Why you should never buy a car without a tachometer

>
> You're asserting it, calling it a "fact" without a shred of evidentiary
> support, and telling us we should believe you because you heard it from
> really knowledgeable people.
>
> You will have to do a great deal better than that if you wish to be taken
> seriously. Just saying "Otto cycle! Otto cycle! Otto cycle! Otto cycle!"
> isn't going to cut it.

Hey, I'm accepting it as a fact, as nobody has posted any evidence to
dispute it, just more mantras of "slow down to save fuel" and useless
information such as "BUT MY CAR GETS 55MPG at 40MPH". (uhhh . . . yes, it
might . . . now what's your point?) -Dave

> Horseapples. There are so many design, implementation and operational
> factors that go into fuel efficiency that picking a random percentage of
> "redline RPM" (for which there is absolutely zero standard definition
> other than "wherever the carmaker prints the red line on the tach, if so
> equipped") and calling it the most efficient speed is grossly untenable.
>

So you think that other aspects of car design can change the physics of the
Otto Cycle engine? Do tell . . . -Dave

> Dave, this 40% of redline RPM efficiency value you read about refers to
> "fuel conversion efficiency," not "fuel mileage." They are two vastly
> different things and are not equal or directly representative of each
> other by any stretch of the imagination. In fact they have different
> units.
>
> All that the "fuel conversion efficiency" value means is that for every
> gallon of fuel you're burning, you're getting the most possible power
> out of the engine at that particular RPM.

And meanwhile, the car is? . . . (class, help him out a little here)

Nobody?

Moooooooooooooving. Eating up miles. -Dave

"Dave C." > wrote:
>> > The Otto Cycle internal combustion engine is most fuel-efficient near
>40% of
>> > redline.
>>
>> It isn't. My car will probably get around 40 mpg at 40 to 45 mph. At
>> 60 to 65 mph it gets around 26 to 27 mpg. At 75 to 80 mph it gets
>> around 23 to 24 mpg.
>>
>> My car has both instantaneous and average mpg readings with which to
>> verify these figures.
>
>OK, again, yet someone else who is posting data points based on MPH. Where
>is the 40% RPM speed, and what does it actually DO there (as opposed to what
>a car computer estimates) -Dave

I posted my car's fuel economy at 40% of redline: 20-21 mpg.

My engine computer's estimate is *excellent*. It is based
on measurement of engine airflow, exhaust mixture, and on
known (and learned) injector flow behavior. I have
correlated my trip computer's fuel usage estimate to pump
fills on long road trips, and found it to remain within 1%
of cumulative pump readings over thousands of miles.

What's *your* measurement methodology, Dave?
--
Chuck Tomlinson

Dave C. wrote:
> > > The Otto Cycle internal combustion engine is most fuel-efficient near
> > > 40% of redline.

> Arif Khokar wrote:
> > It isn't. My car will probably get around 40 mpg at 40 to 45 mph. At
> > 60 to 65 mph it gets around 26 to 27 mpg. At 75 to 80 mph it gets
> > around 23 to 24 mpg.
> >
> > My car has both instantaneous and average mpg readings with which to
> > verify these figures.

> OK, again, yet someone else who is posting data points based on MPH. Where
> is the 40% RPM speed, and what does it actually DO there (as opposed to what
> a car computer estimates)

The car's engine's redline is 7000 rpm. Forty percent of that is 2800
rpm. At 2800 rpm in top gear, the car's speed is about 70 mph. The
car's fuel efficiency is clearly not as high at that speed as it would
be when the engine is running at around 1800 rpm which corresponds to
around 45 mph.

"Dave C." > wrote in message
link.net...
>> >
> http://home.earthlink.net/~graham1/M...stion.htmQuote
>> > from above:" A modern Otto cycle engine tends to be most efficient
>> > at40%
>> > to 45% of its"red-line" r.p.m.
>>
>> Ah. You read it on the interweb, therefore it's true.
>>
>> Just so we're clear on the source you're citing as "authoritative".
>>
>
> Well I've read it several places. I have no reason to doubt that it's
> true,
> when it perfectly agrees with what I have observed over the past few
> decades. It seems to be the speed kills crowd that is most determined to
> spin this into something *I* claim is true, as opposed to facts as
> documented by people more knowledgeable than me. -Dave

Only one poster replying to you really belongs in the
"speed kills" crowd, so that's not it.

