electronic throttle

On 3/6/2010 6:26 PM, Bill Putney wrote:
> jim beam wrote:
>> On 03/06/2010 11:48 AM, Ed Pawlowski wrote:
>
>>> ...Emergencies do happen and a driver should be competent enough
>>> to shift into neutral.
>>
>> or stomp the brakes - which are three times more powerful than the
>> engine,...
>
> I don't necessarily disagree with the rest of your post, but that part
> of your post is definitely incorrect. Have you ever played with your
> power brakes while simultaneously pressing the accelerator? Anything
> more than one or two initial stabs at the brakes depletes the vacuum
> stored in the booster, and with even slight power simultaneously being
> demanded of the engine, the vacuum is not enough to directly power the
> brakes, much less re-charge the vacuum in the booster.
>
> People don't believe that, but try it on your car: On a deserted road at
> highway speed, stab the brake pedal a couple of times while holding the
> gas pedal down a little bit to load the engine slightly (this works
> anywhere from slight to WOT throttle). I guarantee you (unless your
> brake booster gets its vacuum from something besides the intake vacuum -
> like a separate electric motor-driven vacuum pump) that after two or
> more stabs at the brake pedal, the braking power will be extremely low -
> so low that the engine will have no trouble overpowering the brakes. No
> vacuum in the booster essentially equals no brakes.
>
> Also, once the booster is depleted of vacuum during that experiment, the
> vacuum charge in the booster will remain depleted until a second or so
> after the throttle is released - IOW - deplete it and continue applying
> the throttle (again - doesn't have to be anywhere near WOT) for several
> seconds. Every once in a while, while still applying the throttle, try
> the brakes again. You will not have any effective braking until *after*
> you release the throttle.

I certainly believe you. You can get a feel for the amount of reserve
vacuum boost on your car by simply repeatedly pressing down on the
brakes without starting your engine. If your brakes are working
properly, you'll feel the pedal getting firmer until you'll only be able
to move the brake pedal a couple of inches of deflection. At that point,
you'll have used up all your vacuum reserve. I figure that you should be
able to get around 3 stabs at the brakes with mostly full boost. This
means you'll only get maybe two chances for full braking after the
initial attempt at braking. That's the breaks I guess. :-)

>
> I urge anyone who doesn't believe what I claim above to try it before
> commenting.
>

jim beam wrote:
> On 03/06/2010 08:26 PM, Bill Putney wrote:

>> I don't necessarily disagree with the rest of your post, but that part
>> of your post is definitely incorrect. Have you ever played with your
>> power brakes while simultaneously pressing the accelerator? Anything
>> more than one or two initial stabs at the brakes depletes the vacuum
>> stored in the booster, and with even slight power simultaneously being
>> demanded of the engine, the vacuum is not enough to directly power the
>> brakes, much less re-charge the vacuum in the booster.
>
> i have done this. with the engine off, the vacuum remains until the
> pedal is released - thus if you stomp the pedal and keep it there, you
> don't need to keep replenishing the vacuum. and you will stop the car.
> with the engine running, there is no vacuum issue, and the brakes are
> still powerful enough to stop the car. on my honda anyway.

I have real trouble believing that a large majority of people would, in
a sudden inadvertent acceleration situation, be content to press the
brakes one time and not try to pump them once or twice. After that, the
brakes will be almost totally ineffective because of loss of vacuum.

>> People don't believe that, but try it on your car: On a deserted road at
>> highway speed, stab the brake pedal a couple of times while holding the
>> gas pedal down a little bit to load the engine slightly (this works
>> anywhere from slight to WOT throttle). I guarantee you (unless your
>> brake booster gets its vacuum from something besides the intake vacuum -
>> like a separate electric motor-driven vacuum pump) that after two or
>> more stabs at the brake pedal, the braking power will be extremely low -
>> so low that the engine will have no trouble overpowering the brakes. No
>> vacuum in the booster essentially equals no brakes.
>
> with respect, i think you're confusing vacuum with fade...

