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the lowest octane you can run without detonation will produce the most HP. higher octane is for higher compression and/or advanced timing. imo, 93 octane is a waste with stock internals. run 91 and get it dyno tuned.

i'm generalizing here, but it's what i did.
 

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I cant even find 93 here in Arizona. I had Bama throw me a tune for 91 octane only. I feel better about running 91 in my car all of the time, rather than running Regular. In just daily driving, how much better of different will 91 be than say 87 or whatever Regular is?
 

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the purpose of higher octane is to prevent detonation or "pinging". if the manufacturer recommends 87 octane and you have stock compression and a stock ignition timing curve, you gain nothing with higher octane fuel. as a matter of fact, you may lose a pony or two because it burns slower. it really is that simple.

i have stock compression, but have added most other bolt ons and have an SCT tuner set for 91 octane fuel. basically, it advances the timing. had i not done that, i'd still be running 87 octane.

the other advantage of 87 octane is that a ton more people use it so it's always fresh. fuel does have a shelf life, which is why we add stabilizers when it sits over a month or so.

lastly, with prices being what they are, well, you know...
 

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hmm so i got a completely stock 03 gt engine wise n the previous owner only put 91 n so do i, so if i put 87 will there be any difference? i put 91 cuz 1 the orignal owner did and 2 every car in my house has the premium fuel recommendation but mine doesnt but i still do. is a tune a necessity for 91 gas or it doesnt matter? but i noticed in my previous mustang the car drove smoother with 91.
 

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it doesn't hurt to put 91 in it, the question is, does the car require it and is it worth the extra money. if it's stock, look in the owners manual and see what they recommend. that's the minimum you should go with.
 

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ive always ran 89 in mine, its inbetween and ive never had any problems or noises.
 

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I had a tick forever until i started using premium and the tick is gone.
 

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The difference probably wouldn't even be noticeable. I did a comparison of running my custom 93 tune vs no tune at all at the track and the difference was 1 tenth and 1mph. 93 in the tank on both runs.
 

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very interesting thread, guess i've never in my years taking time to my
own testing.

@ dynoDan: so what your saying is a car (just about full-bolt ons) plus
SCT tuned..... running the 87 octune tune will be just the same as the
93 octune tune????

Also how then does having gears effect what gas your usings in conjuction
w/ ur sct tuner??
 

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at a minimum, you have to run the octane that allows your engine to run without predetonation. if you're running high compression or have your timing advanced, you will probably have to run higher octane fuel. if you don't have either, you probably don't need it.

you can look in the owners manual to see what the recommended octane rating is for a "stock" engine. adding bolt-ons may or may not have an affect on that. if your engine breaths better, you may be filling the cylinders a little more which could affect the effective compression ratio. unlikely it's enough to make a substantial difference, but that's the general idea.

a "93 octane tune" advances the timing, so you should run that fuel.

i'm going to generalize here, but bolt-ons don't make enough of a difference to require higher octane fuel. it may run a tad better if you do, but i doubt if it would be required.

changing the tune, changes the timing advance, so you should go with their recommended octane rating.

gears simply increase your mechanical advantage between the engine and the ground. no affect on required octane.

make sense?
 

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Seems some folks here dont understand octane rating. I am no master mind on the subject of octane, but its commen sense that higher octane wont mean more hp for all engines, especially a stocker with low compression.

BTW @MacDizzle, the reason for the tick was because your engine didnt like the Heptane percentage in your lower octane. Higher octane such as 91 means 91% isooctane and 9% heptane (Heptane is the zero point of the octane rating scale. It is undesirable in petrol, because it burns explosively, causing engine knocking, as opposed to branched-chain octane isomers, which burn more slowly and give better performance.)
So the higher the octane the less heptane. BUT heptane also acts as a solevant and thats why you want the lowest possible octane your car requires.

