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Old 24 Dec 2009, 11:28 am
jim beam
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Default torque and power

ok, in the interests of clarity, NOT name calling or derision, here is
the simplified version of why power matters in the acceleration of a car:

from Newton's second law, F=ma, we know that acceleration = force
divided by mass. so to accelerate our massy car, we need to apply
force, which is why we need torque.

but, i can apply torque by hand - for instance, i can apply 600 ft.lbs
to the axle stud of my honda with a breaker bar, extension tube, and my
lardy rear end, no problem. but that's got going to give me an 11s
quarter mile. why? because, and this is the bit that's necessary to
understand, that force is an instantaneous number, not a continuous one.

illustration:
if you drop [downwards] a ball out of a window, it accelerates until it
either hits the ground [or reaches terminal velocity if your window is
high enough] courtesy of the constantly applied force called "gravity".

but if you throw the ball sideways, it's lateral acceleration stops
instantly the ball leaves your hand.

i.o.w, to keep a body accelerating, you need to /keep/ applying force.
to keep applying force, you need to do work. i can't win anything at
laguna seca with my lardy ass and a breaker bar because i can't work
that hard.

so, you need force applied over time. force over time is work, and
another word for work over time is power. clear as mud!!


one more thing:
we can get into torque curves vs power curves, but the essential point
to understand is that a lower torque being delivered at a higher rate
can achieve more than a high torque at a low rate - torque alone does
not make a fast car.

http://en.wikipedia.org/wiki/Formula_One_engines
toyota F1 v8 delivers 202 ft.lbs.

http://automobiles.honda.com/accord-...fications.aspx
honda accord v6 delivers 254 ft.lbs.

but the powers are 740 and 271 hp respectively, and one goes [and
accelerates] considerably faster than the other.
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Old 26 Dec 2009, 07:30 pm
JRStern
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Default Re: torque and power

On Thu, 24 Dec 2009 09:28:14 -0800, jim beam <me@privacy.net> wrote:

>ok, in the interests of clarity, NOT name calling or derision, here is
>the simplified version of why power matters in the acceleration of a car:
>
>from Newton's second law, F=ma, we know that acceleration = force
>divided by mass. so to accelerate our massy car, we need to apply
>force, which is why we need torque.

...


Jim, thanks, but I know all this.

My physics 101 is rusty enough I don't want to try to launch into the
issues of acceleration versus power, which is what the torque and
power curves make easy to see.

The physics 101 stuff is all over the net, but I couldn't find
anything about automotive applications except for that one site that
worried about shift points - which were all given as redline for the
five or six cars I looked at. I guess with modern engines, electronic
ignition, VTEC cams and all, it's all engineered out. Judging from
the numbers there - and what it feels like to drive the cars - the
torque curves are way flat these days for even little engines, that
wasn't true in the 1960s European sub two-liter engines. The area
beneath the curve (given a fixed max height) was a big deal then, and
the slope of the curve determined where optimal shift points were.
Just doesn't seem to matter anymore. But unless I've forgotten all of
this, max *acceleration* is still at the max torque point, not the max
power point.

Just for fun I've managed to run my Accord EX I4 (auto) up above 6k
now in gears 1 thru 3, works well enough in 1 and even 2, 3 it feels
rather rougher. Acceleration still feels about linear from 3500 on
and the engine upshifts the moment you give it any slack, no doubt
Honda trying to keep wear down and mileage up. So it goes, more than
sufficient for my purposes, if I want more I should try the Si with
manual shift, I fully understand.

I probably looked at thirty pages the other day, and all the ones that
tried to link the basic physics to automative terms had gross
misstatements - but I didn't save the links. So, I'll look again.

http://www.google.com/search?hl=en&s...&aq=f&oq=&aqi=

This one seems not to misstate, but neither does it really get to the
point:
http://www.carkeys.co.uk/features/technical/636.asp

This looks good (long, reading ...)
http://ateupwithmotor.com/terms-and-...rformance.html
Has the line:
"Therefore, in normal driving, the shape of the torque curve is often
more important than maximum power."

Others ... well, if I run across a really good one, I'll post.

J.


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  #3 (permalink)  
Old 26 Dec 2009, 11:22 pm
jim beam
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Posts: n/a
Default Re: torque and power

On 12/26/2009 05:30 PM, JRStern wrote:
> On Thu, 24 Dec 2009 09:28:14 -0800, jim beam<me@privacy.net> wrote:
>
>> ok, in the interests of clarity, NOT name calling or derision, here is
>> the simplified version of why power matters in the acceleration of a car:
>>
>>from Newton's second law, F=ma, we know that acceleration = force
>> divided by mass. so to accelerate our massy car, we need to apply
>> force, which is why we need torque.

> ..
>
>
> Jim, thanks, but I know all this.


glad to hear it. because when you said, "it's torque that equates to
acceleration, not horsepower",
Message-ID: <uf83j5l8cfhmojsgqa6im27qe4qvsug7ka@4ax.com>

you had me believing you were completely ignorant.


>
> My physics 101 is rusty enough I don't want to try to launch into the
> issues of acceleration versus power, which is what the torque and
> power curves make easy to see.


rusty???


>
> The physics 101 stuff is all over the net, but I couldn't find
> anything about automotive applications except for that one site that
> worried about shift points - which were all given as redline for the
> five or six cars I looked at. I guess with modern engines, electronic
> ignition, VTEC cams and all, it's all engineered out. Judging from
> the numbers there - and what it feels like to drive the cars - the
> torque curves are way flat these days for even little engines, that
> wasn't true in the 1960s European sub two-liter engines. The area
> beneath the curve (given a fixed max height) was a big deal then, and
> the slope of the curve determined where optimal shift points were.
> Just doesn't seem to matter anymore. But unless I've forgotten all of
> this, max *acceleration* is still at the max torque point, not the max
> power point.
>
> Just for fun I've managed to run my Accord EX I4 (auto) up above 6k
> now in gears 1 thru 3, works well enough in 1 and even 2, 3 it feels
> rather rougher. Acceleration still feels about linear from 3500 on
> and the engine upshifts the moment you give it any slack, no doubt
> Honda trying to keep wear down and mileage up. So it goes, more than
> sufficient for my purposes, if I want more I should try the Si with
> manual shift, I fully understand.
>
> I probably looked at thirty pages the other day, and all the ones that
> tried to link the basic physics to automative terms had gross
> misstatements - but I didn't save the links. So, I'll look again.
>
> http://www.google.com/search?hl=en&s...&aq=f&oq=&aqi=
>
> This one seems not to misstate, but neither does it really get to the
> point:
> http://www.carkeys.co.uk/features/technical/636.asp
>
> This looks good (long, reading ...)
> http://ateupwithmotor.com/terms-and-...rformance.html
> Has the line:
> "Therefore, in normal driving, the shape of the torque curve is often
> more important than maximum power."


let me translate for you: "torque matters to freds that can't be
bothered to spin the engine."


>
> Others ... well, if I run across a really good one, I'll post.


on the basis of the above, i don't think we'll losing sleep on that one j.r.

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