engine RPM vs MPG experiment

[DK]

Impreza 1993, EJ18 engine, manual.

Exclusively city driving, average trip about 5-7 miles. Was trying to
drive keeping RPMs close to 2,000 or to 3,000. For the most part
in practice it meant choice between 4th and 3rd gear.

Same pump, fuel contains "up to" 10% ethanol, same weather, two
full tanks for each "condition". The milage was consistent between
two tanks in each case within 0.3 MPG. Results:

~ 1,800-2,300 RPM = 26.7 MPG
~ 2,500-3,200 RPM = 28.1 MPG
(No, EJ18 is not a marvel of fuel economy )

Based on what I read before, I expected the opposite result. In any
case, the difference is no more than 5%, so it's not really worth
it to keep attention to. Lower RPM is less noise and less power.
What's better for the engine in the long run?

DK

 

[Guest]

Impreza 1993, EJ18 engine, manual.

Exclusively city driving, average trip about 5-7 miles. Was trying to
drive keeping RPMs close to 2,000 or to 3,000. For the most part
in practice it meant choice between 4th and 3rd gear.

Same pump, fuel contains "up to" 10% ethanol, same weather, two
full tanks for each "condition". The milage was consistent between
two tanks in each case within 0.3 MPG. Results:

~ 1,800-2,300 RPM = 26.7 MPG
~ 2,500-3,200 RPM = 28.1 MPG
(No, EJ18 is not a marvel of fuel economy )

Based on what I read before, I expected the opposite result. In any
case, the difference is no more than 5%, so it's not really worth
it to keep attention to. Lower RPM is less noise and less power.
What's better for the engine in the long run?

DK

Not surprising as a result, smaller engines are set up to make better
power in the higher revs, the cylinder filling is just so much better
at those revs it offsets the pumping/friction losses.

I'd have to guess an engine wears out after part X rubs past part y
for the umpteen-billionth time and it just wears out.

As long as you're not lugging the engine, having the car travel in the
highest gear possible will minimize the number of 'rubs' per mile
driven.

Dave

 

[runcyclexcski]

Impreza 1993, EJ18 engine, manual.

Exclusively city driving, average trip about 5-7 miles. Was trying to
drive keeping RPMs close to 2,000 or to 3,000. For the most part
in practice it meant choice between 4th and 3rd gear.

Same pump, fuel contains "up to" 10% ethanol, same weather, two
full tanks for each "condition". The milage was consistent between
two tanks in each case within 0.3 MPG. Results:

~ 1,800-2,300 RPM = 26.7 MPG
~ 2,500-3,200 RPM = 28.1 MPG
(No, EJ18 is not a marvel of fuel economy )

Based on what I read before, I expected the opposite result. In any
case, the difference is no more than 5%, so it's not really worth
it to keep attention to. Lower RPM is less noise and less power.
What's better for the engine in the long run?

DK

I would believe this if you could average over many more, not just the
2 tanks.

 

[hippo]

Impreza 1993, EJ18 engine, manual.

Exclusively city driving, average trip about 5-7 miles. Was trying to
drive keeping RPMs close to 2,000 or to 3,000. For the most part
in practice it meant choice between 4th and 3rd gear.

Same pump, fuel contains "up to" 10% ethanol, same weather, two
full tanks for each "condition". The milage was consistent between
two tanks in each case within 0.3 MPG. Results:

~ 1,800-2,300 RPM = 26.7 MPG
~ 2,500-3,200 RPM = 28.1 MPG
(No, EJ18 is not a marvel of fuel economy )

Based on what I read before, I expected the opposite result. In any
case, the difference is no more than 5%, so it's not really worth
it to keep attention to. Lower RPM is less noise and less power.
What's better for the engine in the long run?

DK

Longer trips will do the most to increase your engine life and minimis
wear! Most engine damage occurs in the first few miles/Kms when the engin
is not properly warmed up. If you run a longer distance for your test, yo
may notice more variety in your figures. Cheers

 

[John Varela]

Not surprising as a result, smaller engines are set up to make better
power in the higher revs, the cylinder filling is just so much better
at those revs it offsets the pumping/friction losses.

