Does Higher Octane make a difference?

[Revving2Redline]

Apparently so, or at least in my case. I filled up with QT 91 e0 in Gladstone Missouri. Current fill is Redline 5w30 too. The compression is high enough to maybe warrant it,but has ran most its life on non ethanol 87. Was kinda shocked but then again I have had numbers like this before. Anyone else have stories of similar findings?
Got over 46 mpg avg over 100 miles. Highest was mid 47's.

 

[kschachn]

22 to 47? If that is what you are referring to then no, it wasn't due to the octane rating LOL.

Yes it can advance the timing and be a bit more efficient but the overall energy content of the gasoline is the same, correcting for how the octane rating is acheieved.

Two fuels that are otherwise same in composition but with a different octane rating do not come anywhere near close to that kind of difference. Physics matters.

Oh and before you mention it again, it wasn't due to the Redline oil either.

 

[Surestick]

Some brand's premium is E0 while their regular is E10 so that might increase the mileage you get from a tank a bit too.

 

[Graham Piccinini]

I got a 2000 Honda CR-V, 5-speed and AWD. Ancient B20z2 honda 4-cylinder, no VTEC.
When I drive on I-40 between Charlotte and Asheville with 87 in the fuel tank I have to do quite a bit more downshifting to stay at speed limit, or to simply keep up with traffic.
With 89 or 93 in the fuel tank the CR-V requires less downshifting and seems to have more pep (aka torque) below 4000 rpm.
No difference between fuel grades when it comes to fuel economy, and at sea level no performance difference either. Performance only seems to be positively improved at higher altitudes.

 

[kschachn]

We get some real misunderstanding on here when it comes to what a fuel's octane rating means (and what it does not mean) but if the OP is relating that to a more than doubling of the fuel economy then this is probably the biggest.

 

[KS-Ryan]

The 2 best tanks on my Civic HX were on 91 E0, on the same stretch of I-70 in Colorado (48.2 and 47.3). I do think the car likes the 91 E0 on long highway runs, but for day to day, it's not worth the higher price. I've been running QT 87 E0 instead (since fuel is so cheap right now).

 

[MRC01]

An engine's best efficiency is usually when running the lowest octane that doesn't ping or cause the computer to pull timing. Most of the octane boosting additives (ethanol, xylene, toluene, etc.) have lower energy content than the gasoline they displace, which means high octane gas typically has slightly lower energy content. So most engines get their best efficiency with low octane gas. Yet some engines having high BMEP will take advantage of high octane to run a more aggressive tune, which can increase efficiency and overcome the slightly lower energy content of high octane gasoline, and be more efficient overall.

The differences are usually so small they're hard to measure becasue they're lost in the noise of day to day variations in wind, terrain, air density, temperature, humidity, and how you drive.

 

[y_p_w]

An engine's best efficiency is usually when running the lowest octane that doesn't ping or cause the computer to pull timing. Most of the octane boosting additives (ethanol, xylene, toluene, etc.) have lower energy content than the gasoline they displace, which means high octane gas typically has slightly lower energy content. So most engines get their best efficiency with low octane gas. Yet some engines having high BMEP will take advantage of high octane to run a more aggressive tune, which can increase efficiency and overcome the slightly lower energy content of high octane gasoline, and be more efficient overall.

The differences are usually so small they're hard to measure becasue they're lost in the noise of day to day variations in wind, terrain, air density, temperature, humidity, and how you drive.

Most flex fuel engines will generate maximum performance with the maximum ethanol allowed for E85. And their MPG relative to the energy content will be higher than with E10. But it's pretty great when you're running 100+ octane E85 to do that.

 

[nicholas]

Apparently so, or at least in my case. I filled up with QT 91 e0 in Gladstone Missouri. Current fill is Redline 5w30 too. The compression is high enough to maybe warrant it,but has ran most its life on non ethanol 87. Was kinda shocked but then again I have had numbers like this before. Anyone else have stories of similar findings?
Got over 46 mpg avg over 100 miles. Highest was mid 47's.

