Does 87 octane vs 89 octane really make a difference?

[Ludville1]

I did a hand calculated milage analysis for 87 for 2 weeks then 89. Posted it somewhere in here on someone elses thread. I got about 2mpg less on 87, and like he just said... didnt run as smooth. That being said Im running a 93 tune now and will probably never go back. Its about 4dollars more every time I fill up.. which is less than a good coffee.

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^^^This!

 

[Ludville1]

My 2014 Hemi has 140,000 miles
Drive like new, pull very strong!
For the first 1000 miles I was noticing the engine pin noise with 87oc, so I changed start using Shell 93 oct. and never looked back!! I think the gas mileage may be 1 mpg better, but I don’t even care about that! I love to whenever I feel like, just flooring it and not worry about the poor engine pinning! Besides, it have been a good investment, since I have never spend 1 dine on mechanic, just oil changes & tires.
I have a crew cab Laramie.
PS, once I used Wal-Mart 93oct and start to Pin again... so, screw the cheap gas! Not worth it.

^^^This gas is all that my baby gets!

 

[Bobbildr]

If you run 87 and your truck runs fine then stick with it. If you run 89 and you feel it runs better then stick with 89. Holy crap. Drive the trucks and enjoy them. Very simple.

 

[Random_Walk]

Gent who has a 3.6 FlexFuel engine wanders by, peeks in, wonders what all the fuss is about...

I do know that, at least with the baby motors, we get specifically told in the owner's manual that if we drive E85 (which has a way higher octane rating than 87 or 89), fuel mileage drops into the toilet. Now if that's due to the 105+ octane rating, or due to the ethanol...? I leave that up to y'all. Either way, I won't touch the stuff.

Word from the cheap seats: 87 works just fine for my needs, and I occasionally toss in around 1/3-1/4 tank of 89 (no particular reason for the amount, except that I usually fill up when I'm down to 2/3-3/4 tank... just the way it works out most times.)

 

[East0352]

Your engine is designed to be able to detect pre-detonation (knock) and will adjust timing and therefore power and fuel mileage to prevent it. If you run 89 octane as the manufacturer suggests, you are able to fully harness the potential of your engine. That being said, when I was living in Houston, due to the amounts of ethanol they required even in premium, I did not get better than 16-17 on the highway and averaged about 13.9 mpg. Since moving to Missouri, Ive been getting 23mpg on the highway and averaging 14.9mpg, (I make a three hour highway drive once a month and never get on besides this). I recently made a 5 hour round trip, all highway, and by the end my overall average was 18mpg. 2016 Ram 2500 6.4L ~48k miles.

 

[East0352]

Gent who has a 3.6 FlexFuel engine wanders by, peeks in, wonders what all the fuss is about...

I do know that, at least with the baby motors, we get specifically told in the owner's manual that if we drive E85 (which has a way higher octane rating than 87 or 89), fuel mileage drops into the toilet. Now if that's due to the 105+ octane rating, or due to the ethanol...? I leave that up to y'all. Either way, I won't touch the stuff.

Word from the cheap seats: 87 works just fine for my needs, and I occasionally toss in around 1/3-1/4 tank of 89 (no particular reason for the amount, except that I usually fill up when I'm down to 2/3-3/4 tank... just the way it works out most times.)

All due to ethanol. Corn syrup sucks as a fuel.

 

[tomb]

IT says in the BOOK-87 is just ok--89-recommended--I have always used 89 whats an extra few dollars per fill --as for anything higher than 89 0ctane you may be doing more harm than good unless you have aftermarket goodies that require a higher octane

 

[Batt4Christ]

I run both octanes and can’t really tell much difference. Haven’t noticed any knocking with 87. I usually run 2 tanks of 89 to one tank of 87. If I’m filling up at Sams it’s 93 or 87. Use both here as well.

I'm not sure what kind of garbage 87 octane gas folks are running to get audible knock. The computer adjusts very quickly to detonation and adjusts timing appropriately. If the gas is sufficiently poor quality, this can result in a bit of mpg drop.

89 octane requires less timing adjustment by the computer, this should marginally improve performance (and maybe mpg), though via hand calculation over the last 30 miles I've found the only consistent mpg difference is with using no ethanol fuel- regardless of the octane rating, and even then it just is measurable.

If you get noticeable knock with a modern, computer-driven automobile, you got seriously bad gasoline.

 

[Nick54]

I've got a 19 Laramie Longhorn and have run 87 from the beginning, however I felt the truck was sluggish on initial throttle opening. I decided to run a couple tanks of 89, sluggish throttle resolved. Next fill up will be 87 to affirm that its octane related.

