Maximum Rpm


The HiPerMath equations are:

cubic inches = (3.14)(bore / 2)2(stroke)(number of cylinders)

maximum rpm due to mass = 0.0056(cubic inches)2 - 8.89(cubic inches) + 11527


What you do:

Choose carburetor or throttle body sizes, intake runner diameter, intake valve lift, intake valve duration, intake valve size, bore and stroke to produce the maximum rpm you want.


Factors to consider:

1. Maximum rpm will be determined either by the mass of the moving parts in the engine or the air passages (throttle body, intake runners, intake valves and exhaust valves).

2. The higher the ci (cubic inches or in3) displacement you have, the lower the maximum rpm. Engines with longer strokes and larger pistons have more mass rotating, so there is more stress on the crank. The crank can only handle so much stress, so rpm is limited.

Small bore and stroke allow for a high revving engine. It is possible for the small bore and stroke engine to draw in as much air as a large bore and stroke engine, but the air will be drawn through at much higher rpms.

3. The larger the carburetor or throttle body size, intake runner diameter, intake valve lift, intake valve duration and intake valve size the higher the maximum rpm. Air velocity is limited to the speed of sound, which is 767.58 mph (miles per hour) or 67547.4 fpm (feet per minute). Large carburetor or throttle body size, intake runner diameter, intake valve lift, intake valve duration, or intake valve size all cause the air to move slower and reach the speed of sound at a higher rpm. This is good if you want your power to come in at higher rpms. Large diameters and lifts allow the engine to breath, increasing maximum rpm at the cost of the power coming in later.

Small carburetor or throttle body size, intake runner diameter, intake valve lift, intake valve duration, or intake valve size all cause air to move faster and reach the speed of sound at a lower rpm. This is good if you want your power to come in early and you want big torque. Larger bore and stroke do the same thing, since each stroke of the piston draws more air.


Definitions:

ci = cubic inches or in3
stroke = the distance the piston travels in the cylinder.
bore = the diameter of the cylinder.
rpm = revolutions per minute