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Discussion Starter #1
After reading a few threads here, i decided to ask an expert...

i have an engineering friend who works in a fuel test lab and who designs equipment that tests fuel at refineries. i figured he'd be a pretty good source. :D

In a nutshell, octane has to do with the ignition point of fuel. Nothing to do with rate of burn. For example, if you have a sharp edge or chunk of carbon on a piston, it can get very hot, to the point it acts like a glowplug. If that happens, fuel can auto-ignite within the cylinder. Then, if the plug fires, you can get multiple flame fronts that collide, which you hear as "ping".

The higher the octane rating, the higher the resistance to fuel auto-igniting.

(on a side note, most filling stations have two tanks, one for lower octane, such as 87 and one for the highest octane, like 91 for example. To get 89 octane, they mix the two.)

The "performance" of fuel is something altogether different and is the rate at which it converts chemical energy to mechanical energy, or in other words, the rate of heat release. For those engineers out there, this is the area under the pressure vs. time curve. Unfortunately, there is no specification for this for auto fuel.

So, in other words, higher octane DOES NOT produce more horsepower. With that being said, there can be different "recipes" for different grades of fuel that may produce more or less horsepower. But it's not because of the octane rating.

I hope this clears it up a bit.
Don
 

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OK. Your assessment is absolutely correct. Almost everyone knows this already. Higher octane does absolutely nothing unless you are tuned for it (meaning you advance timing enough to risk detonation with lower octane fuel). Supercharged, turbocharged and high compression motors require higher octane to prevent premature detonation due to the higher cylinder tempature from increased pressure in the cylinder.
 

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Octane=resistance to detonation
 

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Discussion Starter #4
OK. Your assessment is absolutely correct. Almost everyone knows this already. Higher octane does absolutely nothing unless you are tuned for it (meaning you advance timing enough to risk detonation with lower octane fuel). Supercharged, turbocharged and high compression motors require higher octane to prevent premature detonation due to the higher cylinder tempature from increased pressure in the cylinder.
if everyone knew it already, why did you take the time to reiterate??? :D

i simply posted it as i've seen a few threads lately that have had some misinformation in them regarding octane, mainly that higher octane produces more horsepower. they have nothing to do with each other, which was the point of this.

i apologize if it's redundant.
 

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For example, if you have a sharp edge or chunk of carbon on a piston, it can get very hot, to the point it acts like a glowplug. If that happens, fuel can auto-ignite within the cylinder. Then, if the plug fires, you can get multiple flame fronts that collide, which you hear as "ping".
To expand on this glowplug issue....

One of the things I have always done on performance engine builds is polish the combustion chambers and piston tops/edges. Getting rid of casting flash and rough surfaces can make a significant difference in octane requirement. Some professional builders claim doing this can even allow for as much as 1 additional point in compression over non-polished chambers.
 

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my favorite part of all this was the:

"when flame fronts collide!"


:rockon:rockon:rockon:rockon:rockon:rockon:rockon

- good song title.
 

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Too Soon, Junior!
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I'm not sure I agree on the rate of burn part. I'll try to dig up the SAE paper but I remember there being a section that talked about the slower flame propagation velocity of 93 vs 87. If you think about it, it does make sense. Because higher octane fuels require a higher temperature and pressure to combust it, you get a slight increase in timing loss by switching to 93 from 87.
 

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Reps and Thanks. I almost posted this question, but figured I'd do some more research first. Too many people I know put premium in their cars every 4 tanks or after their motorcycle has sat all winter to "freshen it up" or clean it out... From what I've read that isn't going to burn any cleaner or faster than 87 (unless the car requires it) So thanks for the write up!
 

