Real Horsepower of Muscle Cars

The reality is that muscle car engine horsepower ratings were inflated. They were obtained under the most optimistic situations. Measurements were usually obtained with no accessories, no air filter and a dyno exhaust that were often consisted of large diameter piping and dual high capacity truck mufflers. Accessory drive parasitic loss can easily account for 30-35 hp alone. There’s a test of a 440 Magnum on Comp Cams website. That engine dyno’d 356 hp driving only the water pump and alternator, no air cleaner and dyno exhaust. In their race bulletin back in those days Chrysler even admitted that the 440 Magnum rating was optimistic and closer to 320 hp (I assume as installed in the car.). The later SAE net ratings were closer to reality. The drop in horsepower was not due as much to the drop in compression (about 4% per point) but due to more real world manner in which the measurements were obtained.