Calculating Horsepower

Using bore and stroke and RPM, caculating horsepower is a breeze

| September/October 1980

  • Calculating horsepower
    Calculating horsepower

  • Calculating horsepower

Many times a collector would like to know the horsepower of an unmarked engine. If one knows the bore and stroke, which are easily measured, an estimate of the horsepower rating of an engine can be obtained by using the given chart. Instructions are as follows:

1. Draw a straight line connecting the measured values of bore and stroke, on columns A and B. (An example is the dotted line connecting 5" diameter bore and stroke of 8".) The result on column C is the displacement (200 cubic inches).

2. Connect this point (on column C) with the speed of the crankshaft in RPM, given in column D. (In the example, the 200 cubic inch point is connected by the second dotted line with 500 RPM.) The resulting estimated horsepower is given in column E. (Example, 5.5 HP.)

Since the horsepower developed by an engine is directly related to crankshaft RPM, the collector must either measure the RPM for normal operation, or estimate, if the data is not available from markings or literature.

The chart, which is called a nomograph, is based on the formula:

HP = (BORE)2 X STROKE X RPM/18,000