91 octane

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Frank Enstein

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High octane gas has less heat energy(lower BTU content) in it because they use lighter solvents with higher ignition points and slower burn rates. Used to help build refineries, petroleum engineers everywhere that loved talking about stuff like that. The alcohol used in the blend has at best 75% of the BTU's that gasoline has but has an octane rating of about 106. Lower BTU's, less fuel mileage. Engines built to run only on E85 can have much higher compression to increase fuel mileage and power but still lose fuel mileage compared to similar gasoline only engines. What I am trying to say, if your engine is designed to run on 87 octane, there is no benefit from using higher octane without a different tune up. Just info.

Nope.

The chemical make up of pump gas changes literally weekly to try to match next week's weather. Race fuel is as close as can be made consistent from batch to batch allowing a finer tune-up than pump gas allows. Light aromatics burn very fast and can degrade octane rating. Light aromatics have much lower flashpoint temps and are much easier to ignite with a spark or heat/pressure. The "Winter" blends of fuel have a higher amount of the lighter, easier to ignite light aromatics. They don't have as much BTU/gal as heavier ones. That is part of why your fuel economy goes down in the winter. One reason why diesels get better fuel economy is diesel fuel is made up of heavier chemicals. Fuel economy drops off for diesels in the winter as well because they have to add lighter chemicals to raise the cetane rating (the opposite of octane rating so they will start when it's cold outside).
 

Hipster

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Read it again, slowly

Sorry man I just can buy into your theory that the spark event happens and higher octane waits to ignite. Maybe it's your simplified way of explaining it. Most would be in agreement higher octane ignites at the same time with a slower burn and slower moving flame front albeit fractions of a second.
 

Pinger

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It's the resistance to spontaneous explosion (detonation) in the 'end gas' (the as yet unburned portion of the fuel/air charge) that differentiates hgh and low octane fuels.
Octane is the inverse of cetane. Imagine if the end gas was a composition of air and diesel (high cetane, low octane). The rising pressure in the chamber (and the heat created from it) would spontaneously explode the end gas before the flame front could reach it.
 

thinger2

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Yep, What I stated was a pretty dumb and simplistic way of trying to explain it.
Sometimes I get a little too carried away with trying to talk to people like they are 5 instead of just deferring to the actual pros on this site.
In my own defense, I deal with architects and city officials all day which is kind of like showing card tricks to a dog.
I cant promise that I wont be a dumbass again, but I will eventually admit it, if I have too, I guess...
 

Frank Enstein

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Yep, What I stated was a pretty dumb and simplistic way of trying to explain it.
Sometimes I get a little too carried away with trying to talk to people like they are 5 instead of just deferring to the actual pros on this site.
In my own defense, I deal with architects and city officials all day which is kind of like showing card tricks to a dog.
I cant promise that I wont be a dumbass again, but I will eventually admit it, if I have too, I guess...
I wouldn't say dumb and simplistic. Perhaps misinformed. Now city officials on the other hand...
 

thinger2

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I wouldn't say dumb and simplistic. Perhaps misinformed. Now city officials on the other hand...
I just now this minute got off of a zoom where they are trying to decide if they should use the gold shovell or the gigantic pair of ****** scissors.
Rant of the day incoming.
Im a long time decades long management office guy.
Buy union, contract union jobs
If you dont wake up in the morning and go to bed at night without making sure that you have done every damn thing you can do to get everyone home safe each and every day?
Get the **** out of this business.
Thats my rant for today, its been a long day
**** em
 

Pinger

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Yep, What I stated was a pretty dumb and simplistic way of trying to explain it.
Sometimes I get a little too carried away with trying to talk to people like they are 5 instead of just deferring to the actual pros on this site.

If the process of detonation and the role octane plays in preventing it is now clearer in your mind then the forum has succeeded (in sharing knowledge). Wouldn't have happened without your posts.
 
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