Re: Why you should never buy a car without a tachometer


Ted B. wrote:
> > What nonsense. I know you want to impress yourself and others with
> > your knowlege of automobile terminology by salting your messages with
> > "Otto Cycle Engine". Lets stick with the basics here. You are
> > asserting that a car that runs at 40% of the RPM indicated the tach
> > will get the best gas milage.
>
> No, I'm not asserting it. I'm simply repeating facts as stated by people
> who know a lot more about the Otto Cycle engine than either one of us do.
> There's a HUGE difference, there.
>


I think you missed the part about me writing engine simulation software
in our earlier exchange. If you'd like to discuss the Otto Cycle and
what that precisely means and entails, and how it might or might not
have any bearing on what you currently believe about engines and fuel
economoy, feel free to ask.


> >For many cars on the road today that
> > would would be well in excess of 90mph.
>
> In some cases, YES, that's possible. I haven't experienced it myself, but
> have no reason to doubt that it could happen. For my current car, the most
> fuel-efficient speed is 3500RPM, 43% of redline, and 78MPH (take your pick,
> all are correct). Several cars I've owned have been most efficient
> somewhere above 70MPH. To think that a car MIGHT be most fuel-efficient
> above 90MPH is not too hard to imagine. That's only about a 13MPH
> difference from my own car, and it's not exactly a sports car.
>


If I'm not mistaken, what you're stating is that one can multiply the
redline by around 40% and take a look at the speedometer when
travelling at that engine speed in top gear. Whatever speed you're
running at that point is going to give the optimum fuel mileage,
correct? And the reason for this is because of the scientific fact
that an Otto Cycle engine attains peak fuel economy at that engine
speed (40% of redline). (Note that that figure is most likely
referring to engine speed and fuel conversion efficiency, not that in
combination with vehicle speed and fuel mileage.)

Consider the consequences of that if it is or were indeed true. This
literally means that if you swap out your transmission for a direct
drive unit (1:1 ratio), and assuming your car came stock with a 3:1
differential ratio, your "40% of redline" speed has now tripled. Your
optimum speed was 78mph before, which I still maintain is rather
coincidentally tied to your 43% value.

With the new driveline unit your optimum fuel mileage now occurs at
234mph if what you're saying is correct. Congratulations, the fuel
crisis has now been put off for quite some time. ;-) All we needed
were higher gears all along, go figure.

This reminds me of a time in high school when a buddy wanted to put a
2.32 rear end ratio in his Camaro so he could cruise along at 2000rpm
in excess of 100mph. Unfortunately he did not realize that the engine
did not produce enough power at 2000 rpm to allow that speed to be
reached, even at full throttle. But I digress.

Anyway, this should continue to be a fun debate so long as nobody here
is the sort of person who would insist his MD was wrong about his
"broken leg" diagnosis. ;-)

By the way, the link you posted for your source of information did not
work (probably too long and it got abbreviated when looking here
through Google). Could you please post it on two lines or with a space
half way in between so I can copy/paste it together and view the site?


Thanks,
Todd Wasson
Performance Simulations
http://www.PerformanceSimulations.com

.

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