No - I'm not. While you could certainly induce fade with a certain
prolonged script of usage of the brakes, what I'm talking about is true
over what I would say would be the real world typical scenario (before
the fade issue becomes real - which - yes - it would over a longer
period, but not likely if the 2 or 3 stabs had already occurred in the
relatively short period that I would expect). It is a fact that the
vacuum cannot recharge with almost no vacuum in the intake - it doesn't
recharge by magic. I guarantee you that after a third stab of the
brakes on an engine vacuum-driven power brake car, the brakes will loose
the fight with the engine - fade has nothing to do with that over the
first few seconds that we would be talking about (during which the first
2 or 3 stabs would occur real world).

i've
> experienced that too, one particular time on a major hill in san
> francisco approaching a busy intersection. yes, it's scary stuff. but
> when i changed the pads on my civic from after-market to oem, all fade
> problems disappeared. even fully loaded, repeatedly decelerating from
> speed. [i learned my "honda oem is best" lesson that way.]
>
>
>>
>> Also, once the booster is depleted of vacuum during that experiment, the
>> vacuum charge in the booster will remain depleted until a second or so
>> after the throttle is released - IOW - deplete it and continue applying
>> the throttle (again - doesn't have to be anywhere near WOT) for several
>> seconds. Every once in a while, while still applying the throttle, try
>> the brakes again. You will not have any effective braking until *after*
>> you release the throttle.
>>
>> I urge anyone who doesn't believe what I claim above to try it before
>> commenting.
>
> i have. my results and comments are as above.

And both of us could design the script to create either end result we
wished. I submit that in the real world, most people would try pumping
the brakes in desperation if the sudden acceleration scenario actually
happened - the depleting stabs would occur before fade became a factor.

--
Bill Putney
(To reply by e-mail, replace the last letter of the alphabet in my
address with the letter 'x')

dsi1 wrote:

> I certainly believe you. You can get a feel for the amount of reserve
> vacuum boost on your car by simply repeatedly pressing down on the
> brakes without starting your engine. If your brakes are working
> properly, you'll feel the pedal getting firmer until you'll only be able
> to move the brake pedal a couple of inches of deflection. At that point,
> you'll have used up all your vacuum reserve. I figure that you should be
> able to get around 3 stabs at the brakes with mostly full boost. This
> means you'll only get maybe two chances for full braking after the
> initial attempt at braking. That's the breaks I guess. :-)

My recollection is that boost is noticeably diminished after the second
stab, greatly diminished by the third stab - engine overpowers brakes
for most common vehicles.

--
Bill Putney
(To reply by e-mail, replace the last letter of the alphabet in my
address with the letter 'x')

On 03/07/2010 08:10 AM, Bill Putney wrote:
> jim beam wrote:
>> On 03/06/2010 08:26 PM, Bill Putney wrote:
>
>>> I don't necessarily disagree with the rest of your post, but that part
>>> of your post is definitely incorrect. Have you ever played with your
>>> power brakes while simultaneously pressing the accelerator? Anything
>>> more than one or two initial stabs at the brakes depletes the vacuum
>>> stored in the booster, and with even slight power simultaneously being
>>> demanded of the engine, the vacuum is not enough to directly power the
>>> brakes, much less re-charge the vacuum in the booster.
>>
>> i have done this. with the engine off, the vacuum remains until the
>> pedal is released - thus if you stomp the pedal and keep it there, you
>> don't need to keep replenishing the vacuum. and you will stop the car.
>> with the engine running, there is no vacuum issue, and the brakes are
>> still powerful enough to stop the car. on my honda anyway.
>
> I have real trouble believing that a large majority of people would, in
> a sudden inadvertent acceleration situation, be content to press the
> brakes one time and not try to pump them once or twice.

why? if the car in front of you suddenly jams on their brakes, do you
apply then release your pedal? i think your answer is "no" - unless
you're skidding and know what cadence braking is. and if you know what
cadence braking is, you should know that to stop a car with the engine
on full throttle, you apply the brakes hard and quickly - you don't
monkey about with multiple brake applications that can cause excess
heating and fade.


> After that, the
> brakes will be almost totally ineffective because of loss of vacuum.

no. fade maybe, but vacuum is always present if the engine is running.
even if it's not, you still have vacuum reserve for three applications.


