Pictured is a crank with rods and pistons. The diameter of the crank determines the stroke of your piston.

What you do:

Choose a stroke that give you the power you want and the rpms (revolutions per minute) you want.

Factors to consider:

1. Stroke is the distance the piston travels from the bottom to the top of the cylinder and it is measured in inches.

2. The longer the stroke, the more torque your engine will have. Longer strokes draw more air into the engine per rpm, so they develop horsepower faster (at lower rpms), which means they have more torque.

3. The longer the stroke, the lower the maximum rpm of the engine. Longer strokes require longer rods and larger cranks. Longer rods and larger cranks have more mass. The more mass you try to spin, the lower the rate at which you can spin it before it breaks.

The HiPerMath equations are:

displacement = number of cylinders x 3.14159 x (bore / 2)2 x stroke

maximum rpm due to mass = 0.0056 x displacement2 - 8.8889 x ci + 11527

4. Stroke also determines piston speed, which is a factor in air velocity.

The HiPerMath equations are:

average piston speed in inches per minute = 2 x stroke x rpm

average piston speed in miles per hour = 2 x stroke x rpm x 60
                                                                            12 x 5280

air velocity through intake valve in feet per minute = 2.616 x 2 x stroke x rpm x 3.14159 x bore2
                                                                                      average open intake valve area x 12 x 4


This is a close up of a crank. From left to right are a rod journal, the main journal that gets fastened to the block and another rod journal.


displacement = how many cubic inches (in3) your engine is.
stroke = the distance the piston travels in the cylinder.
bore = the diameter of the cylinder.
rpm = revolutions per minute