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I'm not sure I agree on the rate of burn part. I'll try to dig up the SAE paper but I remember there being a section that talked about the slower flame propagation velocity of 93 vs 87. If you think about it, it does make sense. Because higher octane fuels require a higher temperature and pressure to combust it, you get a slight increase in timing loss by switching to 93 from 87.
X2
I was (also) certain I read in some technical tuning manuals that higher octane fuels have a more controlled and slower rate of burn. Hence, a lower octane fuel does not actually require as much ignition advance when tuning for MBT. If the rate of burn is the same, regardless of octane, then why are you able to run more advance on higher octane? It appears the burn rate is mostly a function of the fuel blend & design and typically correllates with Octane. Here's what I found on the subject via Google search:

http://www.2strokeheads.com/tech-octane-detonation.htm
 

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Discussion Starter #13
I'm not sure I agree on the rate of burn part. I'll try to dig up the SAE paper but I remember there being a section that talked about the slower flame propagation velocity of 93 vs 87. If you think about it, it does make sense. Because higher octane fuels require a higher temperature and pressure to combust it, you get a slight increase in timing loss by switching to 93 from 87.
i understand what you're saying. i thought the same thing as it does seem to make sense, so i asked that exact question during my "interview". (I spent about an hour on the phone with him and we're going to get lunch sometime soon to continue the conversation.)

from what i was told, the octane rating only pertains to the ignition point. there's a bunch more variables that go into the formulation for other reasons, such as rate of flame travel. Quite often, an oil company will make other changes that have an affect on burn rate for their fuels that have higher octane. They can afford to do that as they charge more for that fuel. The "recipe" or chemical formulation of each of the oil companies fuels are different and as closely guarded as the colonels 7 herbs and spices.

I posted part of an article from Hot Rod magazine a short time ago where they tested some fuels and octane boosters. They mentioned that they were surprised to find that when testing some race fuel, (100+ octane) they produced more power by retarding the timing due to the fact that flame propagation was faster in that particular fuel, which supports what I've been told.

So, what does all this mean????
1. You don't need to spend money on higher octane fuel than your car requires.
2. Fuels from different companies are different. Find a brand you like and stick with it.
3. Get your car dyno tuned with a particular fuel and be sure to experiment with retarding the timing as well as advancing it, to achieve maximum HP.

The person i was talking to (his name is Jeff) basically said automotive fuel is a hodge-podge of ingredients and varies from supplier to supplier, from barrel to barrel and from season to season. The reason for that is that there's very few specifications that refineries are required to meet for automotive fuel as opposed to aviation fuel, for example. The best we can do is to find a brand you like and stick with it.

Lastly, Jeff has been all over the world at different refineries and has tested a myriad of automotive, aviation, diesel, etc. fuels. This is his livelihood. I'm not saying he's the worlds foremost expert, but i do believe what he tells me. I guess you can take it for what it's worth.

I'll post again if i find any additional information after we have lunch.
 

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You didn't mention the little caveat that I always hear that, "euro cars have to run higher octane because the have higher octane 'regular' in Europe," which isn't true--we just have a different form of measurement: (r+m)/2

I'm sure most of you know this but hey, what the hell. I need to build up my post count so I can get into the forums with nude pics. :D
 

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Discussion Starter #15
on a side note (one of the many tangents our conversation went to...)

want to run a GREAT race fuel???
convert to compressed natural gas (CNG) (not to be confused with propane)
120 octane and you'll have a perfectly homogenous (and obviously vaporized since it's a gas) mixture by the time it ignites in the cylinder.

Not to mention, it's the cleanest burning fuel on the planet!

the only problem is that not too many race tracks carry CNG... :(
 

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Even though none of my cars/trucks require anything more than 87 octane, I have found I can actually get slightly better MPG (2-3) out of 91 or 92 Octane Premium (as long as it doesn't have Ethanol). Obviously, it's not the octane, but probably a better quality blend of gas. I have also noticed a difference in MPG between name brand fuel like BP, Shell, Texaco, etc. and the cheap discount convenience store gas.
 

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Too Soon, Junior!
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on a side note (one of the many tangents our conversation went to...)

want to run a GREAT race fuel???
convert to compressed natural gas (CNG) (not to be confused with propane)
120 octane and you'll have a perfectly homogenous (and obviously vaporized since it's a gas) mixture by the time it ignites in the cylinder.

Not to mention, it's the cleanest burning fuel on the planet!

the only problem is that not too many race tracks carry CNG... :(
We're actually building a 30kW generator right now that runs on natural gas. It's based off of a Mazda F engine and will run 14:1 compression at 1800RPM constantly for 6 months at a time. The chemical properties of natural gas is pretty nifty.
 
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