octane and mileage

[dasa]

hi folks,

just got back from a long road trip, like so many of you, i'm sure (in u.s.,
that is). going from sf to la, about 400miles, i've got better than 30
miles per gallon (us), on 2000 obs - it has about 40k miles. seems
surprising, as last time i made the same trip, when the car had about 20k,
i've got about 28 mpg. i would have thought that engine break-in would have
been done by 20k, but perhaps not. btw, on the return trip, i tried premium
gas (91 octane), and the mileage went down to about 27mpg. go figure. (the
manual states use of 87 octane or higher)

dasa

 

[FNO]

hi folks,

just got back from a long road trip, like so many of you, i'm sure (in u.s.,
that is). going from sf to la, about 400miles, i've got better than 30
miles per gallon (us), on 2000 obs - it has about 40k miles. seems
surprising, as last time i made the same trip, when the car had about 20k,
i've got about 28 mpg. i would have thought that engine break-in would have
been done by 20k, but perhaps not. btw, on the return trip, i tried premium
gas (91 octane), and the mileage went down to about 27mpg. go figure. (the
manual states use of 87 octane or higher)

dasa

That's not really surprising. The knock detectors in the engine management
system will advance the spark as far as they can with no knock. On 91
octane, that is a lot further than on 87. The car will make a bit more
power, but fuel mileage goes down. On the turbo cars, it makes a huge
difference.

 

[Kim Groves]

Does this mean higher or lower octane for turbos to get the maximum mpg?

 

[Carl 1 Lucky Texan]

I think it would take several tankfuls to see the 'real' difference
between the 2 octanes. The ECU needs a few 'cold' starts to make its
adjustments. Though in general, a relatively low compression engine will
not be able to take advantage of higher octane unless it is turbo/super
charged.

Carl
1 Lucky Texan

 

[Yousuf Khan]

hi folks,

just got back from a long road trip, like so many of you, i'm sure (in u.s.,
that is). going from sf to la, about 400miles, i've got better than 30
miles per gallon (us), on 2000 obs - it has about 40k miles. seems
surprising, as last time i made the same trip, when the car had about 20k,
i've got about 28 mpg. i would have thought that engine break-in would have
been done by 20k, but perhaps not. btw, on the return trip, i tried premium
gas (91 octane), and the mileage went down to about 27mpg. go figure. (the
manual states use of 87 octane or higher)

Is your vehicle with a high-performance or regular engine? The high-perf
engines are the ones that require premium gas, and the regular ones are the
ones that require regular gas. In general, putting premium gas in a regular
engine will not help it one bit, because the engine computer is tuned
conservatively. In a high-perf engine, the computer is tuned to make as much
power as possible before knock starts with premium gas; if you put regular
gas in a high-perf engine, the computer will tune itself even further back
because knock starts earlier with regular gas.

If your car has a high-perf engine computer then it might actually get
better mileage with regular gas, by tuning itself back.

So which engines are the high perf ones? I usually consider any of the
engines producing more than 200HP as the high-perf engines, such as the 2.0L
or 2.5L turbo H4's, or the 3.0L H6.

Yousuf Khan

 

[Yousuf Khan]

Does this mean higher or lower octane for turbos to get the maximum mpg?

Lower octane will generally cause the engine computer to scale back its
performance, therefore it will give you better mpg.

One caution though, you don't want to scale all of the way back to the
lowest grade fuel with a turbo, because the computer will be so busy
preventing knock that it might actually do some damage to the engine; but
it's upto you to experiment. I'd go back to a mid-grade as a first
experiment.

Yousuf Khan

 

[dasa]

the car has non-turbo 2.2, sohc engine used for many us subaru models up to
2001 (mine in particular is 2000 impreza outback sport). i just wanted to
see if there will be any difference with higher octane gas. as many of you
noted, it didn't do any good. in fact, the mileage went down, and i didn't
feel any difference in terms of power. perhaps it may make difference after
few tanks, assuming the ecu will actually advance the timing according to
the higher octane gas? i doubt the ecu (in my car, in particular) had even
been programmed to take advantage of higher octane gas, but who knows... i
will stick with regular grade (87). anyways, i am pleased that the engine,
as it ages (40k), seems to get mileage better than epa rating. i wonder if
synthetic engine oil i have been using actually contributed to this...?
again, who knows.

 

[Edward Hayes]

Not unusual for mileage to go down measurably if a car requiring regular (87
octane) is run on 89 or higher octane. One of the major auto magazines ran a
test with several vehicles and found this is a fact. The magazine was out in
the last year or so. eddie

 

[FNO]

Does this mean higher or lower octane for turbos to get the maximum mpg?

