91 octane and tuning question
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Joined: Sep 2008
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91 octane and tuning question
Will be receiving our new 75 today. The previous owner runs 93 octane, which unfortunately is almost non existant here. Will it run fine on 91 octane? And if it needs to be tuned to 91, how would I go about doing that? This is my first experience with older engines but I'm definitely learning. 
Here are some specs on the engine:
355 (0.30 over)
268/488 lunati cam
882 heads with 194 valves
600 cfi carb
Not sure what the compression ratio is
Appreciate any help.
Here are some specs on the engine:
355 (0.30 over)
268/488 lunati cam
882 heads with 194 valves
600 cfi carb
Not sure what the compression ratio is
Appreciate any help.
Le Mans Master
Joined: Apr 2011
Posts: 6,284
Likes: 1,171
From: Hermosa
What is you elevation? I run mine on 85 octane due to high altitude it allows me to run on lower octane. Try to run on something without ethanol if possible. Around here (western SD) that is 85 or 91 octane. 87 has ethanol in it and would cause the engine to run leaner without changing jets on the carb.
You can retard the timing to stave off detonation if it seems to be on the edge. You can enrichen your mixture (bigger main jets in the carb). You can run cooler plugs. Put in Electric fans to give the whole engine better cooling.
9 to 1 is not very high so you may be able to run low grade with no problems, but if you do hear pinging you can try some of these easier fixes or just bump up to 91 octane.
You can retard the timing to stave off detonation if it seems to be on the edge. You can enrichen your mixture (bigger main jets in the carb). You can run cooler plugs. Put in Electric fans to give the whole engine better cooling.
9 to 1 is not very high so you may be able to run low grade with no problems, but if you do hear pinging you can try some of these easier fixes or just bump up to 91 octane.
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Melting Slicks
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From: N.Richland Hills Texas
Tech Contributor
Joined: Aug 1999
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From: At my Bar drinking and wrenching in Lafayette Colorado
At the stock compession ratio, you can't get detonation no matter where you set the timing... but do set the timing right (36 total) and it will run just fine on standard 87 octane fuel. Running higher octane on a low compression engine gains you absolutely nothing.
Lars
Drifting
Joined: Aug 2009
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From: Enid Oklahoma
Sure it does....It gains you carbon deposits. :O)
Drifting
Joined: Aug 2009
Posts: 1,635
Likes: 8
From: Enid Oklahoma
Keep sayin it Mike someday it will come true. Not a myth anywhere but here. Just because you don't believe it does not make it a myth. Using correct octane makes for a more efficient burn(correct? based on compression and timing? right?). Using too high of octane does not allow for efficient burning of the fuel(or is this a misundertsnding on my part?).(not getting into how octane works) Watching any open fire. When it is cold and not burning efficiently it puts off a lot of particulates (carbon, since most things are carbon based) when it gets going and burning hotter(more efiiciently) there is a lot less particulate contamination and carbon since most of its fuel is being consumed. It is the same inside of a combustion chamber. It may not exist in your mind, and it may not cause huge issues most of the time, but it can happen. Running extremely rich will carbon crap up as well, but running too high of an octane leaves a lot of the fuel unable to burn completely in a lower comression motor and turns basically to soot or carbon more so than when proper octane is used, most is sent out the pipes but not all especially over time. I don't think you will convince me otherwise, but give it a shot I am frequently wrong and willing to be enlightened even though I am hardheaded most of the time.
Last edited by 81pilot; Jul 25, 2012 at 12:09 PM.
Race Director
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Keep sayin it Mike someday it will come true. Not a myth anywhere but here. Just because you don't believe it does not make it a myth. Using correct octane makes for a more efficient burn(correct? based on compression and timing? right?). Using too high of octane does not allow for efficient burning of the fuel(or is this a misundertsnding on my part?).(not getting into how octane works)
The octane rating of gasoline has nothing to do with an 'efficient' burn. The only significance of the rating is the resistance to detonation and nothing else. Nothing to do with burn speed/flame front speed, flame temperature, ethanol content, stability in storage, Reid vapour pressure, amount of cleaning additives, amount of energy it contains, etc. etc. or any other story you may have heard.
This means that once the gas/air mix has been ignited there is NO DIFFERENCE whatsoever in how the fuel burns.
Your example of hot vs. cold fires (?) has no relevance and makes little sense other than the mention of rich/lean mixtures. If you're trying to say that octane rating is connected with flame temperature, this is incorrect. If you're stating that a carb has to be rejetted to correct the mixture, that's way off too.
It's possible that you also believe the common myth that the octane rating is a reflection of how fast a fuel burns. I've always been mystified by this one. If it were true- and that high octane fuel burns more slowly as per myth- please explain how that avoids detonation?
Burning Brakes
Joined: May 2011
Posts: 838
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From: Louisville Ky
octane is a burn inhibitor. it slows down the combustion, preventing detonation, and thus does not burn as effieciently. you can expect more carbon/unburned fuel.
if you have the emissions test, which is more likley to pass -low octane, ethanol, or 93 octane?
with 9 to 1, 87 octane is normally fine.
if you have the emissions test, which is more likley to pass -low octane, ethanol, or 93 octane?
with 9 to 1, 87 octane is normally fine.
Drifting
Joined: Aug 2009
Posts: 1,635
Likes: 8
From: Enid Oklahoma
Putting your sarcasm aside, your basic understanding of combustion vs. octane rating is way off which has lead to your need to layer additional myths on top to make the whole story work.
