Short answer: Generally speaking, (not always, but generally)
when it comes to gasoline engines, horsepower measures how well an engine
retains its torque (which you can think of as acceleration ability) as
the engine speed increases. A high horsepower car will pull hard and keep
pulling hard for a long time when you step on the gas. A low HP car will
not pull as hard, and the pulling will fall off very rapidly as engine
speed rises.
As for Saab’s advertising, it’s true that a 3000 HP engine that
produced almost no torque would be worthless. However, gasoline engines
simply don’t work that way. Torque and HP come as a package deal in a
gasoline engine. More powerful engines produce more of both. Less powerful
engines produce less of both.
Certanly, having more torque at lower engine speeds is nice. Most
people drive at low engine speeds so as to minimize the noise produced
by the engine. With more torque at lower engine RPMs, the car will feel
more tractable and be easier to drive in traffic.
But there’s no such thing as a free lunch. More torque at low speeds
is obtained by stealing from torque at high speeds, or vice-versa. So a
car that’s great around town may be a dog on the highway, and vice versa.
The only way to increase torque at all speeds is to add more horsepower.
Long Answer: This is going to be a looooong and highly detailed
explanation. I hope you’re interested in this subject in more than just
a superficial way.
There are two issues we need to cover here - horsepower in general,
and horsepower as it applies to cars.
First of all, what is horsepower in general? Well, technically speaking,
horsepower is torque divided by RPM. The formula for calculating horsepower
is:
HP = (torque * rotational_speed) / 5252
This formula only holds when torque is measured in ft-lbs and
rotational speed is measured in revolutions per minute. If you measure
them with some other units (say, newton-meters and radians per second)
then the 5252 correction number has to be changed to account for it.
Rotational Speed is probably pretty self explanatory, but you may not
have a mathematically rigorous understanding of torque.
Okay, so what’s torque? Torque is twisting force. You calculate it by
multiplying a force time a distance. For example, suppose you have a
bolt in the wall that you want to screw in. You also have a wrench that
is about a foot long. If you put the wrench on the bolt perfectly
horizontally, then weld a one-pound weight to the handle of the wrench
exactly one foot from the center of the bolt, then the wrench is exerting
one foot-pound (ft-lb) of twisting force, or torque, on the bolt.
Of course, as soon as the bolt starts turning, the wrench will turn
too, and some of the weight of the weight the handle won’t be working to
turn the bolt any more, it’ll be working to slip the wrench off the bolt.
At some point the wrench turns enough to slip completely off the bolt, and
falls on your foot. Ow!
But now think about hooking up an electric motor to the bolt. This
motor is able to constantly exert one ft-lb of torque on the bolt no matter
what angle the bolt is twisted around at. How many horsepower is that
electric motor producing?
Ha, gotcha, that’s a trick question! You can’t know how many HP the
motor is producing because I didn’t tell you how fast the bolt is
turning! Remember the HP formula - To calculate HP you need to know
both torque and RPM. So until I tell you that the electric motor turns
at, say, 60 RPM (one full turn every second), you can’t compute horsepower.
But since I did tell you the rotational speed, we can calculate horsepower.
Working the formula, we have:
HP = ( 1 * 60 ) / 5252
HP = about .01
To make an analogy to the physics concept of work, which is force
but corrected for distance, horsepower is torque, but corrected for
rotational speed.
With that basic understanding in place, let’s move on to how HP
and torque apply to cars…
The first thing you need to understand is that the horsepower numbers
published in car ads are what’s called “peak” horsepower. You remember
that in order to calculate HP, you need to know two things - one,
how fast the engine is spinning, and two, how much twisting force it
can provide at that speed. Now, suppose you had an engine that produced
500 horsepower between 9000 and 10,000 RPMs, but could only product 17
horsepower from 0-9000 RPMs. The average horsepower of the engine would
only be (.9 * 17 + .1 * 500) = 65 horsepower. This car would be very,
very slow. But do you know what they’d advertise the car as? Yup, 500
horsepower! (Fortunatly, real engines generally don’t work this way.)
Similiarly, the torque number quoted is also “peak” torque. That
is, if you measured the torque at all engine RPMs, and then took the
best one out of all of them, that’s what they advertise the torque as.
