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A friend and I were having a discussion about high octane fuels such as 100 octane. we were wondering if the gas would do any harm to a stock engine. I know there would probably be no benefits, but would it be bad for the engine.
 

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Running higher octane fuel than your motor requires is not a good idea. As has been somewhat already stated above, the higher the octane number, the higher amount of heat and time it takes to burn. As a result, running that high octane fuel in an engine that doesn't require it will increase egt's heavily due to the fact your going to have fuel still burning during the exhaust stroke...this can and will lead to a melted piston. On top of that fact you are going to get a hell of alot of carbon building which is going to foul your plugs and reduce the volume of your exhaust ports.
 

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I don't agree with the higher temps in the cylinder, however, due to the slower burn, the combustion process must be done before opening the exhaust valve or they get cooked.
 

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Noone mentioned higher cylinder temps specifically first of all. Higher EGT's on the other hand were. A higher factor in this case being the result of burn still occuring on the exhaust stroke. The same result would be as if you retarded ignition timing..higher EGT's. Carbon buildup on the pistons, to an an extent, create a naturally occuring thermal barrier which reduce crown temperatures in the piston. Excessive carbon buildup, as is the result of running too high an octane of fuel, can have an adverse effect by creating hot spots on the piston. These can certainly create detonation which will raise cylinder temperatures considerably.

An excessive amount of carbon also, over the period of time, will have increased difficulty remaining attached to the top of the piston. These small shards of carbon will create more "fuel" for the fire and will increase combustion temperatures. These shards do not have a cooling or flame front slowing effect that vaporized fuel does, and they are extremely volatile. Pistons that experience this condition often have a cratered or highly pitted piston crown. This is the direct result of constant burn occuring on the top of the piston.

A "certain" seminar took place in June 2003 where the hosts spoke extensively on this issue. Apparently, they (who?..won't say) felt thier dealers should know that part throttle tuning should be less ignored, and that fuel cooling has drawbacks when overdone in forced induction cars. They gave many examples.

Still disagree Lule?
 

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I agree with the higher EGT. I was concerned that the impression was given that octane was an indicator of BTU instead of resistance to detonation.

Fuel mixed with oxygenates such as ethanol and methanol, will raise the octane and create more BTUs per given amount.

As for carbon burning on the piston tops, preignition is a real issue and I agree with that as well.

Maybe our replies were oversimplfied. I did enjoy the parry, however. It is infrequent that I get intelligent rebuttle.
 
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i kno with my engine, JDM D15b vtec, it runs like shite on anything less than 91octane.

the least its ever seen in my car was 89, but tahts because a buddy of mine thought it was the super, when the label clearly said 'Extra'.. enihoo, the engine sees 93octane on a regular basis, but i wont settle for anything less than 91.

i have also heard tho that over in JDM land, they use something like a 98 octane rating.. is there any thruth to that?
 

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As far as i know japan has higher octane for their regular fuel. Thats why some of their engines only in japan have higher compression. I dont know for shure, just what ive heard.
 

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there is a way to calculate which exact octane rating is best for your car and driving conditions.. basically, if you run your car on anything more than what it needs, it will become more and more inefficient just like they've all told you so far.
if i can find that formula ill post it here but i cant remember where i saw it....
 

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Whats funny Skitty, is I sat down one day trying to convince my aunt not to run "the good stuff" in her low mile Buick. I sat basically regurgitated an entire section of "The Internal Combustion Engine in Theory and Practice" by Charles Fayette Taylor. He spoke extensively on fuel octane, different combustion enviroments and shapes, etc. At the end...hoping I hadn't lost her at the on ramp and wasted thirty minutes, I was sure I had supported my point that "running the good stuff" in her application was a waste of money and doing no good at all. ......She still runs "the good stuff". Sometimes no matter how much info you tell people, they mentally are convinced that something works better even though it defies all principle...hell reality. All it shows is a lack of understanding that leads these people to insist on being right. You don't see the gas stations or companies hurrying to educate though.
 

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I filled up with 100 octane this past weekend after about a week and a half after my engine head work. I noticed a difference right off the bat. The car was a lot smoother and felt better. I do have a 12.5:1 cr. I was hesitant at first, but it was suggested to me to for the car to run better.
 

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d16a6 power said:
As far as i know japan has higher octane for their regular fuel. Thats why some of their engines only in japan have higher compression. I dont know for shure, just what ive heard.
This is an old discussion... but to keep it short:

US uses a RON+MON/2 octane rating system. (also called CLC)

Japan uses RON only.

So the numbers are going to be different for the SAME fuel.

91 RON in japan is the SAME as 87 in the US.

i think 98 RON in japan is about 94/95 in the US... so a little higher than what we can get.
 

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Yeah, right on there. Just for those who may be interested, to put it simply, RON is a measurement of the fuels resistance to detonate as a result of increasing compression, MON is the measurement of the fuels resistance to detonate based on increasing intake temperature and ignition timing. We add the two numbers and divide it by two to get a more clear average the the fuels overall resistance to detonate.

I must also clear up one thing, higher octane fuels do not burn slower. In the same conditions fuels of various octane will burn at the same rate.
 

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YAAAARR!!1 Up from the dead!

vtecnitrocrx said:
"The Internal Combustion Engine in Theory and Practice" by Charles Fayette Taylor. He spoke extensively on fuel octane, different combustion enviroments and shapes, etc.
I hate that book. People treat it like a primary source, like it's the King James, and CANNOT be questioned. The first book and half the second goes on... and on... and on... about fuels and their reactions and the thermodynamics of the situation. And - he gets it wrong.

For every pound of gasoline ingested, two pounds of water are produced. Open an engineering book on steam engines sometime. Wet steam expands as much as 1800 times - dry gasses do not do this. Notice how we spend billions of dollars to construct cutting edge nuclear reactors - in order to heat water to turn steam turbines, because it just doesn't get any better. What we have here are all these hot air heat combustion gasses contaminating the working fluid *not* making power. This explains why rich mixtures make more power than stoich; even though rich mixtures, all else being equal, are cooler we have created more working fluid. Why for equal setups running equivalent BTUs of gasoline vs alky, why the alky *always* makes more power; it creates more working fluid and less contaminants per BTU.

It is not Otto or Carnot... it is Open Cycle Reciprocating Steam aka Harris Cycle.

None of this even touches on why octane (knock resistance) has nothing to do with flame speed (burn rate). Just because the high octane fuel you use burns horribly slowly does not mean that is a reflection on octane. Pfft. Common EHN CAS 27247-96-7 is what is used in cetane improver, and will speed combustion reactions with minimal effect on octane - order it in bulk, cheap.

*sob*
 
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