"Jack rabbit" starts give best mileage

Steve wrote:
> 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.

3500 RPM isn't short shifting? <G>

Seriously, I get your point, but my car (Porsche 944) really needs to
be wound up to at least 3500 RPM to keep from lugging the engine in the
next gear. The engine is perceptibly unhappy under power at anything
under 2000 RPM. Of course, it probably has half the number of
cylinders of the engine you're likely imagining.

nate

N8N wrote:


>
> 3500 RPM isn't short shifting? <G>

Well, it is in a Mazda RX-8.... ;-)
>
> Seriously, I get your point, but my car (Porsche 944) really needs to
> be wound up to at least 3500 RPM to keep from lugging the engine in the
> next gear. The engine is perceptibly unhappy under power at anything
> under 2000 RPM.

And the 944 really wasn't designed with economy in mind.

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?
>

like the others have said, your engine is most efficient at about 80%
throttle. The catch is where can you drive like this? In rush hour?

I have an old (90's) C&D or Motor Trend where they tested a Honda Civic
"mileage special" and found the trick was to almost floor it to about 65
and then shut it off and coast to about 35. Rinse, repeat. They got
great gas mileage, and I'm sure about as many raised middle fingers to
match.

Steve wrote:
> 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.


I think folks who say to avoid jack rabbit starts are trying to say,
keep rpms down. Unfortunately, so many interpret this as keeping
throttle opening small, which of course is not the optimum.

N8N wrote:.
>
> Seriously, I get your point, but my car (Porsche 944) really needs to
> be wound up to at least 3500 RPM to keep from lugging the engine in the
> next gear. The engine is perceptibly unhappy under power at anything
> under 2000 RPM. Of course, it probably has half the number of
> cylinders of the engine you're likely imagining.
>
> nate
>

Depends on what you mean by "unhappy". Many cars that are fuel injected
can be lugged very nicely. Not much acceleration, but hey, to save gas
it is worth it.

When the first energy crisis hit, in early 70s, one of the European mfgs
ran a great test. They drove a car on a test track for many tens of
thousands of miles, seriously lugging it (I think it was VW). They then
tore down engine and measured bearing and journal wear. The reason for
this was to explore the old idea that lugging an engine was too hard on
bearings. They found no excessive wear. The idea was to prove that
modern bearing materials, crank materials and machining, and more
importantly, modern lubricants, have solved this problem.

I have read some experts who say, as long as car accelerates smoothly,
without bucking or detonating, it is okay, and that such lugging does
improve milage.

ray wrote:
..
>
> like the others have said, your engine is most efficient at about 80%
> throttle. The catch is where can you drive like this? In rush hour?
>
> I have an old (90's) C&D or Motor Trend where they tested a Honda Civic
> "mileage special" and found the trick was to almost floor it to about 65
> and then shut it off and coast to about 35. Rinse, repeat. They got
> great gas mileage, and I'm sure about as many raised middle fingers to
> match.

No- as long as you keep it in higher gears full or 80% throttle does not
mean fast driving. I short shift all the time now in my stick shift
Neon, and I do get great milage. Yeah, it takes longer to accelerate,
but hey, I am retired now and not in a hurry anymore anyway.

BTW, many of these built for purpose cars in the super high milage
contests do not even HAVE a throttle. Again, like you say, they
accelerate to a precalculated speed, shut off engine and disengage it,
coast to another precalculated speed, re-engage and restart engine and
repeat process.

I use that method which avoids lugging the engine at all costs and keep
the rpm around 2300 and get a sweet 23 mpg or 11L/100km in my Jeep CJ7
with a 258 ci straight six.

It has the aerodynamics of a brick and oversized tires.

I have even forgotten I have a 5th gear because at 65 mph, fifth drops
to 1750 rpm. That lugs the engine trying to push the brick shape
through the wind... I need to use way too much gas pedal to keep the
speed even.

I own 2 Jeeps and do a lot of regular highway trips to the same place,
so get to try different methods for mileage. Going easy always in a top
gear costs me over 5 mpg easy.

Mike
86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00
88 Cherokee 235 BFG AT's
Canadian Off Road Trips Photos: Non members can still view!
Aug./05 http://www.imagestation.com/album/in...?id=2120343242
(More Off Road album links at bottom of the view page)

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?

Don Stauffer wrote:

> N8N wrote:.
>
>>
>> Seriously, I get your point, but my car (Porsche 944) really needs to
>> be wound up to at least 3500 RPM to keep from lugging the engine in the
>> next gear. The engine is perceptibly unhappy under power at anything
>> under 2000 RPM. Of course, it probably has half the number of
>> cylinders of the engine you're likely imagining.
>>
>> nate
>>
>
> Depends on what you mean by "unhappy". Many cars that are fuel injected
> can be lugged very nicely. Not much acceleration, but hey, to save gas
> it is worth it.
>
> When the first energy crisis hit, in early 70s, one of the European mfgs
> ran a great test. They drove a car on a test track for many tens of
> thousands of miles, seriously lugging it (I think it was VW). They then
> tore down engine and measured bearing and journal wear. The reason for
> this was to explore the old idea that lugging an engine was too hard on
> bearings. They found no excessive wear. The idea was to prove that
> modern bearing materials, crank materials and machining, and more
> importantly, modern lubricants, have solved this problem.

