Anyway, the question is: How do Horse Power and Torque differ?
I've read the textbook definiations of HP and Torque, but, what I
want to know is, how do they effect driving? If someone increases
the torque by 5%, what does that mean to me behind the wheel?
What if HP was increased by 5%?
HorsePower is almost meaningless (why else do you think it is the only
number marketeers quote?), Torque is everything. [Well, OK, that's a
bit of an oversimplification...]
Torque is the capability of the engine to turn. It is an amazingly
complicated function of displacement (volume), compression ratio, fuel
mixture, valve timing/opening/duration/etc, engine speed, etc, etc.
It typically plots something [crudely] like
|
| *
T | ** **
o | ** * **
r | ** ** * **
q | *** * ** ***
u | * *
e | ** *
| * ***** *
| ** * *
|** *
+-------------------------------------------------------------
Engine Speed (RPM) ^ ^ ^ Red Line
| |Peak Horsepower
|Peak Torque
Horsepower is, basically, the product of Torque and RPM. As such, what
you see advertised is the PEAK horsepower, which on most production
(consumer) engines is near but below the rated red-line of the engine
(due mostly to the designed-in constraints on the engine - tiny valves,
inadequate fuel-delivery (carb or injection) which "artifically" limit
the "breathing" ability, thus self-limiting the top-end power; typically
most "production" engines that red-line at, say 6000rpm, probably reach
peak power at 4500-5000rpm, optimum shift point well below red-line; the
most notable exception I am familiar with is the Lotus 1600cc (1558, as
rated) engine - with two Stromberg 175CD2 carbs (that is to say, two 1.75
inch throat carbs - can flow more air than many modern 3-5 liter V6-8s!!)
which, once it hits about 3000rpm, climbs continously to 6500 (red-line)
and just begs to go more (I've bumped 7000rpm, before I realized just how
fast I was accelerating...), in race trim that engine is rated 8000 to
9000rpm, yielding an easy 300hp, full-blown Cosworth BDA is around 500hp
(? - it's been years since I read these numbers), with turbo versions
(Formula 1 class) giving 800hp, for a few hours anyways...).
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?
Well, basically, everything. Note that you need the torque curve to answer
this question - peak torque (or horsepower curve/torque, they are linearly
related HP = Torque * RPM) only gives you a single data point, not the
whole driving experience. A flatter torque curve will give you a more
"responsive" feel - it will come off the line quicker, etc. A peaky torque
curve (typically having a higher torque peak but lower "average" torque)
will be slow off the line, but once you hit the peak region (aka "come on
the cam") BOOM off you go. Exhaust-dirven Turbochargers of course compli-
cate things with their lag. Since 99.9983 percent of "civilian" driving is
done at under half of red-line, most people are better served by a flatter
torque curve, optimizing for low-end torque at the expense of peak torque
at the top-end [my opinion]. If, on the other hand you are seriously racing,
then whatever driving is not done at full power is at full braking (to a
first order approximation), so you want peak horsepower = peak torque at
highest possible rpm and just shift to always be running the engine at the
peak.
Are there any good books that explain this kind of information?
There is (or was...) a book titled something like "The Design and Tuning
of Competition Engines" that is very good. Don't remember the author, it
is buried in one of my boxes that I really will someday unpack . . .
Also, as for what tools to buy, I buy Craftsmen tools exclusivly.
They're not as good as Snap-On, but, they cost a fraction of the
cost of Snap-On, and are far easier to exchange on the weekends
(when I do most of my car work). I could understand a professional
mechanic buying them, but, I don't use my tools all day, every day.
Oh, all right, I'll join the fray. 98% of my tools are Craftsman. The
basic tools (screwdriver, sockets, wrneches) are plenty fine for just
about any "home" use, which includes rebuilding Lotus engines [Machine
shop work farmed out...]. Although I ended up with Craftsman Torque Wrenches
that was as much because I needed them and couldn't find a Snap-On dealer
at the time. For real work (say my 1/10,000th micrometer) I would buy a
"real" tool (e.g., Starrett), for, e.g., measuring valve shims.
Anyway, that's my $0.02 worth.
I see your $0.02, and raise $0.03. That's $0.05 to you, bub.
-RDH
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