I recall some years ago BMW did some tests with one of their cars and
found that the best gas mileage was achieved by accelerating at wide
open throttle (WOT) to cruise speed, then holding constant speed. That
would apply only to that model of car, whose engine was designed to run
most efficiently at WOT, and similar cars by BMW and others.

I'd have to guess an engine wears out after part X rubs past part y
for the umpteen-billionth time and it just wears out.

As long as you're not lugging the engine, having the car travel in the
highest gear possible will minimize the number of 'rubs' per mile
driven.

In 1955 I spent six months as a co-op student working at the Ford Motor
Co. proving grounds, then located in Dearborn, Michigan. One of the
things I helped do was to run fuel efficiency tests on Ford and
competitors' cars, all at constant speeds (that is, not from a standing
start like the BMW test above). The highest efficiency was invariably
obtained at the lowest possible speed in the highest possible gear.

Things may have changed in 50 years, but I expect that relationship
still holds.

 

[Tony Hwang]

Impreza 1993, EJ18 engine, manual.

Exclusively city driving, average trip about 5-7 miles. Was trying to
drive keeping RPMs close to 2,000 or to 3,000. For the most part
in practice it meant choice between 4th and 3rd gear.

Same pump, fuel contains "up to" 10% ethanol, same weather, two
full tanks for each "condition". The milage was consistent between
two tanks in each case within 0.3 MPG. Results:

~ 1,800-2,300 RPM = 26.7 MPG
~ 2,500-3,200 RPM = 28.1 MPG
(No, EJ18 is not a marvel of fuel economy )

Based on what I read before, I expected the opposite result. In any
case, the difference is no more than 5%, so it's not really worth
it to keep attention to. Lower RPM is less noise and less power.
What's better for the engine in the long run?

DK

Hmm,
2 tankful? Have you looked at power band curve of your eingine? And
frequency of brake use. My '98 Honda CRV alwasy produce 10L/100Km gas
mileage regardless of season or where I drive in the city. If you drive
your car most of time like that in the city better go out onto turnpike
and burn off all the carbon build up inside the engine.

Two things, every engine has optimum cruising rpm and speed by desing
which will give best efficiency. Of course this is based on sea level
driving. I live near Rockies at altitude of ~2700 feet.

 

[Tony Hwang]

Impreza 1993, EJ18 engine, manual.

Exclusively city driving, average trip about 5-7 miles. Was trying to
drive keeping RPMs close to 2,000 or to 3,000. For the most part
in practice it meant choice between 4th and 3rd gear.

Same pump, fuel contains "up to" 10% ethanol, same weather, two
full tanks for each "condition". The milage was consistent between
two tanks in each case within 0.3 MPG. Results:

~ 1,800-2,300 RPM = 26.7 MPG
~ 2,500-3,200 RPM = 28.1 MPG
(No, EJ18 is not a marvel of fuel economy )

Based on what I read before, I expected the opposite result. In any
case, the difference is no more than 5%, so it's not really worth
it to keep attention to. Lower RPM is less noise and less power.
What's better for the engine in the long run?

DK

Longer trips will do the most to increase your engine life and minimise
wear! Most engine damage occurs in the first few miles/Kms when the engine
is not properly warmed up. If you run a longer distance for your test, you
may notice more variety in your figures. Cheers

Hi,
If that is his usual driving pattern, at least the car needs more
frequent oil change for sure.

 

[John O]

In 1955 I spent six months as a co-op student working at the Ford Motor
Co. proving grounds, then located in Dearborn, Michigan. One of the
things I helped do was to run fuel efficiency tests on Ford and
competitors' cars, all at constant speeds (that is, not from a standing
start like the BMW test above). The highest efficiency was invariably
obtained at the lowest possible speed in the highest possible gear.

Things may have changed in 50 years, but I expect that relationship
still holds.

Nah, the laws of thermodynamics are still the same in Dearborn, last I
checked. They have a bit more eclectic food these days, but that track is
still there. I'll be breaking bread with one of many current Ford engineers
this weekend and I'll ask if they still use that track for such stuff.

-John O

 

[DK]

dk@[EMAIL PROTECTED] (DK) Aug 25, 2008 at 05:16 PM wrote:

Longer trips will do the most to increase your engine life and minimise
wear! Most engine damage occurs in the first few miles/Kms when the engine
is not properly warmed up. If you run a longer distance for your test, you
may notice more variety in your figures. Cheers

I drove the way I use the car for, that's all.

