Why do cars with automatic transmissions have tachometers?

Alan Johnson wrote:

>>Curious as hell about this.
>
>
> Because it's nice to know when your getting ready to "redline" when you have
> your foot all the way to the floor.
>
>
Yeah, but an electronic auto trans will force a shift before "redline"
no matter what you do ANYWAY. Hell- go out and get in many new cars (one
I know for certain- Ford Expedition) and floor it in neutral and see
what happens. OK, I'll tell you- the throttle-by-wire revs it up to
about 3600 RPM and it sits there happy as a clam.. Some aren't quite as
graceful- a GM 3800 I had as a rental didn't have throttle-by-wire, so
it just cuts the fuel and the engine will "surge" around 4000 RPM.

A tach would have been more useful in a 1970 car with an automatic than
in a 2005 car with an automatic (not useful at ALL). Muscle cars
frequently got tachs with automatics, because they do come in useful
when manually shifting an auto. But back then, tachs went primarily in
stick-shift cars where they REALLY belong.

"Alan Johnson" > wrote in message
...
> "Arif Khokar" > wrote in message
> news
>>
>> We read what you had to say the first two times, <idiot.
>
> At least I don't worship some child molesting prophet named Mohammed
>
>

What's the name of your child molesting prophet you worship? Alan Johnson?

Probably cheaper to produce one instrument cluster for everybody in
models that can be bought with either an automatic or a stick, as is
often the case.

At the higher end of the market, it's also useful for the growing
number of automatics that you can optionally run by hand.

With LCD's getting cheaper and faster and brighter, I wonder when (at
least in performance cars) they're going to start superimposing a
virtual indicator on a display of power and torque curves, dyno-tested
for at least the engine and driveline package if not the individual
car.

Cheers,
--Joe
Owner of two tach-less automatics, two tach-less manuals, and, until
recently, one automatic with tach

Ad absurdum per aspera wrote:
>
> Probably cheaper to produce one instrument cluster for everybody in
> models that can be bought with either an automatic or a stick, as is
> often the case.
>
> At the higher end of the market, it's also useful for the growing
> number of automatics that you can optionally run by hand.
>
> With LCD's getting cheaper and faster and brighter, I wonder when (at
> least in performance cars) they're going to start superimposing a
> virtual indicator on a display of power and torque curves, dyno-tested
> for at least the engine and driveline package if not the individual
> car.
>
> Cheers,
> --Joe
> Owner of two tach-less automatics, two tach-less manuals, and, until
> recently, one automatic with tach

The last time I looked (several years ago) it was rather difficult to
find a manual option on most anything but the "sport" models.

I'm waiting for them to do a fully configurable LCD based dashboard
where you can load a "skin" for the instrument style and layout you
prefer. Pretty simple to do and flexible enough that the same guts could
be used on nearly every vehicle in the lineup, just changing the display
bezel for the particular vehicle.

Pete C.

Ad absurdum per aspera wrote:

> Probably cheaper to produce one instrument cluster for everybody in
> models that can be bought with either an automatic or a stick, as is
> often the case.

But given that the bulk of models sold are ONLY available with an
automatic..... its just bling-bling, as another poster already said.

And frankly, you have to be a pretty incompetent driver to actually
*use* the tach to decide when to shift on a routine basis. I always
shift by engine sound, although I will check the tach a time or two the
first time I drive a stick car that I'm not accustomed to (insert
obligatory story about first-time RX-7 driving experience here...). Even
drag racers don't use the tach- they generally use a shift-light
triggered by a digital tach, not the tach itself.