Yes yes and yes.
91 in our 2.0t Santa Fe and in our 13':1 compression Mazda 6.
Notice a big difference in engine smoothness and a small mpg bump.

 

[Amkeer]

I have experimented with this in the 2014 Tacoma a while back. When I ran higher octane fuel over regular it yielded about a 3-4% difference in MPG using high octane over regular. Yes the throttle response and smoothness of operating was noticeable. However, the cost difference between the two doesn't warrant the use of high octane in my vehicles. I stick with 87 in all vehicles.

 

[RadioMannnn]

I have experimented with this in the 2014 Tacoma a while back. When I ran higher octane fuel over regular it yielded about a 3-4% difference in MPG using high octane over regular. Yes the throttle response and smoothness of operating was noticeable. However, the cost difference between the two doesn't warrant the use of high octane in my vehicles. I stick with 87 in all vehicles.

I won't get into the whole debate here, but I do alternate tanks.

If I run the rank down to less than half or half. I gas up with regular. Gas is very inexpensive right now.

A third or quarter needs filling? I'm putting in 93. I don't need to settle for 92 as I found a go-to station, they keep the same spreads no matter what the price is. I do check other stations from time to time.

I get better and smoother car behavior when the 93 goes in, but with my frugality, my car gets 87 just as much as 93 and likes it. This also yields a slightly higher octane result and costs less than always putting in Plus, 89.

My mix achieves an octane somewhere around 90 in the tank.

 

[Malo83]

Nothing but ETHYL in my Buick 2.0T

 

[4WD]

Nothing but ETHYL in my Buick 2.0T

Don’t let your wife catch y’all

 

[Puckrobber]

Haven't used anything but 87 in my Altima, so, can't say much about the higher octane. But, I took a trip from Reno, NV to SE Washington, and got the best mileage in any vehicle I've had. Took this picture after I refilled the tank, hence the 610 miles estimated for it. Nearly 12 hours driven, 662 miles covered, for an average of 43 MPG. Not too bad, I'd say.

 

[IndyIan]

I doubt theirs any car with 87 octane recommended where a fill up on 91 octane would pay for itself? Perhaps if towing?
I was considering a V6 Mitsubishi outlander which has an odd(poorly designed?) engine where 91 is recommended. Being a non luxury SUV everyone runs 87 octane in them without much mileage loss, but when towing many people found that filling up with 91 octane would actually pay for itself with better mileage.
I imagine there's some correlation for cars with lower cruising rpms and mileage gain with higher octane. I think my subaru must run very close to knocking when its running down the hwy at 55 at 1500 rpm, and so higher octane may allow it produce a bit more power at those low rpms safely and allow it to stay at lower rpms under loads. My Focus is spinning about 50% faster at 55 so I don't think higher octane will help as it never needs a downshift on a hwy where I am atleast.

 

[DGXR]

Higher octane *can* make a difference. It depends on the specific engine and its condition. Some engines are designed to make slightly more power on higher octane. And if a "normal" engine spec'd for 87 octane regular has excessive carbon deposits in the combustion chamber (or piston crown), these deposits can increase the compression ratio and require the use of higher octane to prevent pinging.

 

[turboseize]

Higher octane *can* make a noticeable difference in some engines. In others, it doesn't.

Take for example Saab turbos from the 80s: the electronic boost control is dumb by modern standards. It's analogue oldschool electronics, just some resistors and resonance circuits and three potentiometers. Boost follows a set curve, but get's cut back when knock occurs. If no knock is detected, then boost is slowly risen until there is some knock eventually. Boost is reduced, and then slowly risen again... Now sensitivity of knock is influenced by octane number and charge temperature (which is linked to ambient temperature and speed).
These engines can be safely run on 91 RON, but then they won't have any power. To access nominal power, you'll need 97RON at ~20°C ambient temperature. If temperature drops, and if you increase octane rating above 97RON, even more fun can be had.
My first 900 T16S struggled to reach merely 200km/h on hot summer days, and did not even reach it's official Vmax of 210km/h - but with Shell Optimax in the tank (hottest **** back in the day...) and -19°C it ran out of gears. With the old gearbox with the longer differential and a 7 primary, this translates to ~240km/h. Same stock engine and boost controller, just different air temperature and high octane fuel.