Perhaps it is because people who have MPG calculators
in the dash see different data, they disbelieve you.

Bernard
>
>

Bernard Farquart wrote:
> "Dave C." > wrote in message
> link.net...
> >> >
> > http://home.earthlink.net/~graham1/M...stion.htmQuote
> >> > from above:" A modern Otto cycle engine tends to be most efficient
> >> > at40%
> >> > to 45% of its"red-line" r.p.m.
> >>
> >> Ah. You read it on the interweb, therefore it's true.
> >>
> >> Just so we're clear on the source you're citing as "authoritative".
> >>
> >
> > Well I've read it several places. I have no reason to doubt that it's
> > true,
> > when it perfectly agrees with what I have observed over the past few
> > decades. It seems to be the speed kills crowd that is most determined to
> > spin this into something *I* claim is true, as opposed to facts as
> > documented by people more knowledgeable than me. -Dave
>
> Only one poster replying to you really belongs in the
> "speed kills" crowd, so that's not it.
>
> Perhaps it is because people who have MPG calculators
> in the dash see different data, they disbelieve you.
>

This whole thread is making me wish that there were a small,
inexpensive yet accurate flow meter that could be spliced into a 5/16"
or 3/8" fuel line, for those of us who have vehicles without computers
from which to pick injector duty cycle, RPM, and road speed info.

c'mon aftermarket peep holes, get crackin'!

nate

In article ws.net>,
Ted B. > wrote:
>
>The Otto Cycle engine is most fuel efficient at 40-45% of redline RPM.
>There is no way around that, as that's just how the Otto Cycle engine works.

Do you really think repeating that assertion over and over again makes
it any more credible?
--
There's no such thing as a free lunch, but certain accounting practices can
result in a fully-depreciated one.

In article .com>, N8N wrote:

> This whole thread is making me wish that there were a small,
> inexpensive yet accurate flow meter that could be spliced into a 5/16"
> or 3/8" fuel line, for those of us who have vehicles without computers
> from which to pick injector duty cycle, RPM, and road speed info.

Hmm.... I wonder if the flow meters in the product I work on can handle
gasoline... they aren't cheap... but I can probably get my hands on some
that are no longer fit for duty in the machine.

Around 9/14/2005 5:18 AM, Ted B. wrote:

>>Wish the world of fuel efficiency were that simple as applying a
>>constant to rpms.
>>
>>At 60mph the engine is turning 2000 rpm. What's the next guess.....
>
> But it is indeed just that simple!!! Again, you are guessing, and you need
> to collect more facts before any conclusion can be drawn. If your engine is
> really turning 2000RPM at 60MPH, and seems to be relatively fuel-efficient
> at that speed, my best guess would be that it would be even MORE
> fuel-efficient at a slightly higher speed. That's my guess, as I really
> doubt any engine would have a 40% speed of 2000RPM. For that to be true,
> the engine would need to have a max speed of 5000RPM, and the tachometer
> would likely be marked to discourage you from going beyond 4000RPM. That's
> really slow.

<raises hand>

As far as I have been able to tell from research on and offline, my
Mustang has an approximate redline of 4100-4200 RPMs (though the tach
reads up to 6000 and has no marked redline; they used the same tach for
all the various engines), but I can't remember a time when I took it
above 3700 RPMs (that's ~40 MPH in first). My best mileage comes at
around 2400 RPMs and 64 MPH, about where the engine has it's peak
torque. That's 57-58% of redline, if my redline estimate isn't far off.

Bottom line, there's no single number you can point to and say "You'll
get the best mileage at X!!" (where X is speed, RPMs, or what have you)
for every car.

(FWIW, I track my gas mileage at every fillup with a program on my Zire.)


--
~/Garth |"I believe that it is better to tell the truth than a lie.
Almgren | I believe it is better to be free than to be a slave.
******* | And I believe it is better to know than to be ignorant."
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