>
>>> People don't believe that, but try it on your car: On a deserted road at
>>> highway speed, stab the brake pedal a couple of times while holding the
>>> gas pedal down a little bit to load the engine slightly (this works
>>> anywhere from slight to WOT throttle). I guarantee you (unless your
>>> brake booster gets its vacuum from something besides the intake vacuum -
>>> like a separate electric motor-driven vacuum pump) that after two or
>>> more stabs at the brake pedal, the braking power will be extremely low -
>>> so low that the engine will have no trouble overpowering the brakes. No
>>> vacuum in the booster essentially equals no brakes.
>>
>> with respect, i think you're confusing vacuum with fade...
>
> No - I'm not. While you could certainly induce fade with a certain
> prolonged script of usage of the brakes, what I'm talking about is true
> over what I would say would be the real world typical scenario (before
> the fade issue becomes real - which - yes - it would over a longer
> period, but not likely if the 2 or 3 stabs had already occurred in the
> relatively short period that I would expect). It is a fact that the
> vacuum cannot recharge with almost no vacuum in the intake - it doesn't
> recharge by magic. I guarantee you that after a third stab of the brakes
> on an engine vacuum-driven power brake car, the brakes will loose the
> fight with the engine - fade has nothing to do with that over the first
> few seconds that we would be talking about (during which the first 2 or
> 3 stabs would occur real world).

if that is your experience, then i think you must have a vacuum leak.
even with wide open throttle, there is sufficient vacuum in the manifold
to create significant braking assist.


>
> i've
>> experienced that too, one particular time on a major hill in san
>> francisco approaching a busy intersection. yes, it's scary stuff. but
>> when i changed the pads on my civic from after-market to oem, all fade
>> problems disappeared. even fully loaded, repeatedly decelerating from
>> speed. [i learned my "honda oem is best" lesson that way.]
>>
>>
>>>
>>> Also, once the booster is depleted of vacuum during that experiment, the
>>> vacuum charge in the booster will remain depleted until a second or so
>>> after the throttle is released - IOW - deplete it and continue applying
>>> the throttle (again - doesn't have to be anywhere near WOT) for several
>>> seconds. Every once in a while, while still applying the throttle, try
>>> the brakes again. You will not have any effective braking until *after*
>>> you release the throttle.
>>>
>>> I urge anyone who doesn't believe what I claim above to try it before
>>> commenting.
>>
>> i have. my results and comments are as above.
>
> And both of us could design the script to create either end result we
> wished. I submit that in the real world, most people would try pumping
> the brakes in desperation if the sudden acceleration scenario actually
> happened - the depleting stabs would occur before fade became a factor.
>

sorry dude, it's incredibly rare for a panicking driver to pump their
brakes. that's why abs exists - abs does that for them, and only in the
event of traction limit being exceeded..


--
nomina rutrum rutrum

On 03/07/2010 08:16 AM, Bill Putney wrote:
> dsi1 wrote:
>
>> I certainly believe you. You can get a feel for the amount of reserve
>> vacuum boost on your car by simply repeatedly pressing down on the
>> brakes without starting your engine. If your brakes are working
>> properly, you'll feel the pedal getting firmer until you'll only be
>> able to move the brake pedal a couple of inches of deflection. At that
>> point, you'll have used up all your vacuum reserve. I figure that you
>> should be able to get around 3 stabs at the brakes with mostly full
>> boost. This means you'll only get maybe two chances for full braking
>> after the initial attempt at braking. That's the breaks I guess. :-)
>
> My recollection is that boost is noticeably diminished after the second
> stab, greatly diminished by the third stab - engine overpowers brakes
> for most common vehicles.
>

maybe if the engine is stopped and vacuum gone. but if the engine is
running, you still have vacuum. and unless you're driving something
ancient with chronic brakes [there was some scarily under-braked ****
built in the 50's & 60's], brakes stop engines every time.

besides, where does this notion about engines running on full throttle
come from? if the engine computer has freaked so that it won't switch
off, won't switch to neutral, won't respond to throttle sensor input,
etc., why are we assuming it's still responding to crank position sensor
input and still running the code for injecting fuel and firing the plugs???

people need to use their logic and think a little bit before they go
about parroting the not-so-subtle propaganda that's been polluting the
media.


--
nomina rutrum rutrum

jim beam wrote:
> On 03/07/2010 08:10 AM, Bill Putney wrote:
>> jim beam wrote:

>>> i have done this. with the engine off, the vacuum remains until the
>>> pedal is released - thus if you stomp the pedal and keep it there, you
>>> don't need to keep replenishing the vacuum. and you will stop the car.
>>> with the engine running, there is no vacuum issue, and the brakes are
>>> still powerful enough to stop the car. on my honda anyway.
>>
>> I have real trouble believing that a large majority of people would, in
>> a sudden inadvertent acceleration situation, be content to press the
>> brakes one time and not try to pump them once or twice.
>
> why? if the car in front of you suddenly jams on their brakes, do you
> apply then release your pedal? i think your answer is "no" - unless
> you're skidding and know what cadence braking is. and if you know what
> cadence braking is, you should know that to stop a car with the engine
> on full throttle, you apply the brakes hard and quickly - you don't
> monkey about with multiple brake applications that can cause excess
> heating and fade.

