Horsepower and Torque - a Primer (part 2)

[Bruce Augenstein <Bruce.Augenstein@xxxxxxxxxxx>]

The Case For Horsepower

OK. If torque is so all-fired important (and feels so good), why do we care
about horsepower? 

Because (to quote a friend), "It is better to make torque at high rpm than
at low rpm, because you can take advantage of *gearing*. 

For an extreme example of this, I'll leave carland for a moment, and
describe a waterwheel I got to watch awhile ago. This was a pretty massive
wheel (built a couple of hundred years ago), rotating lazily on a shaft
which was connected to the works inside a flour mill. Working some things
out from what the people in the mill said, I was able to determine that the
wheel typically generated about 2600(!) foot pounds of torque. I had clocked
its speed, and determined that it was rotating at about 12 rpm. If we hooked
that wheel to, say, the drive wheels of a car, that car would go from zero
to twelve rpm in a flash, and the waterwheel would hardly notice :-). 

On the other hand, twelve rpm of the drive wheels is around one mph for the
average car, and, in order to go faster, we'd need to gear it up. If you
remember your junior high school physics and the topic of simple machines,
you'll remember that to gear something up or down gives you linear increases
in speed with linear decreases in force, or vice versa. To get to 60 miles
per hour would require gearing the output from the wheel up by 60 times,
enough so that it would be effectively making a little over 43 foot pounds
of  torque at the output (one sixtieth of the wheel's direct torque). This
is not only a relatively small amount, it's less than what the average car
would need in order to actually get to 60. Applying the conversion formula
gives us the facts on this. Twelve times twenty six hundred, over five
thousand two hundred fifty two gives us: 

6 HP. 

Oops. Now we see the rest of the story. While it's clearly true that the
water wheel can exert a *bunch* of force, its *power* (ability to do work
over time) is severely limited. 

At The Dragstrip

OK. Back to carland, and some examples of how horsepower makes a major
difference in how fast a car can accelerate, in spite of what torque on your
backside tells you :-). 

A very good example would be to compare the LT1 Corvette (built from 1992
through 1996) with the last of the L98 Vettes, built in 1991. I'm sorry to
mention the "C" word amongst this august group, but there just isn't a
better example to use. Figures as follows:

        Engine          Peak HP @ RPM   	Peak Torque @ RPM
        ------              -------------
- -----------------

        L98             250 @ 4000      		340 @ 3200

        LT1             300 @ 5000      		340 @ 3600


The cars are geared identically, and car weights are very nearly identical,
so it's a good comparison. 

First, each car will push you back in the seat (the fun factor) with the
same authority - at least at or near peak torque in each gear. One will tend
to *feel* about as fast as the other to the driver, but the LT1 will
actually be significantly faster than the L98, even though it won't pull any
harder. If we mess about with the formula, we can begin to discover exactly
*why* the LT1 is faster. Here's another slice at that formula: 


                                Horsepower * 5252
                Torque  =       -----------------
                                         RPM


If we plug some numbers in, we can see that the L98 is making 328 foot
pounds of torque at its power peak (250 hp @ 4000), and we can infer that it
cannot be making any more than 263 pound feet of torque at 5000 rpm, or it
would be making more than 250 hp at that engine speed, and would be so
rated. In actuality, the L98 is probably making no more than around 210
pound feet or so at 5000 rpm, and anybody who owns one would shift it at
around 46-4700 rpm, because more torque is available at the drive wheels in
the next gear at that point. 

On the other hand, the LT1 is fairly happy making 315 pound feet at 5000
rpm, and is happy right up to its mid 5s redline. 

So, in a drag race, the cars would launch more or less together. The L98
might have a slight advantage due to its peak torque occurring a little
earlier in the rev range, but that is debatable, since the LT1 has a wider,
flatter curve (again pretty much by definition, looking at the figures).
>From somewhere in the mid range and up, however, the LT1 would begin to pull
away. Where the L98 has to shift to second (and give up some torque
multiplication for speed, a la the waterwheel), the LT1 still has around
another 1000 rpm to go in first, and thus begins to widen its lead, more and
more as the speeds climb. As long  as the revs are high, the LT1, by
definition, has an advantage. 

There are numerous examples of this phenomenon. The Integra GS-R, for
instance, is faster than the garden variety Integra, not because it pulls
particularly harder (it doesn't), but because it pulls *longer*. It doesn't
feel particularly faster, but it is.  (part three follows)

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