89 Octane vs 91 Octane

3.8MustangV6

New Member
I am in California and I just heard on the news that Gas prices this summer are going to be high. My Mustang has about 21,000 miles and I have always used 91 Octane. My question is, has anyone seen any difference in performance (slower response, knocking, etc) by using a lower Octane Gas?
 

V6 SSTANG

Part Time Racecar Driver
You have a faily new car, dont worry you can safely use 87. I use 89 just to make myself feel better and im at 84k miles. My moms car didnt like 87 but she has 150k on her expedition so we switched it to 93. On a car like yours 87 will be fine. And yes, gas prices are rising and it sucks.
 

SteveC

Well-Known Member
Octane is the fuel's ability to resist knock. So, a higher octane fuel ignites and burns slower.

You always get the best performance with the lowest octane that you can run without pinging. In other words, if you are using higher than 87, you are just wasting money and hurting performance.

Higher octane can actually cause power loss (though it is minimal).
 

nikko

Admin Emeritus
SteveC said:
So, a higher octane fuel ignites and burns slower.
This is a common misconception.

You are right that Octane rating is the fuel's ability to "resist knock". really, that would be a more accurate statement if you said that higher octane fuel helps the engine resist knock, as fuel does not "knock" on it's own.

"knocking" is of course another term for detonation or pinging, which all refers to the act of fuel igniting in the combustion chamber before the spark plug fires. This can happen at times due to the intense pressure and heat that the fuel is under. At a high enough temperature, fuel will ignite without a spark - octane rating in particular is an indication of how much heat and compression fuel can withstand before spontaneously combusting.

In every scenario here we're talking about a catalyst to get the fuel burning. In any case, the energy and heat produced by a spark plug is hundreds of times that necessary to ignite the fuel - regardless of octane rating. however, once the fuel is ignited, be it by spark or just simple heat from compression, the rate at which it burns is more or less completely independent of octane rating. As is the power it makes. Higher octane gas does not burn slower, and does not produce less power than regular gasoline. On the same coin, however, it does not produce any *more* power either, despite what the oil companies will have you believe. The only thing you gain with higher octane fuel is the ability to resist higher levels of heat and compression before you get knock/pinging/detonation. But, if you're using premium fuel in a relatively low compression engine that does not have a power-adder or anything of the sort (such as your 3.8), the only thing you're gaining with premium fuel is a significantly lighter wallet.

-N
 

Nurserick3

White Trash Racing, Inc.
KTBug said:
It actually ignites at a higher temp. It burns at the same rate though.
YUP!!! The only time you should need a higher octane gas is if you have had a chip upgrade that includes advancing the timing which will take advantage of the higher ignition temp thus creating slightly more compression and a little more horsepower.
 

AZ2KVert

Old guy
Whenever I visit California and fill up on 87 octane, my car starts pinging. I have to use 89 or 91 octane there.

I have no similar problem with 87 octane in Arizona. Run it all the time without pinging. I think it has something to do with the gas mix that is pumped at California stations. The state must play with chemistry to keep down emissions.
 

mark1wat

MStang
nikko said:
This is a common misconception.

You are right that Octane rating is the fuel's ability to "resist knock". really, that would be a more accurate statement if you said that higher octane fuel helps the engine resist knock, as fuel does not "knock" on it's own.

"knocking" is of course another term for detonation or pinging, which all refers to the act of fuel igniting in the combustion chamber before the spark plug fires. This can happen at times due to the intense pressure and heat that the fuel is under. At a high enough temperature, fuel will ignite without a spark - octane rating in particular is an indication of how much heat and compression fuel can withstand before spontaneously combusting.

In every scenario here we're talking about a catalyst to get the fuel burning. In any case, the energy and heat produced by a spark plug is hundreds of times that necessary to ignite the fuel - regardless of octane rating. however, once the fuel is ignited, be it by spark or just simple heat from compression, the rate at which it burns is more or less completely independent of octane rating. As is the power it makes. Higher octane gas does not burn slower, and does not produce less power than regular gasoline. On the same coin, however, it does not produce any *more* power either, despite what the oil companies will have you believe. The only thing you gain with higher octane fuel is the ability to resist higher levels of heat and compression before you get knock/pinging/detonation. But, if you're using premium fuel in a relatively low compression engine that does not have a power-adder or anything of the sort (such as your 3.8), the only thing you're gaining with premium fuel is a significantly lighter wallet.