 

[Batt4Christ]

Every single manufacturer has data showing significantly reduced fuel economy with E-85. Some fudge the numbers a bit to close the gap somewhat, but I've done relatively controlled testing in vehicles from GM, Ford, Fiat/Chrysler and the smallest difference in calculated MPG was in a Ford F-150 with the eccoboost engine. Difference over tune worked out to a reduction in mpg of 22%.

Lowest mpg difference for a GM vehicle tested was a a Chevy Malibu that averaged just under 28% lower mpg.

My wife’s Dodge Grand Caravan looses an average of almost 30% in mog.

Ethanol has less energy per drop and per lb than gasoline.

FUEL. BTU/lb. BTU/gallon
Gasoline. 18,676. 116,090
Ethanol. 11,585. 76,330

One variable that may help account for laws of physics and thermodynamics to seem a bit bent--- ”E-85” technically means ”up to” 85% ethanol.

 

[kurek]

I do know that, at least with the baby motors, we get specifically told in the owner's manual that if we drive E85 (which has a way higher octane rating than 87 or 89), fuel mileage drops into the toilet. Now if that's due to the 105+ octane rating, or due to the ethanol...? I leave that up to y'all. Either way, I won't touch the stuff.

The difference is easy. Petroleum is a series of molecules consisting of hydrogen and carbon only while ethanol is a molecule consisting of hydrogen, carbon and oxygen.

The oxygen takes up space and mass. When you're burning 100% petroleum you get the oxygen for free in the air. When you're burning ethanol you're supplying a little bit of that oxygen in the liquid fuel you paid for.

In layman terms you can think of 1 gallon of E85 like 3 quarts of gasoline and 1 quart of liquid oxygen mixed in with it.

That means you have to burn 4 gallons of E85 to burn the same amount of HC as 3 gallons of gasoline.

The reason ethanol is used as race fuel follows that - it includes oxygen that doesn't have to get pulled through your air intake past your intake valves and that means you don't have to pull as much inert nitrogen through there with it. That means you can develop more horsepower from the same engine for the same reason supercharging works. More oxygen means more fuel, more fuel means more power (as long as it's being burned efficiently)

 

[Jebb]

In follow up to the post above, I don't know this for a fact BUT I suspect at least some fuel companies obtain their octane rating by adding ethanol. In my own experience I get the best mpg running 87 octane. Higher octane (89/91) - in daily driving of the exact same route - seems to yield less mpg. If all you are after is performance, towing, etc., I'd go 91 - otherwise 87. The ECM will adjust for best results either way.

 

[KC5TGQ]

I have a 2019 5.7 Laramie eTorque and have tried both. No difference. I get 18.5 in town and 24.9 on highway.

 

[swapwap]

I think a better quest is do you by ethanol mixed fuel and the difference between some of the gas stations suppliers.

I don’t see a difference in mileage on 87 to 89. Ya 91 runs smoother. But I have seen difference between station brands and additives they put in their fuel including ethanol fuel....

 

[Midnight Storm]

The gas station by my house has the ethanol amounts posted on the pump. 87 adds “up to 20% ethanol” 89-10% and 91-0%

 

[kurek]

That's counter to the theory that refineries add ethanol to boost octane.

Like if they blend 20% ethanol - which has an octane rating of 113 - I'd be wondering more about what exactly the other 80% of that stuff was if it only ended up 87 after being supplemented with that much known good fuel.

 

[Ribtipram]

Although more timing can be pulled to make 91/93 useful manufactures have to make sure the engine is good on all levels of gas. Timing does not have the same effect as high compression for making power. This car runs off miller now thats a waist. They build them so performance can only get better. If you really need 93 or higher and can only find 87 things are only gonna feel worse and everyone would be pissed off. Blowen and supercharged need high oct.

 

[indept]

Although more timing can be pulled to make 91/93 useful manufactures have to make sure the engine is good on all levels of gas. Timing does not have the same effect as high compression for making power. This car runs off miller now thats a waist. They build them so performance can only get better. If you really need 93 or higher and can only find 87 things are only gonna feel worse and everyone would be pissed off. Blowen and supercharged need high oct.

View attachment 180700

Oh no
Tell me they're not selling hi ethenol gas in Hill Valley.....

 

[slacadjuster]

I've been running the recommended 89 octane, but I was curious on whether or not I'm spending the $8~ on nothing. I am currently getting roughly 15 - 16 mpg city driving.

You're spending your $8 on nothing.
I've run 87 in my 2015 Hemi since I bought it brand new....never even a hiccup.