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89 in my Overland (2012) and my late 2008 gives me the best combination of performance and mileage. Just got a 2013 Overland and have yet to fill it from dealer. But I'll go to 89 based on past experience.
 

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Also, using a higher octane than what is recommended, causes the ignition to burn hotter. If you pull your spark plugs and look at the white part above arc, if it's red/pink. It means you're burning the fuel too hot. Use what is recommended. Using higher octane in newer cars may help minimally but the benefits do not out weigh the problems it may cause. If you want more power get it tuned, then think about running it with higher octane. For me, i put a k&n intake for sound. Corsa cat back exhaust for sound, but gained a few mpg. Power range changed, lost full throttle top end, but gained mid range. If it's power you're after, get a hemi lol. My pentastar with 295hp is more than enough. When I want more, I'll get a trackhawk
 

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OK please repeat after me: octane has nothing to do with power (btu/lb). Octane is simply a measure of how fast it burns.

Now a problem with SI (spark ignition) engines is burn time: how long it takes from spark to maximum chamber pressure which you want to reach at somewhere between 7 and 12 degrees ATDC. Everything about the driving experience (power, accelleration, MPG) depends on how close to that "sweet spot" you can stay.

Higher octane means slower burning gas means you need more timing advance to hit that spot (lots of other things like chamber filling vary requirements). Being flex-fuel, the Pentastar can problably handle up to 103 octane so 89 is not a problem, but does it do any good ? Probably not but would take a dyno to tell.

For other reasons (like $$$) most people want to run the lowest octane possible and you might as well. It is possible that higher octane will result in a slightly smoother idle, a bit of variance is typical when right on the edge, why many maps go a little rich at idle.

GM had what was called a Standard Test 20 when dynoing engines. It was popularly known as "Maximum Power and Detonation" because you reach maximum power for any regeme on the edge of detonation. Hard part is not going over the edge since every engine is slightly different.

If you really care about theory may I suggest Sir Harry's "The High Speed Internal Combustion Engine".

ps "up to 10%" can be read as "at least 9%".
You are right about octane level having no impact on btus. The only difference in btu is when ethanol is thrown in.

You are however wrong about saying it changes how fast it burns. I could mention everything I know but I'm just going to quote Exxon here and leave a link:
https://www.exxon.com/en/octane-rating

Octane rating is a measure of a fuel's ability to resist ‘knock’. The octane requirement of an engine varies with compression ratio, geometrical and mechanical considerations and operating conditions. The higher the octane number the greater the fuel’s resistance to knocking or pinging during combustion.
To those of you who may not know what "knocking" is, it is "the premature combustion of the fuel–air mixture in an internal combustion engine."

This can be interpreted that the higher the octane rating, the more precise the engine can run, expecting combustion at more exact intervals(with a lower octane tune, the engine may expect combustion within a 2/100ths of a second window, while with a higher octane, that may reduce to a 5/1000ths of a second window) than it could with a lower octane rating. Of course, this means it is more efficient. Being more efficient, that means you can get more power out of it. This is one of the many reasons a sports car with a V12 engine, needs specialty fuel, because it needs very high resistance to pre-ignition in order to make sure the timing is maintained. More cylinders firing in the same period of time means more need for precision. Of course, all of this also means you actually need the tuning in order to make use of more predictable combustion and efficiency.
 

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Just got my 2017 trailhawk/v6 last week and put in 89 for my first fill up. I don't really trust the ratings for octane anyway. I doubt that 87 rated is always at 87 even or slightly higher. It's probably always a tad lower I would guess. I was thinking about this last night actually and was going to see if there are any testers on Amazon to slip into the fuel stream while filling. Will look one up now.
 

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I run regular unleaded which here is 85 and when in AZ or NV lower altitudes i run regular which is the more "normal" 87.....only vehicles I've ever run anything other then regular aside from some random testing of different grades on a new vehicle to see how it responded are my girlfriends 2015 Charger Scat pack always gets Premium which is 91 here, and our 2017 Focus RS which also gets Premium 91.....or whatever is the highest grade.

I've tried 88 Ethanol free and it didn't really make any difference over the e-10 85 octane other then my wallet was lighter at 40 or more cents per gallon.
 

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I tried 93 in my Wrangler for about a month a couple summers ago and saw no notable difference in performance or MPG though the idle did seem a bit smoother. The cost difference between 87 and 93 has skyrocketed recently so I can't say I'm jumping at the chance to try again. Might try 89 as I have a long trip coming up but I seriously doubt the newer 3.6 will benefit any more than my JK.
 

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Just to throw a curve ball. Anyone uses E-85?? I know right now gas prices are fair, but when they were over $4 a gallon I ran it for daily commute but my MPG sucked.


Sent from my iPhone using JeepGarage
 

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Not using E85 as it's not recommended by Chrysler in the 17 Trailhawk 3.6L, but I am using E15. E15 is 88 Octane here in NC. E15 is about $.04-.06 less a gallon here, where available.

Mileage is pretty close to the same (depending on who is driving) and there seems to be no performance degradation , measuring with the Butt Dyno.
 

