly at Top Dead Center. It fires many degrees of crank rotation before TDC. So, when the plug fires the burn (deflagration) begins while the piston is still moving upward compressing the space. That compression raises the pressures and temperatures even more...

Ok, we're almost done.

...It takes time for the burn to run it's course. The fuel/air mixture that is the farthest from the spark plug (the end gas) has to wait until the "flame front" gets there before it can be consumed. So that mixture around the edges of the combustion chamber just sits there enduring that extreme heat and pressure for a certain amount of time. The more ignition timing advance, the sooner the burn begins and the longer the end gases have to wait. It is because of these long waits, high temperatures, and high compression conditions (pressure!) that fuel decomposes into very unstable chemicals. It's the formation of these unstable chemicals that are responsible for detonation.

Octane ratings define how easily a fuel's end gases decompose under these extreme conditions. The speed at which deflagration takes place also has an impact on octane. The faster a fuel burns the less time there is for preflame reactions to take place. But understand that there is more than one way to achieve a high octane rating. You can have very fast burn speeds, or very stable properties (i.e. not easily decomposed), or some combination of the two. That's why octane is ONLY a measure of resistence to detonation. There are a whole slew of other fuel specifications that are responsible for burn speeds, cold start performance, vapor lock resistance, etc.



ETA: edited for clarity, to add a better definition of "end gases", and properly introduce the concept of "pre-flame reactions".

[chad502ex]

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here's the real formula ppl.


use this it really does work

Xylene 30% by volume

http://www.gnttype.org/techarea/misc/octanebooster.html


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[chad502ex]

Originally posted by GPracer2500
The compression of an engine determines how high of an octane rating its fuel must have.

first off, thanks for your participation.

second, I hope you mean dynamic compression, meaning a running engine's compression not static, right?

[GPracer2500]

Originally posted by chad502ex
first off, thanks for your participation.

second, I hope you mean dynamic compression, meaning a running engine's compression not static, right?

Yes. I specifically did NOT say compression ratio. I left static vs. dynamic (along with all the other factors that affect octane requirement) out of that first post just to keep things simple.

[TMSmx47]

Thanks GPracer2500... that was a good explanation of how it all comes together. My question still lies with the difference in using a 87 octane vs a 93 octane?

87 would produce the end gases faster... So a higher compression that creates all that greater heat makes it detonate. There fore we use the 93 to slow that process down. Correct? Now for instance using the 87 on stock low comp 04/05, wouldn't really hurt but may get a more complete burn? Am I in the area here?

[GPracer2500]

Originally posted by TMSmx47
Thanks GPracer2500... that was a good explanation of how it all comes together. My question still lies with the difference in using a 87 octane vs a 93 octane?

87 would produce the end gases faster... So a higher compression that creates all that greater heat makes it detonate. There fore we use the 93 to slow that process down. Correct? Now for instance using the 87 on stock low comp 04/05, wouldn't really hurt but may get a more complete burn? Am I in the area here?

First, I just made a minor edit to my post above to better explain end gases. End gases exist even when the air/fuel mixture does not severely decompose. End gases are always there even when everything is working normally. End gases are simply defined as the last thing to be consumed in the combustion chamber. But the end gases can decompose through "pre-flame reactions" if the fuel allows it (by way of low octane rating) and if there is enough time for it to happen. It's the products produced through pre-flame reactions that detonate.


To answer your question: As long as the fuel is not detonating then it should burn similarly whether it is high octane or low octane. A high octane fuel should not perform any worse than a lower octane fuel and visa versa assuming no detonation is taking place with either fuel.

But I'm only talking about octane rating here. Many times as octane rating changes some of the fuels other qualities change too. So if there's a difference in performance between two fuels, and deto isn't taking place with either one of them, then it's not the octane rating that accounts for the difference--it's some other characteristic of the fuel.

[TMSmx47]

Thanks GP.. Perfect explanation through and through! I looked around a little more to find exactly what you explained. Like the difference in octane compared to fuel qualities. 87 using lower grade additives then a 93. Both wont give you deto but the 93 will just give you a better flame. The 87 seems like it may not let the end gasses re-burn as easily with pre-flame reactions. 93's octane rating being slighty slower gives it the extra time.. also help the deto... I guess thats how you would explain it in lamens terms..lol!:rolleyes:

I understand what your saying, I just cant put it into my own words:huh