Octane and gas milage

[Blackknight]

I have seen several post that people claim to get better gas mileage using higher octane gas ie. premium over regular.

Is this true or are people/ vehicle computer being confused? Has anyone done a pen and paper calculation to truly calculate any benefit?

FTC claims there is no benefit.

The Low-Down on High Octane Gasoline

Just trying to clear up misnomer. I may need to send this to the guys over at Mythbusters LOL.

 

[illzoni]

As it's been explained to me, by several people I trust, higher octane fuel holds less energy and therefore actually lowers fuel mileage.

 

[CJ3Flyr]

In some cases this article is right; in some not-so-much. The main "player" is the ECM software and the parameters that the ECM can control. In the case of 4Runners (4th gens) the ECM has enough control that an MPG/Power benefit is available to those who are patient (4-5 tanks of fuel). The ECM 'listens' for knock and constantly 'smells' the exhaust among many other things. The ECM will advance the timing and lean the mixture to the parameters prescribed by the O2 sensors and/or it 'hears' the onset of knock. Higher octane fuel allows the timing to advance more and slightly leaner mixture. The inherent design of the engine can recover a bit more of the energy released by higher octane fuel. The slightly slower burn translates to the energy being recovered more efficiently by the piston/crankshaft. (That's PROVIDED there's enough 'room' within the ECM's parameter's.)

Some examples are the following paraphrased entries found in several owner's manuals "To achieve maximum mpg and power Premium Fuel is recommended." Toyota's FJC's, 2nd gen 3.slo's, GX470's and others have that entry.


Is there a point of diminishing returns with regard to higher octane? Absolutely. It's hard to know and is determined by many factors such as compression, stroke, ECM programming etc.



Anything that's as complex as the engineering/theory of modern engines can not be summed up by the Government, especially in one page.



Am I recommending Premium? No. If you want to fine and you may or may not gain something. In 4Runners I would say don't run 1-2 tanks and say it did nothing (to condemn the use of Premium). The 'learning' design of the Toyota ECM takes 4-5 tanks to learn how to properly digest the Premium fuel. Below is one of many summaries what I just summarized.

Welcome to Land Cruiser Owners On Line
:cheers:

 

[CJ3Flyr]

As it's been explained to me, by several people I trust, higher octane fuel holds less energy and therefore actually lowers fuel mileage.

My understanding is slightly less energy but more of it is "usable" if the engine is set up for it...

 

[RonMAIDEN]

Higher octane fuels are a bit more stable and doesn't ingnite under certain temps and performs best in high compression or forced induction engines. With my modded turbo 1st gen DSM I could run 91 on the street but in the heat of a dragstrip enviroment and running 5 more psi I had to bump it up to 101 but anything less and it would detonate pretty easily under WOT.

 

[Blackknight]

.

Welcome to Land Cruiser Owners On Line
:cheers:

Thanks CJ3Flyr..... So I guess the guys over at mythbusters would most def. bust that myth. There is no way they would extend a test for two plus months maybe two tanks max.

 

[Koz]

There is no real difference in the amount of energy between octane’s. The only difference is how each octane prevent engine knock. Depending on the engine design (Mostly compression) different octane will or will not prevent knock. The BEST octane to use is the lowest octane that will prevent knock. Any higher octane is a waste of money and will NOT increase mileage.

Koz

 

[JB.]

There is no real difference in the amount of energy between octane’s. The only difference is how each octane prevent engine knock. Depending on the engine design (Mostly compression) different octane will or will not prevent knock. The BEST octane to use is the lowest octane that will prevent knock. Any higher octane is a waste of money and will NOT increase mileage.

Koz

You're stating categorically that the ECM won't advance timing or lean the mixture in response to premium gas? What makes you so certain?

 

[CJ3Flyr]

Thanks CJ3Flyr..... So I guess the guys over at mythbusters would most def. bust that myth. There is no way they would extend a test for two plus months maybe two tanks max.

