Due to a series of lawsuits most small engine manufacturers no longer publish horsepower (HP) ratings for their small engines.
If you’re old school like me and can only think of small engines in terms of horsepower (HP) it’s handy to know the formulas to determine approximate small engine horsepower.
Definition of Horsepower:
Horsepower = 550 foot pounds per second or 745.7 watts.
Watts doesn’t really matter in our calculations but I’m a nerd so let me be.
Torque Rating Method:
You have to know your engines torque rating to calculate horsepower using the torque rating method.
If it’s not on the engines model plate, most manufacturers publish torque ratings of their engines on their websites.
Once you have the torque rating you can use the following formula to calculate small engine horsepower.
Torque x Speed (RPMs) divided by 5252 = approximate horsepower.
(Example: 1450 Series engine = 14.5 torque, 14.5 x 3600 RPM =522000 divided by 5252 = 9.9390708 HP)
To use the cc method you have to know the cc of your small engine.
Most manufacturers stamp the cc directly on their engines or paint it on the cowling.
If your engine does not have this it’s probably available on the manufacturers website.
Once you know your engine cc you can use the following formula to calculate small engine horsepower.
cc divided 29 = horsepower. (Example: 100cc engine = 100 divided by 29 = 3.44827586207 HP).
The formulas are not giving actual horsepower, the are giving approximate horsepower.