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Discussion Starter · #1 ·
Just moved to a new state and 99% of the gas stations only carry 91. This weekend I found out that very near to my house is a shop that sells race fuel, they have their own pumps that carry ethanol free gas that goes up to 98. While I can’t see myself spending $9.99 a gallon regularly for the 98 would it be worth spending about $1.50 more a gallon to fill up with 96?
 

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2022 Ice Silver BRZ Limited manual.
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That would likely be beneficial if you drive hard often. 91 is a sacrifice. Even if there isnt noticable power gain the added knock resistance is just a safety cushion. You could do thr same thing but blend in some E85 for much less $$ if it is available. That is over 100 octane. The stock fuel system and computer is perfectly capable of using low ethanol blends. Once you go to higher ethanol blends or pure E85 is when tuning is needed.
 

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You could do thr same thing but blend in some E85 for much less $$ if it is available. That is over 100 octane. The stock fuel system and computer is perfectly capable of using low ethanol blends.
I’ve thought about doing this, but any thoughts on how completely the mix blends in the tank? I think octane stability is pretty vital for engine tuning and longevity, especially with the way knock adjustments work on these cars (or at least did for the 1st gen car).
 

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In a nutshell, 93 or above is required to run at optimal timing without risk of pre-ignition (knock). If you use 91, it won't harm the engine, but the ECU may adjust timing to prevent pre-ignition and that'll cause some power loss.

To save some change, you can mix gas of different octane ratings. If a station carries 91 & 98 and you're filling an empty tank, put in four gallons of 98 and fill the rest with 91. That'll give you just over 93. Or if you're mixing 91 & 96, go with a splash over five gallons of 96.
 

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I’ve thought about doing this, but any thoughts on how completely the mix blends in the tank? I think octane stability is pretty vital for engine tuning and longevity, especially with the way knock adjustments work on these cars (or at least did for the 1st gen car).
It mixes just fine. The best way to go about it is to add the 1-2 gallons of E85 first. Then just by pumping in the other gas, it will agitate and blend it up. I do this all the time in my SHO.
 

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Gas prices are too high where I live to put 93 in. I stepped down to 89 and have not noticed a difference. The dealer put in 87 when I took delivery and drove it home 200 miles. Most modern cars have some device to detect octane and adjust the performance accordingly.
 

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Wait. You guys have 87 ? 89 ? 93 ?! In europe we only have choice between 95 and 98 !
Or maybe we don't talk about the same things ?
 

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If the ECU is not tuned for more then 93 (98 RON in Europe), i cannot generate more power.

Too high octane numbers can also result in a loss of power, because its harder to inflame the gas/air mixture.
 

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Well, the ECU 'learns' to an extent, its always trying to maintain optimal performance/emissions of the engine. If you have colder air and higher octane fuel you will find that the engine will eventually start to generate more power, so long as you PUSH the engine so that it is finding those new optimal performance/emissions.

There are 3D _BASE_maps for ignition timing etc, however there is a correction factor which makes dynamic changes based on lots of things, but in particular the lambda oxygen sensor in the header, just before the headers cat. Via this correction factor your final timing on ignition and how much fuel/air is used.
 

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Too high octane numbers can also result in a loss of power, because its harder to inflame the gas/air mixture.
Not true. Octane is the fuels ability to resist engine knock. It has nothing to do with its flash point or ignitability. There is no such thing as too much octane. Although most cars can only benefit from so much .
 

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Gas prices are too high where I live to put 93 in. I stepped down to 89 and have not noticed a difference. The dealer put in 87 when I took delivery and drove it home 200 miles. Most modern cars have some device to detect octane and adjust the performance accordingly.
You are playing with fire. The dealer doesnt give a shit and puts in what is cheap for them. The owners manual and manufacturer of your very high compression engine say it should have 93 octane fuel. It can get by with 91 and even that is a sacrifice. Man, if you you cant afford the gas, dont buy the car. You are doing the car and yourself a disservice.
 

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If not true, why does higher octane self ignite later?
It does not ignite later. What higher octane fuels do is burn slower. It is very important that high compression and forced induction vehicles use high octane fuel. Lower octane fuels will burn off too quickly for the amount of air being forced into the engine causing a lean condition. Also pre-ignition (AKA knock) can happen with low octane. Which means under high heat that fuel burns off so fast that the combustion cycle was not even complete. That will give that horrible knocking and pinging sound and very possibly serious engine damage. The higher the octane the slower the burn rate and the more resistant it will be to pre-ignition. It will still ignite at the same time, just give a slower more complete burn. The result is typically more power. But most computers have their limits as to how much use they can get out of the octane. But never have I seen a vehicle run poorly or sustain damage from too much octane. I cannot say the same thing about the opposite situation.
 

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I didn't really agree with the earlier statement about no such thing as too much octane, but I completely agree with the more detailed follow up about slow vs fast burning fuels.

An engine in a specific config needs what it needs and running extra octane will possibly also have other characteristics like slower burn or less energy per unit of fuel mass (like E85) that also need to be tuned for.
 

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Discussion Starter · #19 ·
OP, where do you live? Here in Denver every grade is 2 octane lower because of the altitude, if your situation is similarly elevation-related there’s no reason to run anything higher than 91.
I actually just moved from Fort Collins to Fayetteville AR so no altitude issues here.
 

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I put 93 in all my cars as safety cushion, but could be snake oil? Then again, half my cars have about 115k miles on stock engines, one being 230k miles, so I don't think it wouldn't hurt?
 
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