It goes back further than that. During WWII, getting enough range out of
the fighters in order to escort the big bombers was an ongoing problem.
Its often reported that the P-51's long range was "the answer," but even
it had to be flown the right way to get that range. Charles Lindbergh
was actually one of the first to demonstrate good long-range flying
tactics and engine management to the Army Air Corps. His method was to
set the propellor control to a *very* low RPM, and then open the
throttle up wide to increase boost to the maximum allowable. It worked
great, and that was in effect "lugging" the engine. The only procedural
change that had to be made was to "clear" the engine briefly at high RPM
every so often to prevent the high lead content from fouling the plugs.
The engineers at Rolls-Royce and Pratt&Whitney both expected a lot more
bearing wear, but it never turned up. As long as the oil film isn't
disturbed, the difference between "lugging" and revving and engine isn't
all that great, because the inertial forces on the bearings go way up at
higher RPM anyway.

Don Stauffer wrote:
> N8N wrote:.
> >
> > Seriously, I get your point, but my car (Porsche 944) really needs to
> > be wound up to at least 3500 RPM to keep from lugging the engine in the
> > next gear. The engine is perceptibly unhappy under power at anything
> > under 2000 RPM. Of course, it probably has half the number of
> > cylinders of the engine you're likely imagining.
> >
> > nate
> >
>
> Depends on what you mean by "unhappy". Many cars that are fuel injected
> can be lugged very nicely. Not much acceleration, but hey, to save gas
> it is worth it.
>

Not this one. It still makes torque under 2K RPM, but it vibrates
badly and feels and sounds like it's being abused. Over 2K, smooth as
a baby's posterior. (well, except for the ongoing saga of the mystery
drivetrain vibration at 80 MPH, but that's a recent development. It
has never run well under 2K.)

> When the first energy crisis hit, in early 70s, one of the European mfgs
> ran a great test. They drove a car on a test track for many tens of
> thousands of miles, seriously lugging it (I think it was VW). They then
> tore down engine and measured bearing and journal wear. The reason for
> this was to explore the old idea that lugging an engine was too hard on
> bearings. They found no excessive wear. The idea was to prove that
> modern bearing materials, crank materials and machining, and more
> importantly, modern lubricants, have solved this problem.
>

Since the 944 has a known issue with rod bearings, I'll take my
chances... Actually VW had an issue with the rod bearings in the G60
engine as well, but that was a different issue; the 944 crank is
drilled wrong so that at high RPM the centrifugal force is keeping the
oil in the crank. The VW issue was simply a "new and improved" bearing
material that wasn't as good as the old one. In either case a rebuild
by a knowledgeable shop should fix the problem (but to my knowledge the
944 has never been rebuilt, and I don't feel like doing it any time
soon. I did roll new shells in SWMBO's G60 and the annoying noise went
away.)

> I have read some experts who say, as long as car accelerates smoothly,
> without bucking or detonating, it is okay, and that such lugging does
> improve milage.

It does *not* accelerate smoothly, and I couldn't tell you whether or
not it's detonating as every piece of trim on the car is buzzing like a
nest full of angry hornets...

I believe the owner's manual specifically suggests keeping the engine
over 2K as well.

nate

Don Stauffer wrote:
> ray wrote:
> .
>
>>
>> like the others have said, your engine is most efficient at about 80%
>> throttle. The catch is where can you drive like this? In rush hour?
>>
>> I have an old (90's) C&D or Motor Trend where they tested a Honda
>> Civic "mileage special" and found the trick was to almost floor it to
>> about 65 and then shut it off and coast to about 35. Rinse, repeat.
>> They got great gas mileage, and I'm sure about as many raised middle
>> fingers to match.
>
>
> No- as long as you keep it in higher gears full or 80% throttle does not
> mean fast driving. I short shift all the time now in my stick shift
> Neon, and I do get great milage. Yeah, it takes longer to accelerate,
> but hey, I am retired now and not in a hurry anymore anyway.
>
I was referring to the driving style for max fuel economy. When you're
puttering along at 5mph at bumper to bumper traffic, you can pretty much
watch the gas gauge on my truck plummet. (I'm not complaining, it's a
truck that's used like one, and I usually don't commute to work in it...)

> BTW, many of these built for purpose cars in the super high milage
> contests do not even HAVE a throttle. Again, like you say, they
> accelerate to a precalculated speed, shut off engine and disengage it,
> coast to another precalculated speed, re-engage and restart engine and
> repeat process.

And that's what I meant about middle fingers. Try driving like THAT on
an interstate and see how happy everyone else is as you zip up to 80 and
then coast back down to 30....

Ray