Altima SE V-6 vs. Accord LX V-6

"David Geesaman" > wrote in message
...
>
> "Lloyd Sumpter" > wrote in message
> news
> > On Tue, 21 Dec 2004 23:33:51 +0000, nospam wrote:
> >
> > > Found this helpful - http://www.vettenet.org/torquehp.html
> > >
> > > Here's my take.
> > >
> > > Power rules for acceleration, and a flat power output curve means
faster
> > > acceleration. A flat power output curve is suggested by a relatively
> > > high-RPM torque peak.
> > >
> > > <SNIP>
> > > Torque x rotational speed does equal power. (Must throw in conversion
> > > factors when using English units: Power [as hp] = torque [as ft-lbs]
x
> > > rotational speed [as RPM] / 5252.)
> > >
> > > In practice, if an engine produces peak torque at a relatively lower
> RPM,
> > > the power curve for that engine is more strongly sloped down from peak
> > > horsepower. It doesn't produce anywhere near the highest horsepower
> except
> > > at its peak horsepower speed.
> >
> > I think you are as-backly-ackwards on this: If HP = Torque x RPM (which
it
> > does, with the fudge-factors you mentioned), then an engine requires
MORE
> > torque at lower RPM to get a "flatter" HP curve. That is, if you want HP
> > to be more or less constant, you need MORE torque when RPMs are lower.
> >
> > So, an engine with lots of torque at low rpm will give more hp at the
> > lower rpm. This will compensate for the fact that, if torgue was
constant,
> > HP goes up with RPM. If torque is achived at a high RPM, then both RPM
and
> > torque are "peaking" at the same time, giving a very "spikey" hp curve.
> >
> > Now, as you said, the flatter power curve (given by torque at LOW rpm)
is
> > better for smooth driving, fewer gear changes, etc. The "spikey" power
> > curve gives more hp, since both RPM and torque are peaking at the same
> > time, but results in lots of gear changes to find that spike.
> >
> > Lloyd
>
> Lloyd is right. Dig up a dyno chart for a rotary engine, which
produces
> almost constant torque at any RPM. The HP is a straight diagonal line
> peaking right at redline. It's not the easiest car to drive fast, and
> requires careful shifting, but the Formula Mazda drivers don't complain
one
> bit.
> If you want a flat horsepower curve, you need to peak your torque
early
> and steadily decrease the torque in upper RPMs.
>
> Dave
>
>

The main reason for my original post was to clarify the physical meaning of
torque vs. power. Torque is a force. Force alone doesn't provide energy or
power. It must be combined with movement (i.e. rotation) to make power.
Power, not torque, is the 'stuff' that accelerates the car. If I could have
an engine that produced maximum power over a wide range of speeds (without
other tradeoffs, like much greater maximum power over a narrower range of
speeds) then this is what I'd want.

Now, as I conceded in reply to Dave, I know nothing about the practical
aspects of engineering a car engine for best acceleration. I am learning
here as I read your replies. Sounds like physical laws governing heat
engines make a flat torque curve the best thing achievable.

My thinking (w/o much knowledge as I've confessed) was that, in real-world
engines of a given displacement and max h.p., if torque peaked at, say, 4000
r.p.m. instead of 3000 r.p.m., you'd get a rise in h.p. that exceeded the
3000 rpm peak-torque engine to that 4000 r.p.m. point, then a flatter h.p.
curve (that's higher than the hp curve for the 3000-peak torque engine) out
to the (say) 5800 peak-hp r.p.m. (Wish I could post a .jpg graph here...).
I figured this was the meaning of cars getting 6-speed xmissions these days
instead of 4-speed. Sounds like this is wrong -- that a real-world engine
that produces max. torque at 4000 r.p.m. would be out-powered by a
real-world engine (larger displacement, or same displacement but better
breathing?) that produces max torque at 3000 r.p.m. but the same max h.p.
at say 5800 r.p.m.

"Raybender" > wrote in message
...
>
>
> wrote:
>
> > Found this helpful - http://www.vettenet.org/torquehp.html
> >
> > Here's my take.
> >
> > Power rules for acceleration, and a flat power output curve means faster
> > acceleration. A flat power output curve is suggested by a relatively
> > high-RPM torque peak.
> >
>
> [snip]
>
> No... you DO NOT want a FLAT POWER curve. Easy to show that
>
> Acceleration = Power / (Mass x Speed)
>
> So if power were constant (Flat) your acceleration would decrease as
engine and
> car speed increased. The whole idea of modern engine design with all the
> variable valve timing etc. is to get a flat TORQUE curve. Because of the

I would rather have a flat power curve -- equal to the MAXIMUM power output
achievable under ANY sort of valve timing... etc. However, you've made
clear that isn't achievable in the real world due to physical laws governing
combustion engines... so I'll accept that a flat torque curve is the best
that can be done.

Torque is a term describing a force applying to an object perpendicularly. In
engine's term, torque is a twisting force at a particular rpm. Connecting these
points together give a torque curve. When referring to torque being applied over
time...during acceleration... the proper term is horsepower. A lot of people
don't realize the proper term.

1. Most gasoline burning street engines make the same ft*lb per liter at
1000-4000 rpm range. This means that they have equivalent horsepower per liter
in that range.
2. According to 1, doubling the engine means doubling the power, in that rpm
range. A 4.0 put out 2x the power of a 2.0 in that rpm range...

As for your question, due to usage of transmission with finite gearing, the
engine that put out more torque...power...at lower rpm accelerate the car faster
in lower rpm. This happens because we try to save gas by driving at a low rpm.
There is no difference at high rpm.
With continuously variable transmissions, they both would be the same since they
both be operating at max horse rpm during acceleration.

wrote:

> It looks like the engines that make max. torque at lower %RPM make a
> relatively higher %HP at that RPM, so this supports your view (I guess...
> it's a small sample and there are lots of details ignored...). ***You're
> saying that if two engines have identical peak HP @ x-RPM, the one that has
> peak torque at lower RPM will (generally) have better usable power output --
> give better acceleraction?*** In other words, does this lower peak torque
> mean the engine was engineered 'better' for acceleration, given a typical
> drivetrain (limited # of gears, no hybrid help) .
>
> Thanks for your input.
>
> ---

In article > , > wrote:

>I would rather have a flat power curve -- equal to the MAXIMUM power output
>achievable under ANY sort of valve timing... etc. However, you've made
>clear that isn't achievable in the real world due to physical laws governing
>combustion engines... so I'll accept that a flat torque curve is the best
>that can be done.

Well, if someone said they'd give me a 200 lb-ft peak torque
engine, I'd ask that it be constant at 200 out to infinity (okay,
as high as possible).

If someone said they'd give me a 200 hp engine, then I'd want it
to be a flat 200 hp at all rpm. So, yes, I'd want max power at
all revs.

So, it all depends on what you are offered :-)

But seriously, as everyone is saying, ICE's by their nature are
much closer to the former (flat torque curve) than the latter
(flat hp curve). Electric motors are closer to the latter.

So which is better? For a given power, take the motor. In
theory, a CVT, or a whole lotta gears, will make the output of the
ICE approach that (if it is designed and controlled to sit near
the peak power rpm). But you'll need a near infinite gear ratio
to match the motor at launch..