To explain this pound feet concept it means applying force (in pounds) to an object to move it a certain distance
(in feet). If you were to tie a 3,000 pound vehicle to the back of a horse in a similar manner to a carriage the horse
should be able to pull it an average of 11 feet in a minute. 3,000 pounds multiplied by 11 feet equals to 33,000 pound
feet of work.
Horsepower is the term used to indicate how much work an engine can perform or how well it can perform this work.
The horsepower of an engine is very important and is given by the manufacturer, reviews and any other magazine or
videos about a vehicle.
The term horsepower was invented by the inventor of the steam engine Mr James Watt as a method of advertising
and marketing his steam engines.
He calculated that the average horse could perform approximately 33,000 pound feet of work per minute (550 pound
feet per second).
A 370 hp engine could do the work of 370 horses. Adding more horsepower to an engine almost certainly results
in a faster car because using the example above 10 hp is 330,000 pound feet of work so the same 3,000 pound car could be
pulled a distance of 110 feet in the minute.
110 feet per minute equates to approximately 20 km per hour which is very slow for a top speed in the 21st century.
That's why most vehicles manufactured in this era have 100 hp and upwards. You should also take into consideration that 500 hp
will not pull the 3,000 pounds 50 times the speed of 10 hp because of aerodynamic
Horsepower is closely related to engine torque. In fact torque and RPM (revolution per minute)
is used along with a constant figure of 5250 to calculate how much horsepower an engine produces. The formula for calculating
Torque x RPM
You can see from this formula why the peak horsepower of an engine is always given at a particular RPM. Peak horsepower
is always a result of engine torque although they may peak at different RPMs in many cases. Let me give some examples and explain
The three examples used above are from one engine with figures I created to explain the relationship between torque, RPM and
horsepower. It is common for the peak torque to be spread across an RPM range and not just at one spot so at the higher end of the
torque peak there will naturally be more horsepower.
| 250 x 3000
5250 = 143 hp
| 250 x 4500
5250 = 214 hp
| 200 x 6200
5250 = 236 hp
Even when the revolutions are too fast to achieve peak torque such as in the third example horsepower can still increase because
the hp requires RPM and torque so if the torque reduces by a small percentage but RPM increases hp will continue to rise but eventually
as the RPM rises higher out of the peak torque range the torque will drop low enough to now reduce the peak horsepower.
The relationship goes even further. Generally an engine designed to have peak torque at lower RPMs will have low horsepower,
usually lower hp than torque (think of some diesel engines or the Bentley Mulsanne). An engine designed
to have peak torque at higher RPMs will have plenty horsepower even if that engine produces relatively low amounts of torque.
The benefits between horsepower and torque is that more horsepower is generally required for a high top speed while more torque is
important to launch the vehicle from a stand still, it also becomes important if you constantly drive up steep and/or long hills and
towing heavy loads.