We aren't talking about what the savvy drive does and knows about. What
I've said still stands for the scenario we are talking about and for
many drivers. We weren't talking about what the best thing would be to do.

>> After that, the
>> brakes will be almost totally ineffective because of loss of vacuum.
>
> no. fade maybe, but vacuum is always present if the engine is running...

Not enough for any effectiveness of brakes. Do you even know what
happens to plenum vacuum at light, moderate, and heavy throttle? If you
knew the numbers, then you wouldn't be saying that an engine running
(with throttle open) will have enough vacuum to run the brakes - because
it simply isn't true. Anyone who has used a vacuum gage to any extent
knows that vacuum plummets when the throttle is open.

> even if it's not, you still have vacuum reserve for three applications.

That part is arguably true. I'd say you're loosing effectiveness after
2, but what's one more jab of the brakes between friends. The result
will be that the booster will absolutely *not* charge back to any usable
level under acceleration. Plenum vacuum is just too low. Again - you
don't seem to know that - if you want to argue that point, then give me
some numbers for vacuum for a common engine of your choice at idle, and
in gear under light, moderate, and heavy acceleration. If the numbers
you come back with are honest, you will prove what I'm saying.

>>> with respect, i think you're confusing vacuum with fade...
>>
>> No - I'm not. While you could certainly induce fade with a certain
>> prolonged script of usage of the brakes, what I'm talking about is true
>> over what I would say would be the real world typical scenario (before
>> the fade issue becomes real - which - yes - it would over a longer
>> period, but not likely if the 2 or 3 stabs had already occurred in the
>> relatively short period that I would expect). It is a fact that the
>> vacuum cannot recharge with almost no vacuum in the intake - it doesn't
>> recharge by magic. I guarantee you that after a third stab of the brakes
>> on an engine vacuum-driven power brake car, the brakes will loose the
>> fight with the engine - fade has nothing to do with that over the first
>> few seconds that we would be talking about (during which the first 2 or
>> 3 stabs would occur real world).
>
> if that is your experience, then i think you must have a vacuum leak...

That is simply not the case.

> even with wide open throttle, there is sufficient vacuum in the manifold
> to create significant braking assist.

That is profoundly incorrect - period.

>> i've
>>> experienced that too, one particular time on a major hill in san
>>> francisco approaching a busy intersection. yes, it's scary stuff. but
>>> when i changed the pads on my civic from after-market to oem, all fade
>>> problems disappeared. even fully loaded, repeatedly decelerating from
>>> speed. [i learned my "honda oem is best" lesson that way.]

>>>> I urge anyone who doesn't believe what I claim above to try it before
>>>> commenting.
>>>
>>> i have. my results and comments are as above.

>> And both of us could design the script to create either end result we
>> wished. I submit that in the real world, most people would try pumping
>> the brakes in desperation if the sudden acceleration scenario actually
>> happened - the depleting stabs would occur before fade became a factor.

> sorry dude, it's incredibly rare for a panicking driver to pump their
> brakes...

That's your *theory*, or you have something to back that statement up?
Admittedly, my theory that some percentage of drivers would pump the
brakes is not provable as far as I know, but I do state it as
conjecture, not as fact as you are stating your argument.

> that's why abs exists - abs does that for them, and only in the
> event of traction limit being exceeded..

You're not talking about the same situation. Trying to stop a vehicle
with runaway acceleration is not the same setup as emergency accident
avoidance as far as paniced driver psychology. (Yes - there are
similarities, but don't turn that similarity into being equal.)


--
Bill Putney
(To reply by e-mail, replace the last letter of the alphabet in my
address with the letter 'x')

jim beam wrote:
> On 03/07/2010 08:16 AM, Bill Putney wrote:
>> dsi1 wrote:
>>
>>> I certainly believe you. You can get a feel for the amount of reserve
>>> vacuum boost on your car by simply repeatedly pressing down on the
>>> brakes without starting your engine. If your brakes are working
>>> properly, you'll feel the pedal getting firmer until you'll only be
>>> able to move the brake pedal a couple of inches of deflection. At that
>>> point, you'll have used up all your vacuum reserve. I figure that you
>>> should be able to get around 3 stabs at the brakes with mostly full
>>> boost. This means you'll only get maybe two chances for full braking
>>> after the initial attempt at braking. That's the breaks I guess. :-)
>>
>> My recollection is that boost is noticeably diminished after the second
>> stab, greatly diminished by the third stab - engine overpowers brakes
>> for most common vehicles.
>>
>
> maybe if the engine is stopped and vacuum gone. but if the engine is
> running, you still have vacuum...