If that is his usual driving pattern, at least the car needs more
frequent oil change for sure.

The oil change has been done regularly at 5,000 miles for the entire
life of the car, which is 15 years and 110,000 miles on it. Frankly,
if it dies on me soon, I wouldn't care that much - I am starting
to get tired of it. On the other hand, its maintanence so far averaged
perhaps $50/month, so for as long as it runs and gets me from point
A to point B, I can't justify buying another one.

DK

 

[AlFire]

to get tired of it. On the other hand, its maintanence so far averaged
perhaps $50/month, so for as long as it runs and gets me from point

15y*12m*$50=$9000 - wow

 

[DK]

15y*12m*$50=$9000 - wow

Well, that ballpark figure is perhaps for the past 5 years when the car
started to "need" things: clutch, timing belt, half-axel, water pump,
rotors/pads. So it's more like $3,000 (including oil changes and
regular maintenance) - far cheaper than buying a new car, which
would have higher insurance rates and carry about 1/3 of the same
maintenance costs.

DK

 

[tom klein]

I recall some years ago BMW did some tests with one of their cars and
found that the best gas mileage was achieved by accelerating at wide
open throttle (WOT) to cruise speed, then holding constant speed. That
would apply only to that model of car, whose engine was designed to run
most efficiently at WOT, and similar cars by BMW and others.


In 1955 I spent six months as a co-op student working at the Ford Motor
Co. proving grounds, then located in Dearborn, Michigan. One of the
things I helped do was to run fuel efficiency tests on Ford and
competitors' cars, all at constant speeds (that is, not from a standing
start like the BMW test above). The highest efficiency was invariably
obtained at the lowest possible speed in the highest possible gear.

Things may have changed in 50 years, but I expect that relationship
still holds.

if the fuel/air mixture were held constant, then the least number of
revolutions would result in the least fuel per distance. (that's just
simple math.)

but modern cars change the mixture, and even the valve timing with
speed and load, so it's quite feasible to have an engine that runs much
more efficiently at higher revs (within reason).

the bmw test also jives with my experiences with a nissan axxess, which
seemed quite happy to accelerate hard up to whatever speed was needed,
and then just cruise along. this wasn't what i had been taught, but it
does make some sense, sort of, to spend less time accelerating. and
it's exactly what cruise control does. (it can be a little scary to
see/hear how hard the cruise control kicks it to get back to speed.)

i haven't tested it thoroughly, but my '02 legacy *seems* to run better
at 3-4k rpm than at 2-3k - both in terms of smoother shifting, better
torque, and (seemingly) slightly better gas mileage. i also think that
it's the smoothest running engine that i've owned, because it doesn't
have the "winding up" sound that you get from most little japanese
engines.

 

[Hachiroku $B%O%A%m%/(B]

Impreza 1993, EJ18 engine, manual.

Exclusively city driving, average trip about 5-7 miles. Was trying to
drive keeping RPMs close to 2,000 or to 3,000. For the most part
in practice it meant choice between 4th and 3rd gear.

Same pump, fuel contains "up to" 10% ethanol, same weather, two
full tanks for each "condition". The milage was consistent between
two tanks in each case within 0.3 MPG. Results:

~ 1,800-2,300 RPM = 26.7 MPG
~ 2,500-3,200 RPM = 28.1 MPG
(No, EJ18 is not a marvel of fuel economy )

Based on what I read before, I expected the opposite result. In any
case, the difference is no more than 5%, so it's not really worth
it to keep attention to. Lower RPM is less noise and less power.
What's better for the engine in the long run?

DK

Where are you? In Mass the pumps say, "Contains 10% Ethanol", but when I
go to Vermont the pumps say "Contains Up To 10% Ethanol".

Now, I thought the premium grades would have the less amounts of Ethanol,
but I may be wrong. It seems to me I recall hearing that Ethanol is an
octane booster, so I'd be willing to bet the 93 octane has the most.

Maybe someone can clarify...

 

[JD]

Where are you? In Mass the pumps say, "Contains 10% Ethanol", but when I
go to Vermont the pumps say "Contains Up To 10% Ethanol".

