Re: Horsepower and Torque - a Primer

[Michiel van Wessem <jmvw@xxxxxxxxx>]

Hello Bruce,

Very interesting post! 

I especially enjoyed the part where you went from work to torque. I found
it a nice puzzle. I finally felt I understood things again when I figured
out that the power needed for a certain acceleration is dependent on speed,
while the torque needed stays the same. The force that pushes the car, as a
function of power, is:

F = P v  (because W = F * d * cos(0) and P = W / t and F=m a)

So at 60 mph you'll need twice the power to give a car the same
acceleration as at 30 mph. Note that power is what consumes energy, not
torque. It seems strange to me to focus much on torque.

Your conclusion:

>The Only Thing You Really Need to Know
>
>Repeat after me. "It is better to make torque at high rpm than at low rpm,
>because you can take advantage of *gearing*." 

In other words, it's better to have a car with an impressive number of
horses, right?

That's for a car that is fast. But there are other things people want in a
car. I imagine that big, torky (spell that as you like) engines, which have
a lot of torque at *low* rpm are popular in American cars because having a
lot of torque at low RPM means that you can accelerate instantly just by
stepping on the gas while you were cruising at low RPM. If you have a nice
manual transmission, you could have downshifted in time, but automatic
transmissions have less gears and often take some time to realize that
you're not kidding. So low end torque is pleasant, so you want a fat engine
for your automatic.

Regards,

Michiel (with 87 325es which has a lot of torque at low RPM, but not much
high, so it doesn't have a lot of hp)

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