Then give me some vacuum numbers for idle, and in gear light, moderate,
and heavy acceleration for a typical engine. If you can show that
moderate to heavy acceleration vacuum levels are anywhere near idle
vacuum levels, then I'll concede.

BUT, ignoring for the moment the numbers, the fact remains that the
booster will not recharge to any usable level with the throttle applied.

> and unless you're driving something
> ancient with chronic brakes [there was some scarily under-braked ****
> built in the 50's & 60's], brakes stop engines every time.

> besides, where does this notion about engines running on full throttle
> come from? if the engine computer has freaked so that it won't switch
> off, won't switch to neutral, won't respond to throttle sensor input,
> etc., why are we assuming it's still responding to crank position sensor
> input and still running the code for injecting fuel and firing the plugs???
>
> people need to use their logic and think a little bit before they go
> about parroting the not-so-subtle propaganda that's been polluting the
> media.

I have made no statements in this discussion whatsoever about whether I
believe the reports of sudden acceleration. All I'm talking about is
that *IF* acceleration is going on (for whatever reason) while the
brakes are being applied, there is *not* enough vacuum to recharge the
booster to any usable level, and what reserve is there will be gone if
the brakes are pumped (on that apparently we both agree if they are
pumped more than 3 times, even though I say it's if pumped more than 2
times for some vehicles).

--
Bill Putney
(To reply by e-mail, replace the last letter of the alphabet in my
address with the letter 'x')

On Sun, 07 Mar 2010 08:35:07 -0800, jim beam > wrote:

>On 03/07/2010 08:16 AM, Bill Putney wrote:
>> dsi1 wrote:

>
>maybe if the engine is stopped and vacuum gone. but if the engine is
>running, you still have vacuum.


Absolutely not so with any naturally aspirated internal combustion
engine.

Manifold vacuum depends entirely on the air moving through the engine
and being "throttled" by the butterfly in the intake. This vacuum only
exists because the engine is pumping out air that cannot be resupplied
through the closed or partially closed throttle plate(s).

Throttle open (stuck, actuated improperly, or opened wide by driver)
always equals ZERO (or certainly very low) manifold vacuum.

Low manifold vacuum means zero power assist from a vacuum assisted
brake booster. This gives the driver a very hard brake pedal after
only couple of strokes and surely a major reduction of the amount of
line pressure that the average person could muster with only leg
power. ie: the feeling that the brakes have "failed".

Unless there is some kind of a vacuum backup assist pump.. (some
trucks and buses), not that I'm aware of on Toyotas, or the brakes are
power boosted by some other means, hydroboost etc. there is going to
be little or no power assistance to help the driver control the
"runaway" vehicle if the throttle is in the opened position.

Neutraling the runaway engine, allowing the RPM to rise until there is
some other limiting factor STILL won't produce much, if any intake
manifold vacuum. It would of course help the driver to regain control
(if only with no power assist to the brakes).

Switch "off" may stop the engine and if the still engaged transmission
allows it to stop turning, there still won't be any brake assistance
because there still won't be any vacuum as long as the throttle is
(stuck?) open. Furthermore, if the engine should stop turning when
switched "off", we now have the added element of no power steering
asist, (if the steering is assisted by an engine driven pump).

Hope this somewhat basic expanation helps to explain what the driver
of a "runaway" feels and experiences.

