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Why you should never buy a car without a tachometer



 
 
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  #1  
Old September 13th 05, 02:51 PM
Ted B.
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Posts: n/a
Default Why you should never buy a car without a tachometer

Over the years I've owned my current ride, I have observed that it is most
fuel-efficient at 3500RPM, where 42MPG is easily attainable. I have also
observed that, at 3000 RPM or less, the mileage on my vehicle drops to 35MPG
or lower. My experience with my current vehicle is similar to several other
vehicles I've owned. In every case, I've found that the best fuel economy
is achieved somewhere above 70MPH. In the case of my current vehicle,
3500RPM in top gear equates to about 78MPH. Now I'm sure someone will state
the obvious, which is how do I know that my tachometer and speedometer are
accurate? I don't. But the bottom line is, in top gear, I can achieve
BETTER fuel economy at slightly higher than normal highway speeds.
Actually, the better fuel economy is due to slightly higher engine RPM, and
the higher ground speed is COINCIDENTAL.

I've always known that every engine has a "sweet spot", but didn't know the
exact scientific explanation for my personal observations until just
yesterday. During research on various engine types, I happened to discover
that Otto Cycle engines are most fuel-efficient at RPMs near (not
necessarily at, but near) 40% of redline. I had no reason to disbelieve
that particular scientific fact. I did the math for my own car, and found
that (surprise, surprise) I SHOULD achieve maximum fuel economy at ABOUT
3200RPM, with my own Otto Cycle engine. This agrees almost exactly with my
own observations that 3500RPM (43% of redline) is very fuel efficient, and
3000RPM or less (37% of redline or lower) is not as fuel efficient.
Obviously, my engine is very fuel-efficient near 40% of redline, AS IT
SHOULD BE, and it is less fuel-efficient at lower RPM levels. Note that I
had no idea that my engine SHOULD be most efficient at about 40% of redline
before I observed that it WAS most efficient at 3500RPM. So the results I
observed were not results that I "expected" to observe. The facts are what
the facts are, and now I have the scientific explanation stating YES, it
should be that way.

From what I could gather, the reason for the lower fuel efficiency below
~40% of redline in an Otto Cycle engine has to do with valve timing and
other aspects of engine performance. Most Otto Cycle engines have fixed
valve timing, so the maximum fuel economy is achieved at a certain RPM, and
if you run the engine higher OR LOWER than that RPM, your engine is not
achieving maximum fuel efficiency. Some engine manufacturers have come out
with variable valve timing Otto Cycle engine designs, with limited success
in improving fuel efficiency at lower RPMs. I gather that the success is
"limited" because fixed valve timing isn't the only factor making an Otto
Cycle engine most efficient at a certain RPM.

Anyway, now that I know WHY I'm seeing fuel economy peak at certain RPMs, I
know exactly what to look for in my NEXT vehicle. IF it has an Otto Cycle
engine, it WILL have a tachometer, or I won't buy it. During the test
drive, I'm taking it out on the highway to see how fast the car is
travelling at 40% of the engine redline. The way I USUALLY drive, a car
would be most efficient for me if it hit 40% of the engine's redline between
70 and 75MPH. Ironically, my current car is designed for a less
conservative driver than its current owner is. While I do get close to
80 sometimes, the vast majority of my highway driving is in the range of
70-75MPH, so that is where I need a car to be most efficient. SIMPLE,
right?

So if you want to find a car that will be very fuel-efficient for the way
you drive, simply look at the tachometer. Wherever it ends, muliply the
highest number by (.4). This will give you a good estimate of the most
fuel-efficient speed (RPM) of the engine. Then take it out on the highway
and see if the engine will run at that speed (RPM) at the speed you normally
drive on the highway. If the engine RPM is too high OR too LOW at your
normal highway speed, you might want to find a car with an engine that is
better designed for the way YOU drive. That is, if your goal is to find a
fuel-efficient vehicle. -Dave

On a side note, it's easy to understand why manual tranny vehicles are more
fuel-efficient for use in City driving. You have to get the engine RPMs UP
to maximize fuel economy, and you just can't do that at low speeds in a
slush-box.