Pete C. wrote:

> I'm waiting for them to do a fully configurable LCD based dashboard
> where you can load a "skin" for the instrument style and layout you
> prefer. Pretty simple to do and flexible enough that the same guts could
> be used on nearly every vehicle in the lineup, just changing the display
> bezel for the particular vehicle.
>

What is critical in that case is that they equip the enigne with
calibrated sensors that can provide real data to the "skin" software.
Its no good having a super-duper skin that shows oil pressure, amps,
coolant temp, oil temp, and transmission temp if all you have is a
voltage probe, an oil pressure idiot light switch, and a 3-state coolant
temp idiot light switch :-/

"TLittle" > wrote in message
...
>
> Curious as hell about this.
>
> Was taught by the parents to drive in general on a 65 Chevy with an auto
> tranny.
>
> Then the parents taught me to drive a stick on a 67 Dodge 3-on-the-tree
> Dart.
>
> None of them had a tach.
>
> I presently drive a 5-speed sedan with a tach but I never notice it as I
> shift based on what my ear (and obviously "experience") tells me.
>
> Obviously I understand what the tachometer is for but.....my friends
> and relatives various vehicles are universally automatics and almost all
> - depending of course on the options they took - have tachometers on
> them.
>
> What's the point of the tachometer on a vehicle with an automatic
> transmission?
>
> Seriously, based on my understanding of WHY the tachometer exists, why
> are there tachs on vehicles with automatic transmissions?
>

Well, my feelings on this subject are mixed. Your Otto Cycle engine in your
auto-tranny equipped vehicle will be most fuel-efficient in the powerband
range of certain RPMs. SO, if you know how to find that fuel-efficient
point*, you can try to run your engine at a certain RPM to maximize fuel
economy. Obviously, if you don't have a tachometer, you can't do this.
But then again, for some vehicles, it doesn't matter whether there is a
tachometer or not. Take my wife's car for example. I ran the numbers to
determine where it would be most fuel-efficient. It should be most
fuel-efficient from 3200-3600RPM. Problem is, I discovered that 75MPH was
somewhere below 3000RPM with overdrive enabled. So to get to 3200 minimum
would be ummmmm, not a good idea. With overdrive OFF, RPM jumped up above
4000. Basically, I discovered that my wife's car was geared incorrectly.
So in the case of that specific vehicle, a tachometer is useless. I can't
drive it legally AND keep the engine in the powerband. -Dave

* Various sources say 40% of redline, 40-45% of redline, or 60% of RPM at
maximum horsepower RPM. In the case of my wife's car, this means it will be
most fuel-efficient in a range from 3200-3600RPM.

Dave C. wrote:

> Well, my feelings on this subject are mixed. Your Otto Cycle engine in your
> auto-tranny equipped vehicle will be most fuel-efficient in the powerband
> range of certain RPMs.

> * Various sources say 40% of redline, 40-45% of redline, or 60% of RPM at
> maximum horsepower RPM. In the case of my wife's car, this means it will be
> most fuel-efficient in a range from 3200-3600RPM.
>
>

The implicit assumption in that sort of "rule of thumb" is that you NEED
all the power that the engine can produce. Its a good rule for, say, an
industrial generator engine or irrigation well engine that is expected
to run most of its life at maximum load Cars cruising down the highway
NEVER need all the power that the engine can produce, so they play by a
different set of rules.

Cars have engines that are capable of producing several hundred
horsepower in order to achieve good acceleration, but when cruising
steady-state they require a few tens of horsepower at most. So how do
you run a 300-horsepower engine at an output of 35 horsepower and do it
most efficiently? It turns out that the best way is to "lug" the engine.
IOW, run it so that its way down on the lower RPM side of its torque
curve, but do so with the throttle held relatively wide open at that low
RPM. that's why overdrive works so well. Lugging the engine does a few
things simultaneously:

1) it raises the starting pressure in the cylinders by decreasing the
amount of manifold vacuum. Higher pre-combustion pressure -> more
efficient combustion

2) it allows the spark timing to be retarded. Spark "advance" is really
only needed because when you operate an engine against very high
manifold vacuum (throttle mostly closed) you are LOWERING the starting
cylinder pressure so much that the burn rate is reduced, so that in turn
you need to start the burn very early in order to finish it before the
exhaust valve opens. The downside to spark advance is that a portion of
the combustion cycle is working against the movement of the piston. If
you lug the engine with the throttle open, the pressure is high so the
burn rate is high, and you don't need much spark advance, and therefore
less of the combustion cycle is spent working against the crankshaft.

