From: jws@billy.mlb.semi.harris.com (James W. Swonger)
Subject: Re: Torque, Horsepower and Brake Horsepower
Date: Fri, 24 Jul 1992 14:20:01 GMT
Organization: Harris Semiconductor, Melbourne FL

 If the keepers of the FAQ list want to include this it might be a good idea.

 Torque is rotational force, the force around a circle of a given radius. Its
units are pounds-feet in the English system, pounds of force at a radius of
so many feet. A lever analogy may help. You can have a long lever and a 
small amount of force, or a short lever and more force, and it all comes
out the same. The combination of the two is the torque. Torque is one of the
two primary qualities of a rotating engine, and is the measure of how hard
it will try to twist the load.

 The torque curve is the profile of engine torque with RPM. An engine which
only produces its maximum torque in a narrow RPM range is said to be
"peaky". An engine with a broad torque curve is usually better for typical
driving, because it provides consistent torque regardless of the engine
RPM. Typically the choice of cam and valve size favor either high-RPM
torque and maximum horsepower or low/midrange torque and smoothness. There
tends to be a concentration on the former in sports cars and the latter in
mass-market passenger cars. A V-type engine tends to have the broadest
torque curve. A long-stroke inline engine tends to have low end torque but
a poor top end, vice versa for a short-stroke inline. An engine with a 
tall, narrow torque band requires more and better shifting to get maximum
performance. Torque basically translates to acceleration.

 Horsepower is the measure of work done in a unit time, 550 foot-pounds per
second. The basic definition is not necessarily a good measure of the
output of a rotating engine; it is defined in terms of linear force and
motion. However, there is still power coming out and it must still be in
the same fundamental units.

 To judge the engine by itself, the units of brake horsepower (BHP)
are used to denote a somewhat different method of measurement. If you look
at the units, power = force x distance / time, you will see that two of the
units are the components of torque. The other component, 1/time, is RPM
(although seconds is the proper time scale). Brake horsepower is commonly
measured by applying a drag torque to the engine, which is noted along with
the RPM that torque holds the engine at (or how much torque can be applied
and still maintain a set RPM). The product of the two, RPM and torque,   
corrected for units and efficiency/heat loss in the brake is the brake
horsepower. 

 BHP is not the whole story, of course; there are the power losses in the
accessories (alternator, water pump, fan, A/C) and transmission and 
differential which are dumped as heat and never make it to the tires. The
modern standard for passenger cars measures HP with accessories on, while
in the '60s and '70s the BHP was measured under the most favorable (no
accessory load) conditions. In addition BHP is very dependent on RPM. The
peak HP is only available at the high end of the range and most driving
conditions do not make use of peak HP at all. But peak HP does indicate
how much you could get out of the engine if you really wanted to.
-- 
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