Exactly... Many ECU's, even through the late '90's, wouldn't take advantage of higher octane so the answer is, generally speaking, as Koz stated, use the lowest possible that doesn't knock. Today we're faced with many ECU's; some do Premium fuel, some don't, some adapt virtually instantly (Caddy Northstar's) and others that take a long time (our Toyota's).

 

[Koz]

You're stating categorically that the ECM won't advance timing or lean the mixture in response to premium gas? What makes you so certain?

Octane rating doesn't measure octane or energy content of a fuel, only its anti-knock properties.

Koz

 

[MikesToy]

I pour 91 octane (Shell). I get 17 in the city and 21+ (pending how fast i drive) on the freeway. To each own.

 

[blacksmithdog]

We bought a new Mini Cooper in the spring of 2009. I didn't realize it when I bought the car, but BMW recommends high test. We record the gas mileage of every tank, and the Mini has 16,000 miles on it now. We'll run 10 tanks of high test, then 10 tanks of medium test. There is no difference in the MPG figures.

 

[JB.]

You're stating categorically that the ECM won't advance timing or lean the mixture in response to premium gas? What makes you so certain?

Octane rating doesn't measure octane or energy content of a fuel, only its anti-knock properties.

Koz

Not seeing a connection between question and answer.

 

[snhrph]

To me the question is not which gas grade provides more MPGs, but which one is more economical?

With that in mind, the premium grade gas would have to offer 10% to 15% improvement when compared to regular gas to make sense economically. I don't see that happening...

 

[Koz]

Not seeing a connection between question and answer.

As far as premium fuel, more than enough octane will not trigger a timing adjustment. When the octane is too low the ignition system will sense the potential knock and adjust the timing to avoid damage. This is why using too high an octane is a waste. You need higher octane for engines that are specifically tuned to use it. Toyota/Lexus designs and tunes their modern engines right at point where higher octane is recommended. If you use an octane that is borderline and there is a high load put on the engine you may lose efficiency because the timing will be adjusted to prevent damage to the engine.

I think what confuses is the timing and air/fuel mix is also adjusted to compensate for different operating conditions such as temp, engine RPM, throttle position, etc.

Koz

 

[Jeff Kleb]

Octane


The octane rating (i.e., anti-knock components) necessary for a particular engine has a lot to do with the compression ratio and the ignition timing - as well as the load or driving technique.

I have a 2002 Nissan Sentra that runs fine - and never, ever "knocks" at all; even when I use el cheapo off-brand fuel. It's just a four-banger that always gets great mileage no matter what grade of gasoline I might choose.

My 2004 Chevy Silverado with a 5.3L V8 runs fine on 87-octane unleaded regular fuel, but will "knock" from time to time when accelerating or going up a steep hill. I hate to hear that sound, but know that an occasional ping won't destroy my engine. Every now and then, I'll top off my tank with some "mid-grade" fuel and the problem goes away. I have tried running all three grades of gasoline and my fuel economy remains fairly constant.......so I might be wasting 20 cents a gallon by going to the "good stuff" all the time.

The "pinging" or "knocking" is known as pre-igntion. It happens when the piston is at the top of the second stroke (of four), known as compression. The air & fuel mixture is compressed tightly and the spark plug fires (third stroke of four - power), which forces the piston downward. Pre-ignition happens when the air & fuel mixture "ignites" on its own before the spark plug fires. Of course, this results in less power and inefficient combustion.

We use to call it "dieseling" - since that is how a compression-ignition or diesel engine operates all the time; as opposed to requiring a spark to set off combustion. That's why diesel engines have that clatter - small explosions going off inside the combustion chamber.
:wink2:

Diesel engines need "cetane" - which is the opposite of octane, since you WANT it to knock in order to run.
:guitar2:

 

[Koz]

My 2004 Chevy Silverado with a 5.3L V8 runs fine on 87-octane unleaded regular fuel, but will "knock" from time to time when accelerating or going up a steep hill. I hate to hear that sound, but know that an occasional ping won't destroy my engine. Every now and then, I'll top off my tank with some "mid-grade" fuel and the problem goes away. I have tried running all three grades of gasoline and my fuel economy remains fairly constant.......so I might be wasting 20 cents a gallon by going to the "good stuff" all the time.