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Unless the vehicle has the "FlexFuel" badging, E85 shouldn't be used...but you all knew that... ;)
 

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OK please repeat after me: octane has nothing to do with power (btu/lb). Octane is simply a measure of how fast it burns.

Now a problem with SI (spark ignition) engines is burn time: how long it takes from spark to maximum chamber pressure which you want to reach at somewhere between 7 and 12 degrees ATDC. Everything about the driving experience (power, accelleration, MPG) depends on how close to that "sweet spot" you can stay.

Higher octane means slower burning gas means you need more timing advance to hit that spot (lots of other things like chamber filling vary requirements). Being flex-fuel, the Pentastar can problably handle up to 103 octane so 89 is not a problem, but does it do any good ? Probably not but would take a dyno to tell.

For other reasons (like $$$) most people want to run the lowest octane possible and you might as well. It is possible that higher octane will result in a slightly smoother idle, a bit of variance is typical when right on the edge, why many maps go a little rich at idle.

GM had what was called a Standard Test 20 when dynoing engines. It was popularly known as "Maximum Power and Detonation" because you reach maximum power for any regeme on the edge of detonation. Hard part is not going over the edge since every engine is slightly different.

If you really care about theory may I suggest Sir Harry's "The High Speed Internal Combustion Engine".

ps "up to 10%" can be read as "at least 9%".
This!

Just run what is suggested, your butt dyno has no idea what is going on. The only time I have seen it worthwhile in a street non tuned car to worry with higher than suggested is maybe in hotter climates to reduce the possibility of Knocking or premature Detention. But it's been a while since reading that info back in the early 2000's on various racing boards. Things are much better on the Air/Fuel ratio side these days.

When I had my Z28 tuned the tuner made me sign a form that said I understand the use of anything less than 93 octane could damage my engine and they were not responsible for me doing that since he had my stoich ratio as lean as we felt safe to get the max amount of power out of my setup.

In my 2014 Regal GS it suggest 91/93 with the Turbo motor and I have seen it does pull a lot of timing when running 87. But again, the manual and I believe the gas cap has 91 written on it (I know the manual, I am thinking the cap as well).



Now the part that confused me was the 5.7 in MY14 suggest 89 octane. I had never seen a mid grade suggested. I figure they would either tune for 87 or tune for 91/93, but it seems they actually take the time figure the best for each (3.6 = 87 / 5.7 = 89 / 6.4 = 91+
 

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OK please repeat after me: octane has nothing to do with power (btu/lb). Octane is simply a measure of how fast it burns.
This is a verifiably and observably infactual conclusion. It's as if someone could completely ignore the maybe millions or at least hundreds of thousands of people who used higher octane on the drag strip to get better results for over half of a century - REGARDLESS of what engine, REGARDLESS of what combustion ratio, REGARDLESS of what was recommended by the manufacturer. Whether it was a 2 cylinder motorcycle, a 4, 6 or 8 cylinder car, before or after computers, people increased octane to IMPROVE PERFORMANCE.
Not to mention, people with the time and money to do dyno results and performance tests have confirmed this. It's only a matter of if it's worth the money, in your own opinion, for the higher octane.
All 4 cars tested increased wheel horsepower by at least 8 hp.
 

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2020 Jeep Grand Cherokee Summit 3.6L 4x2
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Plus it also depends on the fuel mapping on the vehicle.
 

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This is a verifiably and observably infactual conclusion. It's as if someone could completely ignore the maybe millions or at least hundreds of thousands of people who used higher octane on the drag strip to get better results for over half of a century - REGARDLESS of what engine, REGARDLESS of what combustion ratio, REGARDLESS of what was recommended by the manufacturer. Whether it was a 2 cylinder motorcycle, a 4, 6 or 8 cylinder car, before or after computers, people increased octane to IMPROVE PERFORMANCE.
Not to mention, people with the time and money to do dyno results and performance tests have confirmed this. It's only a matter of if it's worth the money, in your own opinion, for the higher octane.
All 4 cars tested increased wheel horsepower by at least 8 hp.
Higher octane fuel has no benefit if an engine has not been tuned to use it. Repeating, a higher octane fuel has no benefit if an engine has not been tuned to use it.


and...

Octane vs. Horsepower – Separating fact from myth in the debate over which fuel makes more power

I can provide many, many more examples if needed.
 

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Higher octane fuel has no benefit if an engine has not been tuned to use it. Repeating, a higher octane fuel has no benefit if an engine has not been tuned to use it.


and...

Octane vs. Horsepower – Separating fact from myth in the debate over which fuel makes more power

I can provide many, many more examples if needed.
Exactly! As Jeep says in the owner’s manual nobody likes to read - Using higher than recommended octane is of no benefit and can actually reduce performance.
 
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Adding to the fun, some parts of the country use a premium grade fuel without the ethanol. So the 87 is an e10, and the 90/91/93 is an e0. Using the premium in this case, although may not yield any added performance, will yield higher fuel economy as you are burning only gasoline, and not any mixture of alcohol.

-trat
 
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