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  #2  
Old September 13th 05, 03:06 PM
N8N
external usenet poster
 
Posts: n/a
Default


Ted B. wrote:
> Over the years I've owned my current ride, I have observed that it is most
> fuel-efficient at 3500RPM, where 42MPG is easily attainable. I have also
> observed that, at 3000 RPM or less, the mileage on my vehicle drops to 35MPG
> or lower. My experience with my current vehicle is similar to several other
> vehicles I've owned. In every case, I've found that the best fuel economy
> is achieved somewhere above 70MPH. In the case of my current vehicle,
> 3500RPM in top gear equates to about 78MPH. Now I'm sure someone will state
> the obvious, which is how do I know that my tachometer and speedometer are
> accurate? I don't. But the bottom line is, in top gear, I can achieve
> BETTER fuel economy at slightly higher than normal highway speeds.
> Actually, the better fuel economy is due to slightly higher engine RPM, and
> the higher ground speed is COINCIDENTAL.


<snip>

just curious Ted, what do the power/torque curves look like for your
particular engine, and/or what engine is it? Do you have access to a
full BSFC map for your engine? Would be interesting to see.

thanks,

nate

  #3  
Old September 13th 05, 03:31 PM
Ted B.
external usenet poster
 
Posts: n/a
Default


"N8N" > wrote in message
oups.com...
>
> Ted B. wrote:
>> Over the years I've owned my current ride, I have observed that it is
>> most
>> fuel-efficient at 3500RPM, where 42MPG is easily attainable. I have also
>> observed that, at 3000 RPM or less, the mileage on my vehicle drops to
>> 35MPG
>> or lower. My experience with my current vehicle is similar to several
>> other
>> vehicles I've owned. In every case, I've found that the best fuel
>> economy
>> is achieved somewhere above 70MPH. In the case of my current vehicle,
>> 3500RPM in top gear equates to about 78MPH. Now I'm sure someone will
>> state
>> the obvious, which is how do I know that my tachometer and speedometer
>> are
>> accurate? I don't. But the bottom line is, in top gear, I can achieve
>> BETTER fuel economy at slightly higher than normal highway speeds.
>> Actually, the better fuel economy is due to slightly higher engine RPM,
>> and
>> the higher ground speed is COINCIDENTAL.

>
> <snip>
>
> just curious Ted, what do the power/torque curves look like for your
> particular engine, and/or what engine is it? Do you have access to a
> full BSFC map for your engine? Would be interesting to see.
>
> thanks,
>
> nate
>


Y'know, I've searched online to find exactly that information on my engine,
or any similar engine. I've had no luck finding it, so far. I know I've
got the brochure for my car in my house somewhere. I'm going to pull it out
and see if I can get a hardcopy of the power/torque curves. IF so, I should
be able to post specific information on what the curves look like for my
particular engine. I didn't mean to focus on my engine in particular. I
just thought it was REALLY INTERESTING to learn that Otto Cycle engines
achieve maximum fuel efficiency near 40% of redline. (especially as that
agrees EXACTLY with the performance I have observed over many years of
driving vehicles with Otto Cycle engines) Knowing THAT particular data
point makes it a little easier to pick a new car when you are car shopping.
But only if the car has a tachometer. If it doesn't have a tachometer, then
you have no idea what ground speed the engine was made to be most efficient
at. That is, without running many hundreds of miles of tests first.

Best I can come up with at the moment is that the engine in my current car
is a Mitsubishi "4g64" 2.4L inline 4. -Dave


  #4  
Old September 13th 05, 03:43 PM
N8N
external usenet poster
 
Posts: n/a
Default


Ted B. wrote:
> "N8N" > wrote in message
> oups.com...
> >
> > Ted B. wrote:
> >> Over the years I've owned my current ride, I have observed that it is
> >> most
> >> fuel-efficient at 3500RPM, where 42MPG is easily attainable. I have also
> >> observed that, at 3000 RPM or less, the mileage on my vehicle drops to
> >> 35MPG
> >> or lower. My experience with my current vehicle is similar to several
> >> other
> >> vehicles I've owned. In every case, I've found that the best fuel
> >> economy
> >> is achieved somewhere above 70MPH. In the case of my current vehicle,
> >> 3500RPM in top gear equates to about 78MPH. Now I'm sure someone will
> >> state
> >> the obvious, which is how do I know that my tachometer and speedometer
> >> are
> >> accurate? I don't. But the bottom line is, in top gear, I can achieve
> >> BETTER fuel economy at slightly higher than normal highway speeds.
> >> Actually, the better fuel economy is due to slightly higher engine RPM,
> >> and
> >> the higher ground speed is COINCIDENTAL.

