Classic Cycles Technical Resources
85
Example
An engine has a piston displacement of 244.6 cubic centimeters and a combustion chamber volume of 41.5
cubic centimeters. What is its compression ratio?
244.6+38.2 = 282.8 = 7.4 to 1 (rounded off)
Compression ratio = 38.2 38.2
HORSEPOWER AND TORQUE
There is sometimes confusion about horsepower and torque. Horsepower is a measure of how much work
can be done in a given length of time. One horsepower is equal to the work done when a weight of 550
pounds is lifted one foot in one second, or 33,000 pounds lifted one foot in one minute. Torque is merely
twisting force developed by an engine, and is not indicative of how much work can be done, unless engine
speed is known. The relationship between horsepower, torque, and engine speed can be expressed as:
rpm × Torque
Horsepower = 5,252
Example 1
A motorcycle engine develops 30 foot-pounds at 6,000 RPM. How much horsepower does it produce at that
speed?
= 6,000 × 30 = 180,000 = 34.3 foot-pounds (rounded off)
5,252 5,252
It is sometimes desired to know how much torque is developed by an engine when horsepower and rpm are
known. The formula can then be written as:
Horsepower × 5,252
Torque = rpm
Example 2
During a dynamometer test, an engine developed 14.7 horsepower at 3,500 rpm. How much torque did it
produce?
Torque = 14.7 × 5,252 = 77,204 = 22.06 foot-pounds (rounded off)
3,500 3,500
PISTON SPEED
It is at times desirable to know the maximum speed pistons reach as they move in their cylinders. This
peak speed is reached when the piston is midway between top dead center and bottom dead center. The
formula for finding piston speed is:
Piston speed = Stroke length (in inches) × 2 × rpm
12
Example
An engine has a stroke of 2.5 inches. What is its piston speed at 5,000 rpm?
Piston 2.5 × 2 × 5,000 25,000
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