With the DI-APC system in the 1989 Saab 9000, the cars were also able to adjust timing depending on knock, and since introduction of the Trionic engine management system in 1992 they even could adjust timing selevtively per cylinder. In a Trionic car, higher octane fuel allows far more aggressive timing and thus results in greater fuel efficiency.

There is a limit however to how far you can advance ignition. Go to far, and you exert pressure on the piston while it is still coming upwards in the compression stroke...
There is an optimal point for ignition timing per given load and rpm. A turbocharged engine often will be far from optimal, as enough boost can make a mixture knock at whatever richness and timing.
A well designed naturally aspirated engine will very close to optimal timing when running the specified fuel. Go lower and ignition will be retarded, which leads to power loss and worse fuel efficiency. But going higher in octane rating generally won't do any good, it could even hurt fuel economy (higher octane fuels having less energy density).

There are only two possible scenarios, where going higher than specified in a naturally aspirated engine is beneficial:
1) The manufacturer designed the engine for high octane fuel, but then back-specced it to lower grade fuel for marketing reasons. Then timing will be retarded by the engine management system. Using higher octane fuel would bring the engine back to what it was designed for in the first place.
2) In exceptional circumstances, that is abnormally high air temperatures and/or with carbon deposits in the combustion chamber which are leading to knock. Then higher octane fuels than specified might mitigate knock and the associated timing retardation.

 

[Hogwash]

....Yet some engines having high BMEP will take advantage of high octane to run a more aggressive tune, which can increase efficiency and overcome the slightly lower energy content of high octane.....

At one time Toyota used to mention that in the owner's manual with regards to the 1MZ/3MZ and 1GR VVT-i engines. "Minimum octane requirement of 87. But for increased performance use a higher octane"......not exact quote.....but along those lines.

 

[tiger862]

I accidentally put Exxon Supreme+ which is the best fuel they sale according to the ads
I knew something was up when I seen the total cost so I checked when I got home. My observation is power is better at low rpm. I tried midgrade which I got a little bit better fuel mileage but worse drivability so I went back to regular. I quit using econ mode as I had drivability issue I didn't like and was worse on 89. Now with premium for the last 100 miles econ mode is the same drivability as regular unleaded without econ mode. With regular unleaded it downshifted on slightest touch of pedal, midgrade held it to long and super it shifts smoother and at correct time. Perfect example is getting on interstate. Regular it will hold gear until you get about 5k, midgrade would shift at 2k and keep giving gas until it downshifted and hit 6500 RPM. Today with econ on (forgot to turn off) got on interstate and it shifted at 2500 then I gave it gas it downshifted to 4500 RPM then I didn't feel it shift again but I know it did. Is it worth it the jury is still out. Between econ mode (1-2 mpg) fuel (1-2 mpg) I am still little short from breaking even. I could be easier on accelerating but what fun would that be.

 

[Sayonara_Sonata]

Not sure how accurate the old MPG gauge is, but a tank of 93 E10 Vs. 93 E-free consistently shows 4 mpg difference. I went to 50/50 keep me foot in 'er arse and dread naught.

" just different air temperature and high octane fuel."

IAT is important especially with turbo. I force me bonnet open against emergency latch for more cooler air when ambient is 10* C or above. Fourth gear 2750rpm ambient/IAT splits are 9-10*C at steady speed up to about 26C ambient. Below 10C I do not require the extra airflow whilst in motion. The payoff comes in city/urban stop and go driving. Idling I might have 21C/53C split, but when moving und using the proper gear keeping the boost down in short order I'm around 11 or 12C ambient/IAT split.

I do not text, so I leave the auto scanner connected full time for amusement when bored driving. Comparing the boost, RPM & IAT at steady speed gives one an appreciation for proper gear selection.