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"Jack rabbit" starts give best mileage



 
 
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  #1  
Old December 4th 05, 05:01 AM posted to rec.autos.tech
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Default "Jack rabbit" starts give best mileage

I was reading in the Bosch Gasoline Engine Management book tips for
improving fuel efficiency, and was surprised to see wide throttle
acceleration recommended, at least up to 2000 RPM, when you are
supposed to change gears.

Exactly the opposite of what I grew up hearing. Anyone know the
thinking (if any) behind the old advice?

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  #2  
Old December 4th 05, 03:37 PM posted to rec.autos.tech
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Default "Jack rabbit" starts give best mileage

Yea, I think you're nuts and probably are misinterpreting things.

  #3  
Old December 4th 05, 04:51 PM posted to rec.autos.tech
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Default "Jack rabbit" starts give best mileage

How else would you interpret

"Increasing fuel economy...

Accelerate at wide throttle openings and low engine speeds, upshift at
2000 RPM"?

  #4  
Old December 4th 05, 05:17 PM posted to rec.autos.tech
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Default "Jack rabbit" starts give best mileage


Wide open throttle reduces pumping losses. Shifting at 2000 RPM
is hardly a 'jack rabbit' start.

While this technique is efficient, it can only be done in a manual
transmission car. (OK, there are a few manumatics that would allow it.)

Dan

(This account is not used for email.)


"Misterbeets" ) writes:
> I was reading in the Bosch Gasoline Engine Management book tips for
> improving fuel efficiency, and was surprised to see wide throttle
> acceleration recommended, at least up to 2000 RPM, when you are
> supposed to change gears.
>
> Exactly the opposite of what I grew up hearing. Anyone know the
> thinking (if any) behind the old advice?
>


  #5  
Old December 4th 05, 06:53 PM posted to rec.autos.tech
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Default "Jack rabbit" starts give best mileage


Misterbeets wrote:
> I was reading in the Bosch Gasoline Engine Management book tips for
> improving fuel efficiency, and was surprised to see wide throttle
> acceleration recommended, at least up to 2000 RPM, when you are
> supposed to change gears.
>
> Exactly the opposite of what I grew up hearing. Anyone know the
> thinking (if any) behind the old advice?


That old advice is too narrowly focused on maximizing fuel economy.
The minor savings in fuel will be offset by increased wear and tear on
the entire drivetrain. Not to mention the increased tendency of many
drivers practicing this style of fuel economy to suddenly rear-end
other cars.

  #6  
Old December 4th 05, 09:22 PM posted to rec.autos.tech
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Default "Jack rabbit" starts give best mileage

I didn't mean it as a personal attack there Mister.
The answer is in the details. Obviously, if you hung there at low
r.p.m. in each gear you would consume much fuel in a low gear. You do
want to move through the gears to get into a higher gear as soon as
practical. That's where the judgment and feel come in. You want to give
it some decent throttle opening, but not wide open throttle. And 2000
r.p.m. is not too much at all. Moderate throttle opening will get there
fairly fast. Of course it varies by the car and torque curves and such.
Experiment a little and check your mileage.

  #7  
Old December 5th 05, 01:41 AM posted to rec.autos.tech
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Default "Jack rabbit" starts give best mileage

In article .com> "Misterbeets" > writes:
$I was reading in the Bosch Gasoline Engine Management book tips for
$improving fuel efficiency, and was surprised to see wide throttle
$acceleration recommended, at least up to 2000 RPM, when you are
$supposed to change gears.
$
$Exactly the opposite of what I grew up hearing. Anyone know the
$thinking (if any) behind the old advice?

It's well known that engines run more efficiently with wider throttle
openings. The engine is responsible for sucking air in during the intake
stroke, and the less restrictive the plumbing through which the air has
to be drawn, the less energy is required to do the job. A mostly-closed
throttle is very restrictive, of course; a wide open one is not.

Engines also generally run more efficiently at lower revs; you lose
less energy to friction that way. Put it together, and Bosch's
suggestion is pretty obvious; wide throttle opening at low revs
gives better fuel economy.

One thing which works against this is that at or near wide open
throttle, engine management systems typically enrich the air/fuel mixture,
which generates more power but wastes fuel. So in practice, you
don' t want your pedal all the way down if you want fuel efficiency.

As for why lots of "experts" promote a gentle touch on the
accelerator, I'm guessing it's north americans' habit of buying
cars with automatic transmissions. Most automatics, unless they
give the driver the ability to select a specific gear (and the
positions below D on most automatics do not do this; they simply
restrict the number of gears from which the automatic can choose),
can't be made to do the right thing in this case. If you keep the
throttle fairly wide open, the automatic will let revs get fairly
high before upshifting, which is not what you want.

As well, the torque converter works against this, by letting engine
revs rise substantially higher than they would be if there were a
direct mechanical connection between the engine and the transmission.

If you drive standard, you can easily do what Bosch is suggesting.

One other thing to keep in mind is that high load at low RPMs
may reduce engine life.
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  #8  
Old December 5th 05, 04:53 AM posted to rec.autos.tech
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Default "Jack rabbit" starts give best mileage

That's a good point about automatics.

However I wonder whether fuel enrichment at WOT wastes fuel. Old days
with carbs, yes; with modern FI, it needn't, as you can add just enough
to create a corresponding increase in torque and power.

Your high load at low RPM comment reminds me of another statement in
the book:

"Drive in the highest possible gear; even at engine speeds below 2000
rpm it is possible to apply full throttle."

Cool.

Not that I'll be changing the way I drive. Just some interesting points.

  #9  
Old December 5th 05, 05:40 PM posted to rec.autos.tech
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Default "Jack rabbit" starts give best mileage

Misterbeets wrote:

> I was reading in the Bosch Gasoline Engine Management book tips for
> improving fuel efficiency, and was surprised to see wide throttle
> acceleration recommended, at least up to 2000 RPM, when you are
> supposed to change gears.
>
> Exactly the opposite of what I grew up hearing. Anyone know the
> thinking (if any) behind the old advice?
>



That's not "jack rabbit" starting if you never get above 2000 RPM.
That's "short-shifting" or "lugging," and yes it is more efficient. If
you've got a car with a pretty powerful engine, you can't really do this
in first gear. But short-shifting at WOT is more efficient than greasing
it up to 3500+ RPM at part throttle.
  #10  
Old December 5th 05, 05:47 PM posted to rec.autos.tech
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Default "Jack rabbit" starts give best mileage

Misterbeets wrote:

> That's a good point about automatics.
>
> However I wonder whether fuel enrichment at WOT wastes fuel. Old days
> with carbs, yes; with modern FI, it needn't, as you can add just enough
> to create a corresponding increase in torque and power.
>


It doesn't even necessarily waste fuel with carbs, if the carb is
properly tuned. WOT at low RPM does require a richer mixture (up to
slightly richer than stoichiometric), but it also requires (allows, in
fact) less ignition advance so there's less time that combustin pressure
is working against the piston's motion prior to TDC.
 




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