"My 2003 Z06 has 405 hp and is advertised as one of the most powerful production Corvettes ever produced...yet my brother had a 69 that was rated at 425 horsepower...some models had 450 hp....what's the story? What is the conversion factor from today's horsepower ratings to those of the 60's?"
The short answer, is that the '69's horsepower was listed in SAE-gross numbers and your ZO6 power is SAE-net, a big difference. This forms the tip of a big, confusing issue that deserves a major explanation. To do that we need to define "horsepower." Unfortunately, how it is defined depends upon who is doing the defining...
James Watt is to blame here. Watt, you might remember, was the Father of the steam engine and the originator of the term, "horsepower." He created the term in his first attempts to persuade mine owners to substitute his steam engines for horses in hoisting coal. A horse, it was known, could work at the rate of 22,000 foot-pounds per minute but Watt arbitrarily made the unit of horsepower half again as much as the average horse - shrewd marketer that he was.
In the U.S. the Society of Automotive Engineers (SAE) has, since 1903, defined one horsepower as the ability to lift 33,000 pounds one foot in one minute, or 550 pounds one foot in one second.
In Germany, however, the horsepower was established as the ability to lift 45,000 kg (there are 0.4536 kg to the pound) one centimeter (2.54 cm to the inch) in one minute, which translates to 98.629% of an SAE horsepower, and you'll just have to trust us with the math. The Germans, by the way, standardized the horsepower through a system of measurement called "JS," the abbreviation for the German term for horsepower, now classified as "DIN."
The DIN rating has been adopted by the International Standards Organization - ISO - as the European standard. Nowadays we have the JIS, or Japanese Standards Institute. It rates horsepower the same as the ISO, but both are different (not by much, but they're still different) from the SAE. For practical purposes, consider them all equal. Okay so far?
How horsepower is measured depends upon circumstances and history. Back before 1972 American manufacturers used SAE Gross horsepower to rate the output of their engines. To measure horsepower they mounted an engine on a test stand and hooked it up to a dynamometer (more on that later, too). It is important to note that the engines, as tested, had no accessories attached. Power was measured at the bare flywheel and the amount was, by definition, gross horsepower.
In 1972, American manufacturers phased in SAE Net horsepower. This is the standard on which current American ratings are based. This rating is measured at the flywheel, on an engine dyno, but the engine is tested with all accessories installed, including a full exhaust system, all pumps, the alternator, the starter, and emissions controls. Both SAE net and SAE gross horsepower test procedures are documented in Society of Automotive Engineers standard J1349.
Note: Because the test equipment used back in the "old days" to measure gross horsepower is different from today's setup, it is impossible to provide an absolute conversion from one to the other. A rule of thumb, however, is to assume that SAE Net is about 80% of SAE Gross. In the case of Jim's question above, the 1969 Vette's engine put out about 340 horsepower in today's ratings.
Hang on, now, because this gets worse. For decades those of us who've delved through road test magazines and vehicle brochures have used the term "brake horsepower." Simply put, nowadays brake horsepower is nothing more than another way of stating SAE Net horsepower, although before 1972 it referred to Gross horsepower.
However, it's the brake that measures all this, either in the form of a Pony Brake (a device that applies friction to the output shaft) or the Fan Brake (absorbs power by the resistance of a fan in air or water) or the Eddy Brake (uses a metal disk in a magnetic field.) These methods have generally been replaced by the dynamometer.
A dynamometer is nothing more than a heavy-duty electric generator that is either hooked up to the engine's flywheel on the test stand, in the case of the manufacturer, or driven by a car's wheels in the case of a chassis dynamometer. It measures the engine's power by measuring how many watts (yeah, James Watt managed to get the unit of electrical power named after him) of electricity the engine is capable of generating, or how many watts it takes to stop its turning.
We told you to hang on, didn't we? You remember from our little history lesson above that horsepower is really kind of an arbitrary term. After all, when is the last time you measured how many pounds a horse could lift? In engineering circles, engines are rated in terms of thermal efficiency and output in watts (thousands of watts actually, or kilowatts.) As far as the SAE is concerned, one horsepower is the electrical equivalent of 746 watts. The JIS/DIN equivalent is 735.5 watts, but let's not quibble anymore.
Whichever standard one happens to choose (it's really where you live) the measurement technique is the same. The engine, or entire car, is hooked up to the dynamometer and run to full rev. The resultant number of kilowatts measured is converted to horsepower and there you are.
Back in the muscle car era the manufacturers loved to advertise horsepower. It got people into the showrooms and sold cars, as it still does. GM, in some cases, underrated its horsepower figures because of insurance and liability concerns, but always managed to advertise just a little more than the competitors. Power means speed, etc.
Actually, horsepower numbers should be replaced by torque numbers, but that's the subject of another article...