Now, I thought the premium grades would have the less amounts of Ethanol,
but I may be wrong. It seems to me I recall hearing that Ethanol is an
octane booster, so I'd be willing to bet the 93 octane has the most.

Maybe someone can clarify...

There are several ways to boost octane; ethanol is certainly one. Not sure
if 93 in the US is boosted solely by ethanol, but I would doubt it. The
more ethanol you put in, the less energy density in the fuel. Given that
premium is more expensive, while ethanol is cheaper, I suspect that they are
using another hydrocarbon to boost octane.

 

[sexsmithguitars]

Impreza 1993, EJ18 engine, manual.

Exclusively city driving, average trip about 5-7 miles. Was trying to
drive keeping RPMs close to 2,000 or to 3,000. For the most part
in practice it meant choice between 4th and 3rd gear.

Same pump, fuel contains "up to" 10% ethanol, same weather, two
full tanks for each "condition".  The milage was consistent between
two tanks in each case within 0.3 MPG. Results:

~ 1,800-2,300 RPM = 26.7 MPG
~ 2,500-3,200 RPM = 28.1 MPG
(No, EJ18 is not a marvel of fuel economy )

Based on what I read before, I expected the opposite result. In any
case, the difference is no more than 5%, so it's not really worth
it to keep attention to. Lower RPM is less noise and less power.
What's better for the engine in the long run?

DK

Hi DK
this is about another issue, was it you who was having trouble with a
p172 code on your subaru?
my subaru is behaving the same with the same code, what was the
problem.
thanks
GM

 

[DK]

this is about another issue, was it you who was having trouble with a
p172 code on your subaru?

Yes, but that was wife's Forester.

my subaru is behaving the same with the same code, what was the
problem.

In this case, the problem was definitely front oxygen sensor.
The moment it was replaced, the troubles disappeared (and
milage improved somewhat).

DK

 

[sexsmithguitars]

Yes, but that was wife's Forester.


In this case, the problem was definitely front oxygen sensor.
The moment it was replaced, the troubles disappeared (and
milage improved somewhat).

DK

Thanks DK sorry to hijack your discussion, I was at a bit of a loss, i
went through the whole check list, and the front o2 was just replaced
about 8 months ago,
do you think the rear o2 would have that much influence over the way
the car runs? or does it just serve to monitor the catalitic
converter?
Thanks
GM

 

[DK]

Thanks DK sorry to hijack your discussion, I was at a bit of a loss, i
went through the whole check list, and the front o2 was just replaced
about 8 months ago,
do you think the rear o2 would have that much influence over the way
the car runs? or does it just serve to monitor the catalitic
converter?

It certainly did in my case. And it makes sense. You might be thinking
of the rear O2 sensor, which indeed just monitors the cat. converter
and is unimportant for driving.

When I was digging probable causes for P0172, here is one fix
I found that turned out well for at least couple guys: make sure the
ground cable attached to the intake manifold is clean and tightly
screwed. It's a ground for a bunch of sensors and when it is not
connected, many things can go wrong. Or so I read.

DK

 

[sexsmithguitars]

Thanks again,
I'll let you know how i go, will take a while for parts to arrive
though.
GM

 

[clare at snyder dot ontario dot canada]

I would believe this if you could average over many more, not just the
2 tanks.

I just got back from a 6231km road trip with my 2.4 liter PT Cruiser.
Averaged 29MPG. When I was running over 3000 RPM (and 117-120KPH) I
got 34MPG. When running under 2500RPM I got up to 28MPG. A lot of
hills throughout the trip (Waterloo Ontario through quebec (north
shore of the St Laurence) , through New Brunswick, PEI, Nova Scotia,
(including the Cabot Trail), Vermont (including Smuggler's Notch and
Stowe area) Main, New Hampshire, andLake Champlain and Finger Lakes
districts, back to Waterloo Ontario.
The car ran best, and gave the best mileage, when running over 2800
RPM - hills or no hills.
Even on flat runs, like Waterloo to Windsor, at 100KPH (under 2500RPM)
I can not do any better than 28MPG and I have to have my foot into it
deeper than when running at 2800+RPM and 117KPH (72-73MPH)