The octane rating of gasoline has nothing to do with an 'efficient' burn. The only significance of the rating is the resistance to detonation and nothing else. Nothing to do with burn speed/flame front speed, flame temperature, ethanol content, stability in storage, Reid vapour pressure, amount of cleaning additives, amount of energy it contains, etc. etc. or any other story you may have heard.
This means that once the gas/air mix has been ignited there is NO DIFFERENCE whatsoever in how the fuel burns.
Your example of hot vs. cold fires (?) has no relevance and makes little sense other than the mention of rich/lean mixtures. If you're trying to say that octane rating is connected with flame temperature, this is incorrect. If you're stating that a carb has to be rejetted to correct the mixture, that's way off too.
It's possible that you also believe the common myth that the octane rating is a reflection of how fast a fuel burns. I've always been mystified by this one. If it were true- and that high octane fuel burns more slowly as per myth- please explain how that avoids detonation?
The octane rating of gasoline has nothing to do with an 'efficient' burn. The only significance of the rating is the resistance to detonation and nothing else. Nothing to do with burn speed/flame front speed, flame temperature, ethanol content, stability in storage, Reid vapour pressure, amount of cleaning additives, amount of energy it contains, etc. etc. or any other story you may have heard.
This means that once the gas/air mix has been ignited there is NO DIFFERENCE whatsoever in how the fuel burns.
Your example of hot vs. cold fires (?) has no relevance and makes little sense other than the mention of rich/lean mixtures. If you're trying to say that octane rating is connected with flame temperature, this is incorrect. If you're stating that a carb has to be rejetted to correct the mixture, that's way off too.
It's possible that you also believe the common myth that the octane rating is a reflection of how fast a fuel burns. I've always been mystified by this one. If it were true- and that high octane fuel burns more slowly as per myth- please explain how that avoids detonation?
Le Mans Master
Joined: Apr 2011
Posts: 6,284
Likes: 1,171
From: Hermosa
Quote:
Originally Posted by Mike Ward View Post
Putting your sarcasm aside, your basic understanding of combustion vs. octane rating is way off which has lead to your need to layer additional myths on top to make the whole story work.
The octane rating of gasoline has nothing to do with an 'efficient' burn. The only significance of the rating is the resistance to detonation and nothing else. Nothing to do with burn speed/flame front speed, flame temperature, ethanol content, stability in storage, Reid vapour pressure, amount of cleaning additives, amount of energy it contains, etc. etc. or any other story you may have heard.
This means that once the gas/air mix has been ignited there is NO DIFFERENCE whatsoever in how the fuel burns.
Your example of hot vs. cold fires (?) has no relevance and makes little sense other than the mention of rich/lean mixtures. If you're trying to say that octane rating is connected with flame temperature, this is incorrect. If you're stating that a carb has to be rejetted to correct the mixture, that's way off too.
It's possible that you also believe the common myth that the octane rating is a reflection of how fast a fuel burns. I've always been mystified by this one. If it were true- and that high octane fuel burns more slowly as per myth- please explain how that avoids detonation?
I typed a huge reply and lost it! So I will simplify it a little. Higher Octane usually contains more aromatic or parrafin based hydrocarbons which are know contributors to carbon fouling in all forms, injectors, valves, combustion chambers etc.... The metal composition in older engines also lends a hand in carbon fouling. Running too high of octane does not allow a complete process due to the molecular composition of the fuel. When a proper octane is used the state of it changes right before spontaneous detonation, which is a chemical reaction due to pressure changes. Without that change the fuel will not completely burn, and leaves by products that react with the surrounding metal to cause deposits. But all that is irrelevant if people understand to use the proper octane in their application. Sorry about the sarcasm in the previous post.
This article is pretty enlightening on the octane question and even adresses the use of aviation fuel to improve octane rating.
http://www.2strokeheads.com/tech-octane-detonation.htm
Last edited by REELAV8R; Jul 25, 2012 at 03:52 PM.
Race Director
Joined: Jul 2001
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Getting a bit off topic from the OP's question, when I raced motocross this subject used to come up time and time again.
This article is pretty enlightening on the octane question and even adresses the use of aviation fuel to improve octane rating.
http://www.2strokeheads.com/tech-octane-detonation.htm
This article is pretty enlightening on the octane question and even adresses the use of aviation fuel to improve octane rating.
http://www.2strokeheads.com/tech-octane-detonation.htm
I'm not sure that we're off topic, especially when posters keep throwing in stuff that might guide the OP in the wrong direction. The article you've quoted is quite good and is one of many that attempts to dispel the myths. Being that there's lots of similar articles available on the weeb and have been for years, I guess it's obvious that it's pretty much a futile effort expecting that it will make much difference in changing people's minds.
I hear that the flat earth society is alive and well too.
False. It is a DETONATION inhibitor. Please learn the difference.
Last edited by Mike Ward; Jul 25, 2012 at 05:17 PM.
Race Director
Joined: Apr 2000
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From: Who says "Nothing is impossible" ? I've been doing nothing for years.
My 600HP 427ci small block runs on 91 no problem, depending on how long the previous owner put the higher octane in will determine how much money he wasted. Just put in regular gas and save a few bucks. If it pings which I doubt come back and we'll fix it up for you