Time to clear this up again...
Well, when I first starting trying to find an answer to my question,
I thought that it was a dumb question, but, since none of my souces
knew the answer, I guess that it's not so bad.
Anyway, the question is: How do Horse Power and Torque differ?
Horsepower is torque over time. That is,
Torque is what an engine makes. Horsepower indicates
how fast an engine makes torque.
Torque is the raw measure of what your engine produces
when it's running. It is a twisting force. It can be
measured on a chassis dyno, which is a kind of brake;
the dyno measures how much force the engine applies to
the dynamometer wheel. That is where the term "brake
horsepower" comes from.
Brake horsepower is a mathematical abstraction that
represents how much and how well an engine produces
torque. Horsepower can be calculated on a chassis
dynamometer, from the amount of torque measured. Please
note carefully the verbs used: torque is measured,
horsepower is calculated.
To determine horsepower, here's the formula:
BHP = (torque * rpm) / 5252
But you say you've read that, so let's get to some examples.
I guess the question boils down to: What does an increase/decrease
in HP or Torque do to the 'seat-of-the-pants' feel of driving? How
will it effect the off-the-line speed, passing speed, etc?
Engines produce a result: they convert fuel into motion. Depending
on a lot of parameters -- bore, stroke, compression ratio, camshaft
design, valve configuration, number of cylinders, etc -- the way
that engines produce this result can vary.
For your purposes, ignore the difference between torque
and horsepower. There isn't any. They're just different
ways of expressing how the engine produces results.
Most people who talk about "horsepower" and "torque" talk
as if they're two different things, like downforce and
camber. That's nonsense. They're two ways of quantifying
how an engine produces results.
So I don't think you're asking the right question. What I
think you want to know is:
How can engines that have high torque numbers produce low
BHP numbers, and vice versa, and how does that affect driving
cars with those engines?
Some engines -- let's say the 225-bhp L78 Chevy V8 in the
'65 Super Sport I'm driving today -- don't rev very high
but they have a number of cylinders and produce a lot of
power fairly low in the RPM band. I think this motor is
redlined at about 5000 RPM, with peak torque probably about
2250 RPM and peak power probably around 4500. (It might
be a little higher, this is the Super Sport version with
better breathing and higher compression than the cooking
motors.) This engine is connected to a 2-speed automatic
transmission. First gear will hold to about 50 mph; 2nd
gear is useful down to about 15 mph. This engine produces
most of its power -- its peak torque -- at very low RPMs.
As a result, it pins you in your seat when you crack the
throttle plates open, even at a fairly low speed. (The
transmission keeps this car from having a really good
launch; when I want to drag-race, I take the Falcon with
its four-speed and similar, but stronger, motor.) But
power over 5000 RPM falls off, because the engine's
valvetrain isn't designed to run that high.
Other engines -- let's say the 1558cc Lotus Twin Cam in
the Lotus Cortina I used to own -- have small displacement
but are designed to run at very high RPM. In this engine,
there is very little response under about 4000 RPM (well,
with the cams I had) -- that is, you press the throttle
to the floor and you think, this is a Lotus? It's not
as strong as my Midget! Then at about 4000 RPM the most
amazing thing happens. At that speed, the engine seems to
come alive. Because of the length of the intake and exhaust
tracts in that motor, and some other things, it suddenly
enters this synergistic phase where each power pulse seems
to feed the next one with astonishing speed. The car
goes much faster up to about 6500 RPM -- but it doesn't seem
to have the boot in the back that the Chevy does. (Both of
these cars, in stock form, were rated at about the same
0-60 times when new, BTW -- low nines, for the record.)
So when you talk about changing an engine's torque or
horsepower, what you're really talking about is changing
either the quantity of torque that it produces or the rate
at which it produces torque.
The classic example here is the Ford Taurus 3.0 versus 3.8.
Both engines have the same horsepower rating, but the 3.8
has something like 20% more peak torque. Consequently, the
3.8 is about a second quicker to 60 -- the engine produces
more results if you measure the raw output, but the output
over time is about the same because of *where* the engine
produces its power in its RPM band.
An alternative example is an 8V versus a 16V GTI. I drive
an 8V regularly; a month or two ago I drove a 16V for the
first time. At one of the first stoplights in the 16V,
I started off and thought I'd picked third gear -- there
was very little power below 3000 RPM, while in the 8V I have
good crisp response from about 2000 RPM on up. However,
once I crossed that 3000-3500 RPM boundary, the 16V was
off like a rocket. BHP ratings are 90 for my 8V, 123 for
the 16V. But in practice, the difference is more in where
the engine makes power than in how much it makes. And
horsepower is a way of measuring that.
The real way to quantify how an engine will feel is to
use a graph of torque and horsepower curves. In most
engines, horsepower is fairly linear up to a peak, while
torque tends to look like a bathtub.
This isn't an easy topic, so don't give up yet.
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