Horsepower and Torque 
Technical Articles:

Horsepower 
The definition of 1 horsepower is the ability
to do 33,000 poundsfeet of work in one minute. So horsepower is a
measurement of torque over time. The measurement of horsepower was
invented by James Watt when he replaced horses turning a water pump with a
steam engine and wanted to know how many horses the engine could replace
in order to rate the engines power. He figured that a horse could pull
with 180 lbs. of force. The horse traveled in a circle with a 12 ft.
radius, and could make 144 complete revolutions in 1 hour. This means that
the horse traveled 181 feet per minute. 180 lbs. of force times 181 feet
equals 32,580 poundsfeet of work in 1 minute. Watt rounded that up to
33,000 poundsfeet of work in 1 minute. 
Figuring Engine Horsepower 
Watt's definition of horsepower is a horse
pulling on lever, so the power was applied to the lever. An engine does
not work like this, the power is made at the crankshaft. So to know the
horsepower of an engine you will need to imagine a 1 ft. lever at the
crankshaft and figure the circumference of that one ft. lever and how many
rotations per minute it is traveling. A 1 ft lever will have a
circumference of 6.2831853 ft., so the formula for hp would look like
this:
HP = (6.2831853 × RPM × Torque) ÷ 33,000
And can be simplified to:
HP = (rpm × Torque) ÷ 5252
By using this formula you can see that HP and Torque curves will always
cross at 5252 rpm. To help put horsepower into perspective, imagine two
engine that both make 300 ft. lbs. of torque One engine makes it at 3000
rpm and the other makes it at 6000 rpm. Which one makes more power? The
one revving at 6000, because the same amount of torque is being used twice
as fast. Gearing the engine down to the the same 3000 rpm will double the
torque at the final drive making it 600 ft. lbs. If both engines had the
same final drive, they would be able to do the exact same amount of work
(300 ft. lbs. worth), but the 6000 rpm engine will do it in 1/2 the time. 
Torque 
Torque is simple, imagine a 1 ft. lever with
1 lb. of force pushing on it. That is 1 pound ft. of torque. The formula
to figure torque from a known HP is:
Torque = (5252 × HP) ÷ rpm 
Brake Horsepower and Torque 
Dynamometers measure torque by use of a
brake. Brake torque is simply a measurement of how much resistance is
needed to hold the engine at a steady rate of speed. This is commonly
referred to as a step test, usually taken in 250 rpm increments. Brake
Horsepower (BHP) is then figured using the above formula. The problem with
brake torque is that it is not effected a measurable amount by the inertia
of the engines rotating and reciprocating parts. For an acceleration
engine, there is a better way. 
Acceleration Horsepower 
For an acceleration engine, you want to
measure power as the engine is accelerating. You don't only care how much
HP the engine has, you also want to know how fast it can rev up. This will
take into account all of the inertia of the rotating and reciprocating
parts. So instead of a brake holding the engine at a steady speed, the
brake holds the engine at a steady acceleration rate, 300 rpm per second
and 600 rpm per second are commonly used. Hp and Torque readings will be
lower, due to less braking force required, but the mass of all the moving
parts will be better measured. You could have two engines put out the
exact same brake power readings, but if one has much lighter pistons crank
and rods, it have more acceleration power and be quicker at the track. To
put this into perspective, it is easy to lose 6 lbs. from a stock
smallblock Chevy crankshaft and actually make it stronger in the process.
My Camaro has a 3.27:1 1st gear ratio and a 3.50:1 rear gear ratio making
a total ratio of 11.445:1. Losing 6 lbs. rotating weight is like losing
over 68 lbs. off the car in 1st gear. That's just the crank, using lighter
pistons, rods, pins, and aluminum flywheel can get the rotating and
reciprocating mass over 20 lbs. lighter. Now 229 lbs. lighter in 1st gear
is a big difference. You see why people get crazy to loose a few lbs. 
Building Power 
Many engine builders will say that torque is
what wins races and too much emphasis is put on horsepower. It is true
that torque is what actually pushes the car, but torque and horsepower are
directly related, you can't have one without the other. The chart below
shows how you can increase the rpm range of an engine to get more
horsepower, at the cost of torque, but still make more rear wheel torque.
Engine 
Horsepower @ RPM 
Flywheel ft. lbs. @ Peak Horsepower 
Final Drive Ratio 
Rear Wheel ft. lbs. @ 1000 RPM 
First 
500 @ 6000 
437.6 
6:1 
2625.6 
Second 
550 @ 8000 
361.1 
8:1 
2888.8 
In this example, both engine have the same rear wheel speed, but the
higher rpm engine, which makes less flywheel torque, makes more rear wheel
torque. Torque does win races, but you need horsepower to put it to use,
so saying torque is more important than horsepower is really a
contradiction. For a street car, an 8000 rpm engine and steep gears are
not practical, but for all out drag racing, build as much horsepower as
you can and just gear it to suit. If you build the horsepower, the torque
will be there through gearing. The engine with the most average hp for the
rpm range used will have more average power, period. 