To the OP, yes, I +1 that for SOME people, lower octane is better for specific cars that dont require the higher octane (Higher octane ratings correlate to higher activation energies: This being the amount of applied energy required to initiate combustion. Since higher octane fuels have higher activation energy requirements, it is less likely that a given compression will cause uncontrolled ignition, otherwise known as autoignition or detonation.)

BTW, in parenthesis is str8 form wikipedia.
 

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my understanding is that your engine doesn't care what octane fuel you use unless it's too low. Also, octane in itself does not produce horsepower. It allows more complete utilization of the internals of your engine. (This came from one of the engineers at 104+ Octane Booster)

The following statement came from a test Hot Rod magazine did on different octane fuels and fuel additives.

But most of all, we discovered that our presumption that higher-octane fuels burn slower than lower-octane fuels (and therefore require more ignition lead) is largely incorrect. There are too many other fuel-formulation issues at work to assign a general rule about octane. Race fuel tends to have a more powerful formulation than pump gas, regardless of octane rating, because it is denser and can release more power and heat. (Note that we made the best power with 114 octane with the least ignition lead, indicating it had the fastest burn time.) California pump gas is blended with methyl tertiary-butyl ether (MTBE), alcohol, and other ingredients damaging to performance. Knowing what we know now, we’ll always experiment with ignition timing—both higher and lower settings—when we change fuels rather than presuming that more power can be made with more octane due to more timing.

i also have an engineer friend who works at a local company that designs octane testers for refineries. i'm sure he'll be a wealth of information that's way above the level of what most of us find useful. :confused: But, i am going to touch base with him on this.

Anyway, interesting conversation. i always enjoy learning more.
Thanks,
Don
 

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Thank you for clearing that up

Thank you for clearing that up! So ya anyone engine knocks, start running premium, if you do that and it still knocks check your oil for cryin out loud. lol
Seems some folks here dont understand octane rating. I am no master mind on the subject of octane, but its commen sense that higher octane wont mean more hp for all engines, especially a stocker with low compression.

BTW @MacDizzle, the reason for the tick was because your engine didnt like the Heptane percentage in your lower octane. Higher octane such as 91 means 91% isooctane and 9% heptane (Heptane is the zero point of the octane rating scale. It is undesirable in petrol, because it burns explosively, causing engine knocking, as opposed to branched-chain octane isomers, which burn more slowly and give better performance.)
So the higher the octane the less heptane. BUT heptane also acts as a solevant and thats why you want the lowest possible octane your car requires.

To the OP, yes, I +1 that for SOME people, lower octane is better for specific cars that dont require the higher octane (Higher octane ratings correlate to higher activation energies: This being the amount of applied energy required to initiate combustion. Since higher octane fuels have higher activation energy requirements, it is less likely that a given compression will cause uncontrolled ignition, otherwise known as autoignition or detonation.)

BTW, in parenthesis is str8 form wikipedia.
 

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So far I've only ran Bama 87 and 93 octane tunes. I ran the 87 real low so I could fill up with 93, loaded the 93 octane tune, switched backed to 87 and the 87 octane tune a few weeks later.

Their tunes are so conservative that I didn't even notice the slightest seat of the pants feel. So, I just save a few pennies and run 87. Only reason I wanted to even try it was to get the most out of it since I was going up against a friend with a bolt on L98 powered '91 Firebird. The extra few dollars for 93 octane was a total waste for my stock internal engine, advancing the timing can only do so much with stock internals, stock cams, stock heads, etc. Regardless of the gas I took the Firebird 2+ lengths each time in a 50 to 120+ roll. Ran out 4th a little past my 5,500-5,600 limiter the one run to about 120, grabbed 5th and pulled to around 130 and just ran away from it. Probably 5+ lengths the one time.

I might be in trouble soon though. Between his being an auto and easier to launch, and my tires just blowing off on most launches, I may lose from a dig. I'll just slip the clutch a little, mash it once I get rolling, and go to work with jamming them gears. Don't think 87 or 93 octane will make a difference there. :lol
 
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