Turbos are forced induction. Consequently, you have to be very careful with
them. Generally, you cannot use regular (87 octane) at all. In the case of
my car (the WRX STi) it will go into limp mode and damage the engine. The
secret to high MPG in a turbo is to use one grade less than recommended and
keep the thing off boost. You'll need a boost pressure gauge to tell. That
usually means easy accelerations and cruise control.

 

[Carl 1 Lucky Texan]

I can see the knock adjustments being made 'on the fly'(and maybe
RPM/idle adjustments) but are those the only adjustments being made?


Carl
1 Lucky Texan

 

[FNO]

I can see the knock adjustments being made 'on the fly'(and maybe
RPM/idle adjustments) but are those the only adjustments being made?


Carl
1 Lucky Texan
<snip>

It really depends on the car. For most of them, yes. Those are generally
the only adjustments the ECU makes. For the turbo cars, it can also adjust
the boost controller based on where it detects knock. Its first response is
to back off the timing. Its second response is to reduce the pop-off
pressure on the boost controller; effectively reducing compression.

In the STi, the car may also adjust the amount of locking on the center
differential depending on driver input, the lateral G-forces, and the amount
of wheel slip detected.

The pricier you go, the mosre sophisticated the ECU and onboard computers
are. The big thing to remember, is that nowadays, they are fast.
Consequently, gone are the days of simple ignition maps on the these ECUs.
Now, they are adaptive, and they adapt quickly.

 

[TransFixed]

hi folks,

just got back from a long road trip, like so many of you, i'm sure (in u.s.,
that is). going from sf to la, about 400miles, i've got better than 30
miles per gallon (us), on 2000 obs - it has about 40k miles. seems
surprising, as last time i made the same trip, when the car had about 20k,
i've got about 28 mpg. i would have thought that engine break-in would have
been done by 20k, but perhaps not. btw, on the return trip, i tried premium
gas (91 octane), and the mileage went down to about 27mpg. go figure. (the
manual states use of 87 octane or higher)

dasa

Where did you fill up? I recently did the same trip twice (extending a
little further South) and found that the gasoline sold in the valley
(oxygenated?) was awful. With gas purchased in San Diego or LA or the
urban Bay area I got 28-29mpg, with gas filled in the valley I got 26mpg
(Passat 6cyl. 4Motion Wagon [AWD]).

- D.

 

[dasa]

mill valley (marin county) and south san francisco (pennisula). don't know
how the gas is in south bay...

hi folks,

just got back from a long road trip, like so many of you, i'm sure (in u.s.,
that is). going from sf to la, about 400miles, i've got better than 30
miles per gallon (us), on 2000 obs - it has about 40k miles. seems
surprising, as last time i made the same trip, when the car had about 20k,
i've got about 28 mpg. i would have thought that engine break-in would have
been done by 20k, but perhaps not. btw, on the return trip, i tried premium
gas (91 octane), and the mileage went down to about 27mpg. go figure. (the
manual states use of 87 octane or higher)

dasa

Where did you fill up? I recently did the same trip twice (extending a
little further South) and found that the gasoline sold in the valley
(oxygenated?) was awful. With gas purchased in San Diego or LA or the
urban Bay area I got 28-29mpg, with gas filled in the valley I got 26mpg
(Passat 6cyl. 4Motion Wagon [AWD]).

- D.

 

[Dominic Richens]

anyways, i am pleased that the
engine, as it ages (40k), seems to get mileage better than epa
rating.

As a data point, with careful driving (e.g. keep it between 90 - 100 kph,
easy on the acceleration) I can get 37 mpg on my 95 Legacy Wagon (Brighton)
2.2l SOHC - tires inflated to ~35 psi (instead of 32/30).

I recently switched to Ethanol mix (10%) and am interested to see what
effect that has on milage.

 

[Rick Courtright]

easy on the acceleration) I can get 37 mpg on my 95 Legacy Wagon (Brighton)

Canadian (Imperial) gallons, or US?

Rick

 

[Dominic Richens]

Canadian (Imperial) gallons, or US?

US. In Canadian-English that would be 6.4l/100km

 

[Patrick Lee]

I have been told by Subaru mechanics to
use only 87 octane because you don't get
better gas milage with higher octane. The
higher octane causes the engine to run to
rich.

 

[Edward Hayes]

That is true. Car & Driver magazine ran a test and found that running the
recommended octane gave the best mileage. eddie

 

[ricardox]

Here's the article:
http://www.caranddriver.com/article.asp?section_id=4&article_id=3604&page_number=1