On 03/07/2010 09:44 AM, Bill Putney wrote:
> jim beam wrote:
>> On 03/07/2010 08:10 AM, Bill Putney wrote:
>>> jim beam wrote:
>
>>>> i have done this. with the engine off, the vacuum remains until the
>>>> pedal is released - thus if you stomp the pedal and keep it there, you
>>>> don't need to keep replenishing the vacuum. and you will stop the car.
>>>> with the engine running, there is no vacuum issue, and the brakes are
>>>> still powerful enough to stop the car. on my honda anyway.
>>>
>>> I have real trouble believing that a large majority of people would, in
>>> a sudden inadvertent acceleration situation, be content to press the
>>> brakes one time and not try to pump them once or twice.
>>
>> why? if the car in front of you suddenly jams on their brakes, do you
>> apply then release your pedal? i think your answer is "no" - unless
>> you're skidding and know what cadence braking is. and if you know what
>> cadence braking is, you should know that to stop a car with the engine
>> on full throttle, you apply the brakes hard and quickly - you don't
>> monkey about with multiple brake applications that can cause excess
>> heating and fade.
>
> We aren't talking about what the savvy drive does and knows about. What
> I've said still stands for the scenario we are talking about and for
> many drivers. We weren't talking about what the best thing would be to do.
>
>>> After that, the
>>> brakes will be almost totally ineffective because of loss of vacuum.
>>
>> no. fade maybe, but vacuum is always present if the engine is running...
>
> Not enough for any effectiveness of brakes. Do you even know what
> happens to plenum vacuum at light, moderate, and heavy throttle? If you
> knew the numbers, then you wouldn't be saying that an engine running
> (with throttle open) will have enough vacuum to run the brakes - because
> it simply isn't true. Anyone who has used a vacuum gage to any extent
> knows that vacuum plummets when the throttle is open.

then you're making generalizations from your own exceptional experience
that are not representative of most vehicle, and certainly not vehicles
with standard tuning. vacuum decreases significantly at w.o.t, is
sufficient to give brake boost.

besides, how much vacuum do you think you need for a brake booster?
with a 9" diameter diaphragm, [on the small side by modern standards],
that's 63 sq inches. how much pressure delta do you think you need to
double a person's braking force?


>
>> even if it's not, you still have vacuum reserve for three applications.
>
> That part is arguably true. I'd say you're loosing effectiveness after
> 2, but what's one more jab of the brakes between friends. The result
> will be that the booster will absolutely *not* charge back to any usable
> level under acceleration.

you're supposed to apply the brakes and keep them on. only if you
release do you lose vacuum.


> Plenum vacuum is just too low. Again - you
> don't seem to know that - if you want to argue that point, then give me
> some numbers for vacuum for a common engine of your choice at idle, and
> in gear under light, moderate, and heavy acceleration. If the numbers
> you come back with are honest, you will prove what I'm saying.

see above.


>
>>>> with respect, i think you're confusing vacuum with fade...
>>>
>>> No - I'm not. While you could certainly induce fade with a certain
>>> prolonged script of usage of the brakes, what I'm talking about is true
>>> over what I would say would be the real world typical scenario (before
>>> the fade issue becomes real - which - yes - it would over a longer
>>> period, but not likely if the 2 or 3 stabs had already occurred in the
>>> relatively short period that I would expect). It is a fact that the
>>> vacuum cannot recharge with almost no vacuum in the intake - it doesn't
>>> recharge by magic. I guarantee you that after a third stab of the brakes
>>> on an engine vacuum-driven power brake car, the brakes will loose the
>>> fight with the engine - fade has nothing to do with that over the first
>>> few seconds that we would be talking about (during which the first 2 or
>>> 3 stabs would occur real world).
>>
>> if that is your experience, then i think you must have a vacuum leak...
>
> That is simply not the case.
>
>> even with wide open throttle, there is sufficient vacuum in the
>> manifold to create significant braking assist.
>
> That is profoundly incorrect - period.

again, if that's your experience, i think you're working with something
not representative of most vehicles. certainly not anything post 1980.


>
>>> i've
>>>> experienced that too, one particular time on a major hill in san
>>>> francisco approaching a busy intersection. yes, it's scary stuff. but
>>>> when i changed the pads on my civic from after-market to oem, all fade
>>>> problems disappeared. even fully loaded, repeatedly decelerating from
>>>> speed. [i learned my "honda oem is best" lesson that way.]
>
>>>>> I urge anyone who doesn't believe what I claim above to try it before
>>>>> commenting.
>>>>
>>>> i have. my results and comments are as above.
>
>>> And both of us could design the script to create either end result we
>>> wished. I submit that in the real world, most people would try pumping
>>> the brakes in desperation if the sudden acceleration scenario actually
>>> happened - the depleting stabs would occur before fade became a factor.
>
>> sorry dude, it's incredibly rare for a panicking driver to pump their
>> brakes...
>
> That's your *theory*, or you have something to back that statement up?

where is your "theory" that people pump brakes? i saw someone on a
freeway in rain in los angles one time, skating along with their brakes
locked, car gently rotating, and the look on her face was that of
someone trying to break the pedal off she was pushing it so hard.
there's no way that person, as an average driver, was going to let off
that pedal, and thus, even though she was going to crash, she was not
going to lose vacuum.


