The engine displacement is stamped on the engine block.  Use this table to cross-reference your engine model, displacement, and net horsepower. 

* Net horsepower more closely correlates with the power the operator will experience with muffler and air cleaner installed. Net horsepower is measured at 3600 rpm (7000 rpm for engines: GXH50, GXV50, GX25 and GX35) with net torque at 2500 rpm. Actual horsepower can be affected by several factors such as application, environment, and other variables.