Better MPG with 87 than 91 Octane ?

[HTSS_TR]

The '00 MB E430 requires Premium 91 Octane and it had 91 since new, until about 6-7 months ago when weather was cooler I tried 87 octane to see if it reduced power/drivability or if engine developed a knock. No difference in drivability and no abnormal engine sound between the two octanes.

The very first tank of 87 octane increased MPG by about 0.8-1.5 MPG, from average of 21.2-21.8 MPG to 22.0-23.2 MPG. The daily round trip distance is 48 miles with an average speed of 34-37 MPH for the trip, according to trip computer and Ultra Gauge. Every tank of 87 octane gave similar MPG, from 22.xx MPG to 23.xx MPG.

I then tried 2 tanks of 91 octane, the MPG went back down to 21.4-21.7 MPG. Last week I went back to 87 octane and MPG went back up again. The weird thing is engine rev as smooth as with 91 octane and very quiet too.

I think I stay with 87 octane for this E430.

I tried lower octane in '04 S2000 and acceleration suffered immediately and MPG went down by more than 1 MPG, from average of 22.2-22.8 to less than 21. The S2000 needs 91 octane to have good power.

 

[Dieseldoctor]

With less than a 1 mpg difference i'd suspect driving habits and or conditions are at fault normally i think the 87 octane has slightly higher BTU per gallon something like 114K vs 112K for 93 octane not sure what 91 octane is sold i've personally never seen it i'm assuming it contains eather ethanol or something else. We have 93 octane which doesnt contain 10% ethanol the fuel economy usually increases about 3-6% mainly due to the higher btu per gallon.

 

[Eddie]

I also think other things are influncing your milleage.

 

[Torino]

Originally Posted By: Dieseldoctor
With less than a 1 mpg difference i'd suspect driving habits and or conditions are at fault normally i think the 87 octane has slightly higher BTU per gallon something like 114K vs 112K for 93 octane not sure what 91 octane is sold i've personally never seen it i'm assuming it contains eather ethanol or something else. We have 93 octane which doesnt contain 10% ethanol the fuel economy usually increases about 3-6% mainly due to the higher btu per gallon.
++++1 John--Las Vegas.

 

[RamFan]

That is certainly odd, my Charger recommends mid-grade but can use regular so I did an experiment and the mid-grade turns in better mileage. Not by much though, only .5-2 miles better.

Here is my MPG Tracker

 

[asiancivicmaniac]

I don't know about increasing mileage, but 93 doesn't increase my mileage enough to be distinctly noticeable compared to 87 and the car is tuned for and recommends 93. Normally it's .5-1 mpg better when 93 is used.

 

[lexus114]

If I dont use Premium (as recommended) Mine suffers in millage and power.

 

[Rickey]

Be sure you compare apples with apples. If the 91 is gasohol and the 87 is gasoline, that would explain it.

Normally a vehicle with a sufficiently developed engine management system will be able to optimize ignition timing etc for increased octane.
This will result in increased engine efficiency.

Rickey.

 

[SteveSRT8]

The above only applies to vehicles tuned for 91 or more.

No automobile specifying regular 87 will advance timing to properly utilize premium fuel.

BTW, those are some great mileage figures for a V-8!!!

 

[HTSS_TR]

Nowhere in California you can find pure gasoline, all grades have about 10% Ethanol. The 87 octane was used since last year winter until last month, since we started having summer gas in early April, I tried 91 octane to see if the gas mileage will be better, but not.

Most engines spec'ed with premium can use lower octane with reduced power and probably less MPG, I'm suprised to see the decrease in MPG with 87 octane in this car while the drivability with sane speed is unchanged. I think the reduced power is so small such that neither my wife and I detected anything.

I'm sure that the drivability/acceleration with 87 octane will show its ugly head if we go to Vegas in summer, especially when passing Death Valley going up steep hills at more than 100 MPH with ambient temperature above 120F.

 

[Rickey]

Originally Posted By: SteveSRT8
The above only applies to vehicles tuned for 91 or more.

No automobile specifying regular 87 will advance timing to properly utilize premium fuel.

BTW, those are some great mileage figures for a V-8!!!


I have seen several exceptions, I own one as a matter of fact.
How does an ECM even "know" what octane is in the tank?
Simple, it advances timing while listening for knock, that's what the knock sensor is for.
That's also one of the ways fuel economy is maximized.
If an 87 vehicles operating map were tuned to never knock on 87 then the knock sensor would never be needed. Right?

My 87 octane speced Cruze ECO (turbo 1.4) shows knock retard on 87,93 and 100.

.

(100 UL is available in Thomasville NC at 109 and I-85)

In other words it does tune advance etc for the octane in the tank.
This is done by the ECM advancing timing until knock is detected then it backs off a little.
No, it's not broke, it's hard to argue with 44.3 MPG on 87.
And yes I have a data logging scan gauge.

On a particular long steep grade in 6th gear (1900 RPM) 60 MPH WOT at a predetermined point results in a slight drop in mph by the top of the hill with 87. Filling the empty tank with 93 an identical retest on the same hill results in a 3 MPH gain.
This is repeatable.

This particular 87 power-train meets it's specs on 87 and improves on 93.

Rickey.

 

[SteveSRT8]

This (repeating) is only applicable to selected platforms.

It is NOT generally true. Congrats on an exception, but most automobiles including newer will NOT exhibit this behavior. Many PCM's don't even have the timing map to fully utilize premium fuel. Timing is VERY engine/application specific, and gets a lot of attention from powertrain engineers.

You are fortunate to own a GM product that uses advanced engine controls and their 'flex fuel' tuning. Good for you.

But don't apply the same thinking to all cars. You'll be disappointed.

 

[Rickey]

The only way to know for sure whats going on is with a scan tool and experimentation.
For instance my '06 3.5 Impala had knock retard under "dead pull" conditions with 87. The KR was gone with 89 or better on most days.
Ambient temp, humidity, and baro are large factors.
My Solstice GXP would even KR a little under certain conditions with 100 in it.

As you say there are no absolutes with fuel in this regard.
Really you don't know 'till you know.

There is much knowledge to be gained with the aid of a scan tool!
The scan tool disproves lots of "internet hype" regarding fuel octane and other powertrain matters.


Keep in mind the knock sensor is there to aid the ECM in optimizing timing for the fuel and conditions of the moment.
The ECM will incrementally retard ignition timing in response to knock caused by insufficent octane.
It will also "incrementally not retard timing" in the absence of knock in the presence of sufficient octane.
So in effect by adjusting for knock it also adjusts for octane among other things.
In essence this is a feedback loop with knock as one of the inputs to the comparator.
And that Sir is not a generality.
I haven't been an engineer for the last 22 years for nothing.

Modern ECM's are an old drag racers dream come true. Automatically adjusting A/F ratio, ignition and cam timing, etc. to ever changing conditions.

Most folks will never notice what their engine is really doing regarding octane (other factors as well). The ECM has things managed so well.
Even if there is a power or mileage difference caused by octane it is (usually) so small (a few percent) as to be indistinguishable by most observers.
This serves to perpetuate the myth that octane does not matter that is adhered to by some.
(obvious exception for for folks that know the difference)

Rickey.