Understanding the difference between gross and net horsepower ratings is essential for making sense of classic car performance claims.

Gross horsepower was the standard measurement through 1971. It measured engine output with no accessories attached — no alternator, no power steering pump, no air conditioning, no air filter. A dyno reading in ideal conditions with nothing drawing power.

Net horsepower, adopted in 1972, measures the engine as installed in the vehicle with all accessories operational. The same engine measuring 360 gross horsepower typically measured around 260 net horsepower. The engine didn't lose performance — the measurement convention changed.

This transition is why 1971 muscle cars look so much more powerful on paper than 1972 cars. The 1972 cars often used the same engine in the same tune. The comparison requires adjusting for the measurement convention.