-N

This just got me thinking so just out of curiousity. Do you know what is the octane rating of Diesel fuel? Does that come into play with the engine having no spark plug, or is it just because they have such a high compression ratio?
 

nikko

Admin Emeritus
AZ2KVert said:
Whenever I visit California and fill up on 87 octane, my car starts pinging. I have to use 89 or 91 octane there.

I have no similar problem with 87 octane in Arizona. Run it all the time without pinging. I think it has something to do with the gas mix that is pumped at California stations. The state must play with chemistry to keep down emissions.


Could be the reformulated gas used here during winter months.

It has a higher ethanol concentration I believe, among other things. Reduced emissions during cold months when they're usually at their worst. It does seem to have the effect of lowering the octane rating a bit - which seems odd, as 87 should be 87 no matter what the gas is made of.

The common recommendation for this is to, of course, buy up a grade.

Sounds like a conspiracy to me.

I hate the gas in this state. Try running a car that requires premium here... my car is soooo picky about what goes in it I can basically buy gas at like two stations.


Gr..
 

nikko

Admin Emeritus
SteveC said:
It does burn slower. Even a simple google search you can find that out.

Here's just one... the first one... http://www.state.mn.us/mn/externalDocs/Commerce/Gasoline_Octane_Facts_102902052227_OctaneFacts.pdf

High octane gasoline burns slower than low octane gasoline. The slow burn prevents engine knock when
cylinder pressures are high.

That's just wrong.

Negligible difference in "speed of burning" aside, the "slow burn" is not what prevents engine knock. The fact that it doesn't start burning until the plug fires is what prevents engine knock.

Whoever wrote that article and let that sentence slip is lacking a basic understanding of high school chemistry.

And then to go on and say that pinging under full throttle is normal? Yeah. WTF?
 

drew_shipley

Bullitt #5023
nikko said:
Whoever wrote that article and let that sentence slip is lacking a basic understanding of high school chemistry.

And then to go on and say that pinging under full throttle is normal? Yeah. WTF?

:lol:
 

SteveC

Well-Known Member
Nikko, no matter what I'll bring to the table as far as proof, you will counter as wrong. So what even try? You are confusing combustion with burn time.

The fact is that a higher octane is not necessary. I'm not going to spend time trying to argue with a guy that is never wrong. :uhh:
 

nikko

Admin Emeritus
SteveC said:
Nikko, no matter what I'll bring to the table as far as proof, you will counter as wrong. So what even try? You are confusing combustion with burn time.
How does a longer burn time prevent detonation? That's what I was talking about in my last reply.

The fact is that a higher octane is not necessary.
Agreed.
 

cobra232

New Member
nikko said:
How does a longer burn time prevent detonation? That's what I was talking about in my last reply.


Agreed.
longer burn time directly corresponds to a heat of oxidation or heat of diesel ignition. the longer something burns the harder it is to ignite. you said octane doesn't hinder burn speed which it most certainly does. we are only talking milliseconds but it still increases burn rate and increases resistance to diesel combustion heat of ignition.

octane is a longer chain hydrocarbon which means it has a higher diesel ignition. when added to gasoline it raises gasoline's diesel ignition temp and because it is a longer chain hydrocarbon it contains more potential energy, which when ignited gives more power compared to a lower octane content fuel. the power difference may amount to less than 1 hp but it still will produce more power in the same engine compared to lower octane fuel

diesel fuel has a similar additive called cetane which prevents detonation which is a serious consideration in a diesel.

just like the old saying that adding mothballs adds power becauses it increases octane. to 1 extent there is some truth to that but the reality is that mothballs (naptha) like octane has a higher diesel ignition temp than gas and can add alittle power and prevent detonation.

the problem is that naptha sublimes and doesn't readily mix with liquid gasoline
unless sloshed around in the tank quite a bit. throughly mixed (which is near impossible because of sublimation) mothballs has the same affect on gas as octane.

there are other additives than can be used in gasoline to prevent detonation like Nonane, Decane,Heptane, Kerosene,Napthalene,Diesel and pretty much any combustable hydrocarbon that has a longer molecular chain than gasoline

maybe you should take some organic chem classes then you might know these facts. or if your are so smart then apply for that idiots job with the Minnesota dept of commerce since that guy is lacking basic highschool chemistry but seems to have a highly technical job that pays pretty well to be published.

Nikko i usually respect you cause you are a knowledgable person but here you are dead wrong
 
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