Because of the obvious flaws of depending on only a single peak
HP or torque value, more saavy car folks generally like to see a
graph that plots engine RPM and HP, or engine RPM and torque. Or
sometimes even both on the same graph. By looking at this graph, you
can get some idea how the engine behaves in general. For instance, if
the torque curve is very smooth and relatively flat, the engine has a
similiar amount of torque at all RPMs, and the “peak” torque value they
quote will be a much more realistic estimate of the average torque value
of the engine. If, on the other hand, the torque chart looks like a
mountain peak, the peak torque will be much higher than the average
torque, and the engine will pull very hard at certain speeds but be
much weaker at others.
HP charts are harder to read, because HP depends on both engine
RPM and torque. Generally, horsepower will rise as torque and RPM
do, and when torque starts to fall, so will HP, though less steeply.
Since HP is the product of torque and RPM, the HP curve will not
start to trend down until the torque falls off so badly that the
continually increasing RPM can’t make up for it.
These graphs are sometimes called “dyno charts” because the way
to make them is to hook your car up to a large machine called a
“dynomometer.” The dynomometer measures the torque and RPM at the
drive wheels of the car, and uses that information to calculate
the torque and HP graph.
You will notice that ads for high-performance cars (or motorcycles)
virtually never include these dyno charts. Apparently the manufacturers
are afraid their customers might discover that most performance engines
are tuned for high peak power at the cost of low-end power, instead of
less peak power but more consistent average power. To make matters
worse, it’s often the case that the peaks in the HP and torque
curves must come at very high RPMs. Meaning you have to rev the
living crap out of the engine all the time, which most people don’t
enjoy. Also read http://www.zhome.com/ZCMnL/tech/torqueHP.htm
However, having a relatively peaky torque curve isn’t necessarily a
bad thing. As long as the torque is reasonable through most of the
RPM range, the peak can be an exhilirating burst of acceleration,
and make the car fun to drive. The problem comes in when the height
of the peak is so extreme compared to the rest of the torque curve
that the car has no power except for just a second. The same is
generally true for HP curves. The steeper the HP curve, the more
exciting the car will be to drive, but the more the engine will
feel weak during lower-speed driving. Also, a peakier torque curve
will generally require a closer ratio transmission to keep the
engine running in that narrow RPM band where it produces good
power. This in turn will mean a lot of shifting. Again, this
can be thought of as “more fun.” Or it can just be annoying
depending on how you like to drive.
Lastly… remember the peak torque and HP numbers we talked about
earlier? Those are measured at the output shaft (crankshaft) of the
engine. But the engine isn’t hooked directly to the wheels. First
it goes through the transmission, and then in most cars it goes
through a differential, before it gets to the wheels. These
intermediate steps are important for two reasons.
First, the transmission trades RPM for torque. So when your engine
is spinning at, say, 1000 RPM, your wheels are only spinning at,
say, 500 RPM. However, because you’re trading RPM for torque, you
gain torque at the wheels by using a transmission! Different gears
trade different amounts. There is a “perfect” amount of torque at
the rear wheels. This is enough torque to make the car accelerate
as fast as possible without breaking the wheels loose. When the
wheels break loose, you actually get slightly slower acceleration.
Oddly enough, the transmission does not create or destroy horsepower.
If you think about this, it makes sense. Remember that HP is torque
times RPM. So if you only trade torque for RPM in equal amounts, the
product of torque and RPM remains constant.
Well, actually, I’m lying. The transmission doesn’t create HP, but
it does destroy a little bit. The gears of the transmission (and
in almost all cars, the gears in the differential) are swimming in
lubricating oil. The friction between gears and the drag of the
gears turning through oil wastes some of the engine’s energy. That
energy can’t go to the rear wheels. This “drivetrain loss” can be
significant. On most cars, anywhere from 10-20% of the engine’s
power is lost going through the transmission, driveshaft and
differential. This is significant. My Nissan 300ZX twin-turbo has
300 horsepower at the engine… but by the time it gets to the rear
wheels, I only have 260 horsepower left to actually make the car go.
But remember those HP numbers in the car ads? Those are measured right
at the engine, not at the wheels. So they’re a double scam. The advertisers
never publish rear-wheel HP numbers, not even peak numbers, because they’re
so much lower. Never mind that they’re the only numbers that really count.
You can have 9 million horsepower at the crank, but if only 17 makes it
to the rear wheels, your car will be dog-slow. Moral of this story, don’t
trust the car ads.
Phew… okay, I’m done. Correction and comments welcome, especially
from Anthracite who seems to know about this stuff too.
-Ben