> >
> > <snip>
> >
> > just curious Ted, what do the power/torque curves look like for your
> > particular engine, and/or what engine is it? Do you have access to a
> > full BSFC map for your engine? Would be interesting to see.
> >
> > thanks,
> >
> > nate
> >

>
> Y'know, I've searched online to find exactly that information on my engine,
> or any similar engine. I've had no luck finding it, so far. I know I've
> got the brochure for my car in my house somewhere. I'm going to pull it out
> and see if I can get a hardcopy of the power/torque curves. IF so, I should
> be able to post specific information on what the curves look like for my
> particular engine. I didn't mean to focus on my engine in particular. I
> just thought it was REALLY INTERESTING to learn that Otto Cycle engines
> achieve maximum fuel efficiency near 40% of redline. (especially as that
> agrees EXACTLY with the performance I have observed over many years of
> driving vehicles with Otto Cycle engines) Knowing THAT particular data
> point makes it a little easier to pick a new car when you are car shopping.
> But only if the car has a tachometer. If it doesn't have a tachometer, then
> you have no idea what ground speed the engine was made to be most efficient
> at. That is, without running many hundreds of miles of tests first.
>
> Best I can come up with at the moment is that the engine in my current car
> is a Mitsubishi "4g64" 2.4L inline 4. -Dave


Really I was just curious if there was anything magical about 40% or
redline, or if that happened to coincide with the torque peak, which
I've always heard was a good approximation of where the BSFC map is
most favorable even under part throttle conditions. Having a nasty
Real Job(tm) I can't be looking for that info just at the moment

nate

  #5  
Old September 13th 05, 03:49 PM
external usenet poster
 
Posts: n/a
Default

Interesting. How did you calculate that?

  #6  
Old September 13th 05, 04:34 PM
Ted B.
external usenet poster
 
Posts: n/a
Default

>> Best I can come up with at the moment is that the engine in my current
>> car
>> is a Mitsubishi "4g64" 2.4L inline 4. -Dave

>
> Really I was just curious if there was anything magical about 40% or
> redline, or if that happened to coincide with the torque peak, which
> I've always heard was a good approximation of where the BSFC map is
> most favorable even under part throttle conditions. Having a nasty
> Real Job(tm) I can't be looking for that info just at the moment
>
> nate
>


Well, I found some numbers for the Nissan 3.5L V6 (aka Maxima) engine. This
engine has been one of the best automotive engines in the world for several
years, so I think it's a good "example" to use.

Redline is 8000, so maximum fuel efficiency should be about 3200RPM, for
that particular Otto Cycle engine (8000 X .4). Peak horsepower is 265 at
5800RPM and peak torque is 255 at 4400RPM. Nissan does use variable valve
timing, but this would tend to favor lower RPMs, as higher RPMs don't need
the valve timing trick to produce more power or torque.

SO, peak fuel efficiency RPM for that particular Otto Cycle engine seems to
be below -both- the horsepower and torque peaks, which makes sense. You
don't need MAXIMUM horsepower or torque to keep a car moving, but more
torque will be helpful for acceleration.

Now I want to hear from Nissan Maxima 3.5L owners . . . what speed (MPH) is
equivalent to 3200RPM engine speed in your Nissan Maxima in top gear (O/D
enabled, if it's an automatic)? If you pay attention to RPM or MPH, have
you noticed an increase in fuel efficiency at that speed??? Since it's an
Otto Cycle engine with an 8000RPM redline, it should be most fuel efficient
around 3200RPM (8000 X .4). IS IT most fuel-efficient near 3200 RPM? -Dave


  #7  
Old September 13th 05, 04:41 PM
Ted B.
external usenet poster
 
Posts: n/a
Default


> wrote in message
oups.com...
> Interesting. How did you calculate that?
>


How did who calculate what? -Dave


  #8  
Old September 13th 05, 06:03 PM
223rem
external usenet poster
 
Posts: n/a
Default

Ted B. wrote:
>>>Best I can come up with at the moment is that the engine in my current
>>>car
>>>is a Mitsubishi "4g64" 2.4L inline 4. -Dave