The proof is in the pudding. If you look at that (otherwise useless :-)
) tach in any modern car, you'll see that when cruising in O/D at 70
mph, the engine is only turning 2000 RPM or even less. And this is
usually with engines that have redlines of 5000-7000 RPM and torque
peaks up at 3000-4000 RPM. By any rule of thumb relating to their
MAXUMUM power, the engines are being operated way, way off optimum. But
for the NEEDED power, they're right in the sweet spot.

>
> The implicit assumption in that sort of "rule of thumb" is that you NEED
> all the power that the engine can produce. Its a good rule for, say, an
> industrial generator engine or irrigation well engine that is expected
> to run most of its life at maximum load Cars cruising down the highway
> NEVER need all the power that the engine can produce, so they play by a
> different set of rules.


Ummmmm, no. The most fuel-efficient RPM of an Otto Cycle engine is well
below the RPM that will produce the most horsepower. "Lugging" the engine
is not fuel-efficient, as the engine RPM is too low. At low RPM, too much
energy is wasted as heat. (a lesser percentage of energy is converted to
kinetic energy while the engine is lugging) Overdrive is meant to improve
fuel efficiency at higher speeds. (OVER drive speeds) It can't do this by
"lugging" the engine. It does this by reducing the RPM into a more
fuel-efficient range while still NOT lugging it. Good example: My car is
at 3500RPM/43%(redline) and .6 (of max horsepower RPM) at 78MPH in
OVERdrive, where it easily gets 42MPG. That's just the way it was designed.
If I downshift it to drive, the most fuel-efficient
3500RPM/43%(redline)/.6(of max horsepower RPM) equates to exactly 55MPH. I
have no idea how fuel-efficient it is at 55MPH in drive, as there is nowhere
near me where I could safely test that. Even the highways posted at 55MPH
have average traffic speeds well in excess of 70MPH, and it's just not a
good idea to hold a steady speed 20MPH below the flow in any lane. BUT, I
suspect my vehicle would be quite fuel-efficient at 55MPH, as that is how it
was designed. The most fuel-efficient RPM of my engine happens to fall
right AT 55MPH, when the car is in drive gear. No way was THAT a
coincidence, so it had to be by design.

Yet since I learned how to find the most fuel-efficient RPM of an engine, I
have discovered that a lot of vehicles are geared incorrectly. My wife's
car is one of them. Her car should be most fuel-efficient at ~ 3400RPM, yet
it is hard to get her engine to even hit 3000 (without driving fast enough
to lose your license), which is the point of maximum torque. A lot of cars
I've driven have been similarly geared incorrectly. In one extreme example,
I drove a chevy with a 4-cylinder engine that rarely hit 2000RPM. It was
horrible, as far as fuel economy goes. Now I know why.

If "lugging" the engine was such a good idea, I'd expect to see more 7 or
8-speed trannies, both automatic and manual. That is, there should be more
"over"drive gears to keep the engine running really slow. But if the car is
geared correctly, only one overdrive gear is needed. This allows the car
engine to max out the EPA estimate at 55MPH (where it is tested, in drive
gear), and yet still get decent fuel economy closer to speeds that the car
is actually driven. Thus, more overdrive gears would be redundant on a
street-legal car. -Dave

Steve wrote:
>
> Pete C. wrote:
>
> > I'm waiting for them to do a fully configurable LCD based dashboard
> > where you can load a "skin" for the instrument style and layout you
> > prefer. Pretty simple to do and flexible enough that the same guts could
> > be used on nearly every vehicle in the lineup, just changing the display
> > bezel for the particular vehicle.
> >
>
> What is critical in that case is that they equip the enigne with
> calibrated sensors that can provide real data to the "skin" software.
> Its no good having a super-duper skin that shows oil pressure, amps,
> coolant temp, oil temp, and transmission temp if all you have is a
> voltage probe, an oil pressure idiot light switch, and a 3-state coolant
> temp idiot light switch :-/

In a lot of cases the computer has the real info, but they chose to just
stick a cheap idiot light on the output.

Pete C.