The "pinging" or "knocking" is known as pre-igntion. It happens when the piston is at the top of the second stroke (of four), known as compression. The air & fuel mixture is compressed tightly and the spark plug fires (third stroke of four - power), which forces the piston downward. Pre-ignition happens when the air & fuel mixture "ignites" on its own before the spark plug fires. Of course, this results in less power and inefficient combustion.
:guitar2:

Knocking at ANY time is detrimental to your engine and needs to be prevented. The shockwaves cause damage that can not be fixed. I suggest you use an octane that will eliminate engine knock under ANY conditions. Do not think it is OK sometimes!

Koz

 

[Jeff Kleb]

Knocking at ANY time is detrimental to your engine and needs to be prevented. The shockwaves cause damage that can not be fixed. I suggest you use an octane that will eliminate engine knock under ANY conditions. Do not think it is OK sometimes!

Koz

I have heard that from more than a few people, but "Mr. Goodwrench" at the Chevy dealership said that the truck has a 'knock sensor' that is supposed to temporarily delay the ignition timing & that my Silverado is designed to run on "regular" 87-octane unleaded gasoline. He said that I am okay if it is only a 'very rare' occurrence - such as towing, hard acceleration or climbing a steep hill. I am with you, though, and don't like hearing that that knocking - even once. I usually back my foot out of it until it goes away.
:violin:

If I were to use 89-octane fuel (instead of 87), I would by forking out another 15 to 20 cents a gallon. The truck ain't exactly a "gas hog" - but it is not cheap to drive, either. I will top it off tomorrow (taking a four-hour road trip with my son) with higher-grade fuel and am going to see how well it does with both mileage and performance. Having dual Flowmasters helps with the sound...that's for sure! I love the beautiful music of a small-block V8 engine.
:frog:

P.S. I filled up my International 8600 (tractor) big rig today with diesel fuel - where the pump had "40" as the cetane rating for ultra-low-sulfur diesel (what we must use now).

 

[DRC]

Interesting discussion. As snhrph put it, its all about the economics. It really does not matter if you get more mpg unless the cost equation favors the higher octane. If you were getting 18mpg on 87 octane at a cost of 2.80 per gallon and you got 19mpg on 91 octane at a cost of 3.00 per gallon, it still would be cheaper to put the 87 octane in your tank since that extra 23 miles cost you 4.60.

Anecdotally I will say that while towing I have seen a difference in gas mileage between 85 octane and 91 octane. We did a road trip this past June from Chicago to Yellowstone in my 08 v6 pulling a 5200lbs TT with 500lbs in the truck. I have always used regular and in my area that is 87 octane. Once we got onto the highways regular turns into 85 octane. Without noticing this, I did my first 4 fill ups at the 85 octane and was getting terrible mileage. While I was running the 85 octane I was literally getting less than 7 miles per gallon. At one point I went 143 mile on 21.5 gallons of gas. At this point I looked at the octane at the next stop and noticed they only had 2 grades 85 and 91. I filled up with the 91 and continue 91 for the remainder of the 3400 mile trip. After 1 tank of 91 octane I started averaging 10 – 11 mpg. It was like heaven. To go from 7 up to 11. I know this for a fact as I kept every gas receipt and wrote down the mileage between fill ups so I could track this. Overall 3400 miles cost me 912.50 in gas. Ouch!

I say this is anecdotal because I can’t say if the ECM had just adjusted itself to the load by this time or if the gas was the factor but in any case when I tow, I only use 91.

I also did a small experiment, seeing that that I got better mpg with 91 while towing I tried the same thing during regular driving where I average 19mpg. After 5 tanks with 91 octane I compare this to my average and did not see any appreciable difference. Paying the extra cost did not make any sense to me. So for me its 87 all them time unless I am towing then its 91.