You are correct and I admit that I was partially wrong. I have researched this and found the following from an automotive web page explaining Torque and Horsepower; I now understand the reason for much lower horsepower readings on the charts:


"There are many different ways of measuring the power of an engine. Brake horsepower is a common term and refers to the power measured at the crankshaft using a "prony" brake. A prony brake was a simple lever that was connected to the crankshaft with a brake. As the brake was applied, the lever would deflect and power was indicated. Nowadays, computers and strain gauges are used to measure the power.

Rear wheel horsepower is just what it implies - horsepower applied by the rear wheels. A chassis dynamometer is used to measure the power transferred by the drive wheels onto the dynamometer's rollers. Brake horsepower is much lower than the horsepower specs provided by the manufacturers, because of all the power used by the transmission, final drive, and accessories.

Gross horsepower is the power measured at the crankshaft without any engine accessories. This includes removing the air cleaner and ductwork, exhaust system, water pump drive, and any other devices driven by the engine. Prior to 1973, horsepower specifications were listed by the manufacturers as gross horsepower. After 1972, manufacturers provided net horsepower specifications.

Net horsepower is also measured at the crankshaft before it goes into the transmission, but this time the engine is operated as it is installed in the vehicle, complete with accessories and ductwork."