High compression ratio: is higher octane better?

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my DI loves 91+ octane. Better MPGs and it eliminated fuel dilution. It is a no brainer for me. I only run premium in it.

My Escape doesn't notice such an improvement, though. It gets 87.
 
Originally Posted By: TiredTrucker
I will. ...The oil is not being subjected to as many particulates, combustion acids, etc and possible fuel dilution issues when getting above a certain mpg threshold.


Thanks, that explains it for me. I will allow this tank to go down to 1/4 and fill up on 93. See how it feels and report back.

I appreciate all the responses.

Warren
 
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very high compression ratios but the only fuel required is an 87 octane
The required octane is load related.

Anyone old enough to have driven cars without modern electronic management? Remember the spark knock when going uphill, maybe high gear, and foot hard on the throttle? Ease the throttle and the spark knock ended or at least diminished.

Modern engines handle that with the electronic controls. At light load they advance the spark for best efficiency. At heavy load they retard the spark for engine life. Use the 87 octane gas.
 
Ken, I do remember the bad old days of primitive carburettors and static ignition timing, there was plenty of it still around in the 1980s.
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Some modern transmissions are programmed to maximize low RPM load on the engine to minimize fuel consumption. Perhaps this along with more advanced engine controls are why some vehicles respond noticeably to higher octane fuel, especially in stop and go or demanding conditions.
 
In my area, there is 87, 91, and 93. The 91 is a dime more than the 87. My Silverado 5.3L gets a little better mpg on the 91 than the 87, and seems to run a little better. Have not considered the 93.
 
My MR2: Yes, clear as day difference on 93 compared to 87. Slight bump 1.1mpg and it definitely feels better. 99 Outback... minor difference at best. MPG inconsistent over the long-run but many Gen1 OB folks swear by it as it was "required" in '96 but dropped for 97-99 model years. I get a consistent 25mpg on regular and I have to use those grocery-gas points somewhere. '14 Outback, inconclusive. I am getting poor fuel economy now. It has dropped significantly of late (7mpg... yikes) with no obvious defect. Might try premium and see where it goes.
 
In a vehicle with ECU controlled timing and a knock sensor you should always run the lowest octane fuel that doesn't cause knock.
More advance gives you more power&economy, using the correct octane rated fuel is a no-brainer
 
Originally Posted By: Olas
In a vehicle with ECU controlled timing and a knock sensor you should always run the lowest octane fuel that doesn't cause knock.
More advance gives you more power&economy, using the correct octane rated fuel is a no-brainer


Platform dependent. vastly differing strategies from mfgr to mfgr means no blanket statements please.

For example, my car has never knocked even on 87 despite being programmed for 93 octane. Many newer cars also will simply not knock no matter what swill you feed them.

Just depends on the electronics and their style of setup...
 
Well,

I did allow the tank in the 2.4 to fall below 1/4 and filled up with 93 oct from Costco. It absolutely made a difference. Not something that a non-car person would be able to put a finger on, but noticeable to me and my butt calibration senses. I don't expect mpg differences or any more power, but the slightly more refined 'feel' is worth it.
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Is this a myth or does it increase mpg?

Years ago I experimented with a Texaco and Citgo station. I used 87 octane and the Texaco got almost 2 mpg more in a Toyota rav4. It was consistent and proved to me the added cost was worth it....then about 5 cents. Today it fluctuates.
 
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When I ran a tank of 93 octane in my Hyundai SFS 2.0T the gas mileage DECREASED, and I didn't note any change in operational "feel". I've never experienced any adverse engine characteristics while using the recommended 87 octane. I do use quality fuel though. I think Hyundai has done a pretty good job at managing the fuel management / timing in their DI engines to use lower octane fuels. Lower octane = higher energy density in engines designed for them. . .
 
Originally Posted By: Robster
...Lower octane = higher energy density in engines designed for them. . .


Incorrect.
 
Depends on the car as many have said. Some will benefit from it, others may not. I've mapped it on the ej25, and you can see timing being pulled on 87 octane under load in hot weather. Is there a benefit to using premium all the time? Sure, but you have to do the math to determine if it makes sense.
 
I have since filled up a few times...

The SFS 2.0T doesnt seem to mind 87, 89 or 93. We now fill up on 93 and go to 1/2 tank then fill with 87 or 89 then vice versa.

The Sonata 2.4 is another story - it runs best on 89 and 93.
 
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Originally Posted By: Robster
When I ran a tank of 93 octane in my Hyundai SFS 2.0T the gas mileage DECREASED, and I didn't note any change in operational "feel". I've never experienced any adverse engine characteristics while using the recommended 87 octane. I do use quality fuel though. I think Hyundai has done a pretty good job at managing the fuel management / timing in their DI engines to use lower octane fuels. Lower octane = higher energy density in engines designed for them. . .



87 octane and 91 octane have the same energy(btu) except here where 87 octane has ethanol thus has less btu than the pure gasoline that premium is here.
Knock/preignition/detonation can be controlled by pulling timing which lowers an engines power output.
So this lowered output translates to requiring more pedal/fuel to do the required work which in this case is propelling a vehicle.
In my vehicles,all of them get better mileage with premium. All of them.
Why?
Because I need less pedal and therefore less fuel to move the vehicle.
Spark timing equals power. So the more advance you can give the engine the more power it will make,until it pre-ignites.
So there is a fine line.
 
As engines are used they often pick up some carbon which decreases the space inside the combustion chamber and therefore raises the compression ratio. An engine which could tolerate a cetain ignition advance with 87 when new might need 88 or 89 after 100K to
run the same curve. It's a difference you might feel. If you suddenly need 93 to get an "87" engine to run properly, there's something wrong, in many cases something which leaned the mixture.
It used to be a practive among Volvo B18 drivers in the 60s to pull the head and "decoke" it every 20K or so to avoid having to use "high test" to stop the ping which would develop. I'm sure the "coke" doesn't build as fast as it once did, but modern engine management makes sure you don't hear ping by pulling the timing back.
 
Also, the car manufactures have to put that in the OM because of the variety of climates and altitudes that the vehicles may encounter. What I mean is, if you're at sea level, the engine may like one octane. But, if you are at 1 mile above sea level, the engine may require another still.

Or as mentioned, the amount of carbon deposits that will build up on the pistons over time, thus, requiring the engine to use a higher octane to avoid detonation.

And although, engine build quality/consistency is at it's best today(I would assume), no two engines are exactly the same!
 
A few months ago I had one or two stations near me that offered premium E0, though at 91 octane instead of 93. I may experiment a little with that in the Regal.
 
...and to clarify. At no point have I noticed or more importantly, measured, any mpg gains regardless of octane. No quantifying involved, just 'feel'.
 
Originally Posted By: Char Baby
Also, the car manufactures have to put that in the OM because of the variety of climates and altitudes that the vehicles may encounter. What I mean is, if you're at sea level, the engine may like one octane. But, if you are at 1 mile above sea level, the engine may require another still.

Or as mentioned, the amount of carbon deposits that will build up on the pistons over time, thus, requiring the engine to use a higher octane to avoid detonation.

And although, engine build quality/consistency is at it's best today(I would assume), no two engines are exactly the same!
Octane requirement drops with altitude.
 
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