>>
>>Really I was just curious if there was anything magical about 40% or
>>redline, or if that happened to coincide with the torque peak, which
>>I've always heard was a good approximation of where the BSFC map is
>>most favorable even under part throttle conditions. Having a nasty
>>Real Job(tm) I can't be looking for that info just at the moment
>>
>>nate
>>

>
>
> Well, I found some numbers for the Nissan 3.5L V6 (aka Maxima) engine. This
> engine has been one of the best automotive engines in the world for several
> years, so I think it's a good "example" to use.
>
> Redline is 8000, so maximum fuel efficiency should be about 3200RPM, for
> that particular Otto Cycle engine (8000 X .4). Peak horsepower is 265 at
> 5800RPM and peak torque is 255 at 4400RPM. Nissan does use variable valve
> timing, but this would tend to favor lower RPMs, as higher RPMs don't need
> the valve timing trick to produce more power or torque.
>
> SO, peak fuel efficiency RPM for that particular Otto Cycle engine seems to
> be below -both- the horsepower and torque peaks, which makes sense. You
> don't need MAXIMUM horsepower or torque to keep a car moving, but more
> torque will be helpful for acceleration.
>
> Now I want to hear from Nissan Maxima 3.5L owners . . . what speed (MPH) is
> equivalent to 3200RPM engine speed in your Nissan Maxima in top gear (O/D
> enabled, if it's an automatic)?


It is well over 80 mph, and, with my 2 recent speeding tickets, I never
cruise at that speed.


If you pay attention to RPM or MPH, have
> you noticed an increase in fuel efficiency at that speed??? Since it's an
> Otto Cycle engine with an 8000RPM redline, it should be most fuel efficient
> around 3200RPM (8000 X .4). IS IT most fuel-efficient near 3200 RPM? -Dave


I dont have hard data, but no way it is most efficient at that kind of cruising
speed.

  #9  
Old September 13th 05, 06:14 PM
Ted B.
external usenet poster
 
Posts: n/a
Default

>>
>> Now I want to hear from Nissan Maxima 3.5L owners . . . what speed (MPH)
>> is equivalent to 3200RPM engine speed in your Nissan Maxima in top gear
>> (O/D enabled, if it's an automatic)?

>
> It is well over 80 mph, and, with my 2 recent speeding tickets, I never
> cruise at that speed.
>
>
> If you pay attention to RPM or MPH, have
>> you noticed an increase in fuel efficiency at that speed??? Since it's
>> an Otto Cycle engine with an 8000RPM redline, it should be most fuel
>> efficient around 3200RPM (8000 X .4). IS IT most fuel-efficient near
>> 3200 RPM? -Dave

>
> I dont have hard data, but no way it is most efficient at that kind of
> cruising
> speed.
>


Interesting! Is it really well over 80MPH at the most fuel-efficient range
of 3200RPM for the 3.5L V6 Otto Cycle engine? I guess that would make it
kind of tough to gather hard data. Who would keep the engine running at a
speed that equates to well over 80MPH for hundreds of miles? That would be
pretty gutsy in most areas of the U.S. Not unsafe at all, but not a good
idea, either, unfortunately.

From what you just told me though, it would seem that the Maxima was
designed for high-speed cruising, well over 80MPH. So it wouldn't be an
appropriate choice for a conservative driver. I don't doubt many people
think it's a real BLAST to drive, though. -Dave


  #10  
Old September 13th 05, 08:23 PM
John S.
external usenet poster
 
Posts: n/a
Default


Ted B. wrote:
> Over the years I've owned my current ride, I have observed that it is most
> fuel-efficient at 3500RPM, where 42MPG is easily attainable. I have also
> observed that, at 3000 RPM or less, the mileage on my vehicle drops to 35MPG
> or lower. My experience with my current vehicle is similar to several other
> vehicles I've owned. In every case, I've found that the best fuel economy
> is achieved somewhere above 70MPH. In the case of my current vehicle,
> 3500RPM in top gear equates to about 78MPH. Now I'm sure someone will state
> the obvious, which is how do I know that my tachometer and speedometer are
> accurate? I don't. But the bottom line is, in top gear, I can achieve
> BETTER fuel economy at slightly higher than normal highway speeds.
> Actually, the better fuel economy is due to slightly higher engine RPM, and
> the higher ground speed is COINCIDENTAL.