> Admittedly, my theory that some percentage of drivers would pump the
> brakes is not provable as far as I know, but I do state it as
> conjecture, not as fact as you are stating your argument.

with respect, you're projecting your own behavior as representative of
everyone. it's not true.


>
>> that's why abs exists - abs does that for them, and only in the event
>> of traction limit being exceeded..
>
> You're not talking about the same situation. Trying to stop a vehicle
> with runaway acceleration is not the same setup as emergency accident
> avoidance as far as paniced driver psychology. (Yes - there are
> similarities, but don't turn that similarity into being equal.)

indeed, but i'm pointing out the fact that refutes your supposition -
typical panicked drivers do not let off the brake pedal. if they did
and were capable of regaining control of the vehicle in skids, we
wouldn't have any need for abs. instead, we have abs on almost
everything now because the facts show it to help drivers of average
ability that just push that pedal as hard as they can.


--
nomina rutrum rutrum

On 03/07/2010 09:52 AM, Bill Putney wrote:
> jim beam wrote:
>> On 03/07/2010 08:16 AM, Bill Putney wrote:
>>> dsi1 wrote:
>>>
>>>> I certainly believe you. You can get a feel for the amount of reserve
>>>> vacuum boost on your car by simply repeatedly pressing down on the
>>>> brakes without starting your engine. If your brakes are working
>>>> properly, you'll feel the pedal getting firmer until you'll only be
>>>> able to move the brake pedal a couple of inches of deflection. At that
>>>> point, you'll have used up all your vacuum reserve. I figure that you
>>>> should be able to get around 3 stabs at the brakes with mostly full
>>>> boost. This means you'll only get maybe two chances for full braking
>>>> after the initial attempt at braking. That's the breaks I guess. :-)
>>>
>>> My recollection is that boost is noticeably diminished after the second
>>> stab, greatly diminished by the third stab - engine overpowers brakes
>>> for most common vehicles.
>>>
>>
>> maybe if the engine is stopped and vacuum gone. but if the engine is
>> running, you still have vacuum...
>
> Then give me some vacuum numbers for idle, and in gear light, moderate,
> and heavy acceleration for a typical engine. If you can show that
> moderate to heavy acceleration vacuum levels are anywhere near idle
> vacuum levels, then I'll concede.

dude, for the typical vacuum diaphragm, you only need the smallest
vacuum to significantly boost brake pressure. do the math.


>
> BUT, ignoring for the moment the numbers, the fact remains that the
> booster will not recharge to any usable level with the throttle applied.

that is a false presumption. the only way that can happen is if there
is vacuum leak, or if the manifold vacuum is below booster vacuum. but
booster vacuum will never rise above manifold vacuum.


>
>> and unless you're driving something ancient with chronic brakes [there
>> was some scarily under-braked **** built in the 50's & 60's], brakes
>> stop engines every time.
>
>> besides, where does this notion about engines running on full throttle
>> come from? if the engine computer has freaked so that it won't switch
>> off, won't switch to neutral, won't respond to throttle sensor input,
>> etc., why are we assuming it's still responding to crank position
>> sensor input and still running the code for injecting fuel and firing
>> the plugs???
>>
>> people need to use their logic and think a little bit before they go
>> about parroting the not-so-subtle propaganda that's been polluting the
>> media.
>
> I have made no statements in this discussion whatsoever about whether I
> believe the reports of sudden acceleration. All I'm talking about is
> that *IF* acceleration is going on (for whatever reason) while the
> brakes are being applied, there is *not* enough vacuum to recharge the
> booster to any usable level,

untrue. and that statement also presumes all vacuum is lost - also untrue.


> and what reserve is there will be gone if
> the brakes are pumped (on that apparently we both agree if they are
> pumped more than 3 times, even though I say it's if pumped more than 2
> times for some vehicles).

who pumps their brakes when they're trying to stop the vehicle, and the
brakes are not skidding??? you are correct that if all vacuum
disappeared, the driver would have a much tougher time, but i think your
presumption about pumping is not a reflection of reality.


--
nomina rutrum rutrum