89 octane recommend, does it really make a difference?

[RJS]

If that's the case, then should just tune every car to run 87. No need to run higher octane, unless boosted or high compression

That's not what they said. They took an engine that they knew the sweet spot of performance from a lot of testing, Timing, AFM, compression ratio etc and ran it with different octanes. Even when producing 540 HP it could handle 87 octane at those settings leading to the conclusion that octane doesn't matter if the engine can use it at it's sweet spot of performance. Raise compression, add a blower and octane requirements will change.
But the bottom line is an engine that can produce it's full power on 87 octane doesn't need more octane.
I was surprised as was Dolcich but the data doesn't lie and they tried tuning at higher octanes and did not get better results. Fuel value is fuel value if the engine can burn it at it's sweet spot.

 

[silver billet]

Actually the electrical equivalent of octane might be voltage, or chemistry. Lithium ion batterys have to be treated differently than lead acid or nickle cadmium etc.

For our Ram's; the proof is in the logging. Guys have data logged their 5.7 hemi engine running 87 vs 89, and there is a huge difference. The 89 octane doesn't give you more power, it just prevents you from losing it due to reduced timing. So yes, in effect, it "feels" like 89 is more powerful even though the power isn't from the gas itself, it comes from the fact that your engine runs better on it.

All the youtube and forum responses won't change my mind on this; 87 is pretty bad for your engine if you work it hard, though you can get away with it if you sunday stroll your way through life.

 

[Jako]

He summed it up in his last statement. "You should use whatever octane fuel is recommended by your manufacturer."

The HEMI is 10.5:1 compression ratio. 89 octane fuel is recommended for optimum performance and fuel economy by Mopar.

Fuel Based On Compression Ratio

87 Octane
Gasoline with this octane number is proper for engines with compression ratios of 7:1 to 9:1.

89-90 Octane
Gasoline with this octane number is good for engines with compression ratios of 9:1 to 10:1.

93 Octane
Gasoline with this octane number is advisable for car engines with compression ratios of 10:1 to 11:1.

Based on this information our trucks will run optimally with 89-93 octane fuel. Without having to advance or retard the timing.

Thanks for the posting. Some additional info on this forum.

Highest compression ratio for 91 octane?

Friend is considering building a N/A high compression 396 for his Chevelle SS. Whats the safest compression for 91 octane? Would 12:1 be pushing it? We have 93 octane at a lot of the pumps here but he just wants to make sure in case he gets stuck with 91 octane at some random pump

forums.anandtech.com

 

[Scap]

The argument will be the source of the electricity. You don't want to be one of those heathens that get from coal or nuclear. You better have green electricity or pay the penalty.... I mean tax

Texas found out what it's like to be "green". I lost power for 60 hours straight in sub freezing conditions. Solar and wind can suck it.

 

[RJS]

Actually the electrical equivalent of octane might be voltage, or chemistry. Lithium ion batterys have to be treated differently than lead acid or nickle cadmium etc.

For our Ram's; the proof is in the logging. Guys have data logged their 5.7 hemi engine running 87 vs 89, and there is a huge difference. The 89 octane doesn't give you more power, it just prevents you from losing it due to reduced timing. So yes, in effect, it "feels" like 89 is more powerful even though the power isn't from the gas itself, it comes from the fact that your engine runs better on it.

All the youtube and forum responses won't change my mind on this; 87 is pretty bad for your engine if you work it hard, though you can get away with it if you sunday stroll your way through life.

That's exactly the point. If a Hemi is happier with 89 octane than 87 it will detune itself to use 87. When it's happy, higher octane won't make a difference. Well said.

 

[silver billet]

Texas found out what it's like to be "green". I lost power for 60 hours straight in sub freezing conditions. Solar and wind can suck it.

You lost power because your utility was stupid and greedy. Nothing wrong with running wind in Canada or even Antarctica. You're welcome to have your own opinion but your utility won't change if you give them a free pass and pretend the problem was elsewhere.

 

[cclove]

Has anyone actually seen an HP and MPG difference enough to negate the price difference from 87 octane? I just did my first fill up and used 89 just because I was headed out of town and wanted to use what was recommended. Not sure if it's going to be worth it.

The best I've gotten so far is about 18mpg on a trip back from Atlanta, Ga to Charlotte, NC using 89octane...

 

[Timeless]

Not how it works. You won't get as complete of a burn with the higher octane, so depending on where the timing is set you will be sending unburnt fuel through exhaust, and not getting full potential out of the combustion cycle

Reading through these post, I don't think anyone ever really doubted what your saying. The huge waste of money they are referring to is pump pricing between 93 and 89 at most places. No benefits from 93 unless you are tuned for 93. Sure it may not be worse economy than 89, but the pump price difference is a waste of money for the same economy.

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For some reason @HSKR R/T does not agree...so I am really just responding to him. You and I are in agreement it seems.

 

[Fuzznutz]

For some reason @HSKR R/T does not agree...so I am really just responding to him. You and I are in agreement it seems.

Lol I'm just pot stirring. I trash around on 87 all day everyday, and so do millions of others. I've never won a "who's got a bigger one match". And I don't plan to or intend to change anyone's mind here either. I just like to keep engaged with everyone's thoughts lol

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[JLaw297]

I'm not sure how no one notices even some of the spark knock/rattle you can get with 89 after cruising at 45 then punching to mid throttle to beat a light or pass someone. I've used Chevron, BP, Shell, QuikTrip and they all do it. With 93 it's completely gone. Can't imagine how 87 would be...

I use 89 when cheaper and Costco 93 when cheaper than 89.

 

[silver billet]

I'm not sure how no one notices even some of the spark knock/rattle you can get with 89 after cruising at 45 then punching to mid throttle to beat a light or pass someone. I've used Chevron, BP, Shell, QuikTrip and they all do it. With 93 it's completely gone. Can't imagine how 87 would be...

I use 89 when cheaper and Costco 93 when cheaper than 89.

Me neither. Either they don't know what knock really sounds like, or they don't get on it enough, or they're just "audibly challenged". ?

 

[Fuzznutz]

Me neither. Either they don't know what knock really sounds like, or they don't get on it enough, or they're just "audibly challenged". ?

I really don't have any noise, and yes I know exactly what it sounds like. Hear it in the 6.2 gm motors all the time. They do seem to be really picky about it. But so far I honestly don't even have a ping. I already posted a pic of most our pumps around me. It's all minimum octane 87 at many places. Now sure I can find better, and I actually will if I need to. Only thing I can think of is maybe it saying minimum 87 octane on the pump, means obviously that it is at least 87, but possibly it's closer to 89. I don't know? But I'm telling you it's silent so far.

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[JLaw297]

Could be going out on a limb here.. but maybe it's the guys with 3.92 gearing that don't have knock issues. Less low-mid rpm load since gearing is lower? Could be a million things though.. altitude, fuel quality, carbon build up, air temps, etc.

 

[usedup]

I tried pure 91 and saw very little difference, certainly not enough to justify the 20% price increase at the pump. At 6800 miles the average mileage is a whopping 14.8 mpg. 3.21 gears in my '21 Limited 4x4 with E torque V-8. Maybe 9 towing.