Your experience seems to be the opposite of most advice given to
maximize milage. I would like to find out more about how you conducted
your test.

If I understand you correctly you are saying that your car gets better
milage at 78mph (3500rpm) in high gear than it got at lower increments
say: 70mph, 60mph and 50mph?

Were you able to hold all the variables constant for a long enough time
to get a reasonably accurate reading for each increment?

Did you use the car computer to measure the miles per gallon?


>
> I've always known that every engine has a "sweet spot", but didn't know the
> exact scientific explanation for my personal observations until just
> yesterday. During research on various engine types, I happened to discover
> that Otto Cycle engines are most fuel-efficient at RPMs near (not
> necessarily at, but near) 40% of redline. I had no reason to disbelieve
> that particular scientific fact. I did the math for my own car, and found
> that (surprise, surprise) I SHOULD achieve maximum fuel economy at ABOUT
> 3200RPM, with my own Otto Cycle engine. This agrees almost exactly with my
> own observations that 3500RPM (43% of redline) is very fuel efficient, and
> 3000RPM or less (37% of redline or lower) is not as fuel efficient.
> Obviously, my engine is very fuel-efficient near 40% of redline, AS IT
> SHOULD BE, and it is less fuel-efficient at lower RPM levels. Note that I
> had no idea that my engine SHOULD be most efficient at about 40% of redline
> before I observed that it WAS most efficient at 3500RPM. So the results I
> observed were not results that I "expected" to observe. The facts are what
> the facts are, and now I have the scientific explanation stating YES, it
> should be that way.
>
> From what I could gather, the reason for the lower fuel efficiency below
> ~40% of redline in an Otto Cycle engine has to do with valve timing and
> other aspects of engine performance. Most Otto Cycle engines have fixed
> valve timing, so the maximum fuel economy is achieved at a certain RPM, and
> if you run the engine higher OR LOWER than that RPM, your engine is not
> achieving maximum fuel efficiency. Some engine manufacturers have come out
> with variable valve timing Otto Cycle engine designs, with limited success
> in improving fuel efficiency at lower RPMs. I gather that the success is
> "limited" because fixed valve timing isn't the only factor making an Otto
> Cycle engine most efficient at a certain RPM.
>
> Anyway, now that I know WHY I'm seeing fuel economy peak at certain RPMs, I
> know exactly what to look for in my NEXT vehicle. IF it has an Otto Cycle
> engine, it WILL have a tachometer, or I won't buy it. During the test
> drive, I'm taking it out on the highway to see how fast the car is
> travelling at 40% of the engine redline. The way I USUALLY drive, a car
> would be most efficient for me if it hit 40% of the engine's redline between
> 70 and 75MPH. Ironically, my current car is designed for a less
> conservative driver than its current owner is. While I do get close to
> 80 sometimes, the vast majority of my highway driving is in the range of
> 70-75MPH, so that is where I need a car to be most efficient. SIMPLE,
> right?
>
> So if you want to find a car that will be very fuel-efficient for the way
> you drive, simply look at the tachometer. Wherever it ends, muliply the
> highest number by (.4). This will give you a good estimate of the most
> fuel-efficient speed (RPM) of the engine. Then take it out on the highway
> and see if the engine will run at that speed (RPM) at the speed you normally
> drive on the highway. If the engine RPM is too high OR too LOW at your
> normal highway speed, you might want to find a car with an engine that is
> better designed for the way YOU drive. That is, if your goal is to find a
> fuel-efficient vehicle. -Dave
>
> On a side note, it's easy to understand why manual tranny vehicles are more
> fuel-efficient for use in City driving.


My experience is exactly the opposite of yours. The lower the rpms are
kept the better the milage. Many manual transmission drivers don't
know when to shift and usually blow any opportunity to maximize milage.


> You have to get the engine RPMs UP
> to maximize fuel economy, and you just can't do that at low speeds in a
> slush-box.


 




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