In your answer to the reader that was buying the 1977 Ford Thunderbird a few weeks ago, you said the HP rating changed in 1972. It is now measured as "net horsepower" while prior to 1972 it was measured as "gross horsepower." Does that mean that todays muscle cars like the Mustang GT with the 5.0 V8 (412 HP) is measured with every option loaded onto the motor? Would that mean that the engine is actually putting out more HP at the flywheel? Is there a way to convert this measurement back to gross horsepower so pre-1972 muscle cars and todays muscle cars can be compared equally?
I’ve gotten more follow-up questions to this column than any other, so we’ll spend a little time further exploring the answer. It seems that we “classic car folks” really do have horsepower on our minds. The short answer to your questions are No, No, and No.
Now for a slightly longer answer.
Prior to 1972, horsepower figures were quoted as “gross horsepower,” or “brake horsepower” leading many people to believe that horsepower was being measured at the rear brakes. This would include any loss in horsepower from the transmission, driveshaft, differential, etc. In reality, horsepower has always been measured at the crankshaft, even prior to 1972. A “brake” was simply a device that was used to place a load on an engine at the crankshaft.
The basic difference between “gross” or “brake” horsepower, and net horsepower, has more to do with the lax standards prior to 1972, and some semblance of standardization after 1972.
Prior to 1972, test engines were measured using few if any power robbing accessories such as the transmission, differential, water pump, alternator, and power steering. They were often fitted with headers instead of exhaust manifolds, and atmospheric conditions such as temperature, barometric pressure, and humidity were all set at values that would maximize horsepower.
After 1972, an engines power output was quoted as “net horsepower,” and just like “gross horsepower,” it was measured at the crankshaft, so it too does not take into account drivetrain losses. However, it is measured with an air cleaner and an exhaust system installed, as well as belt-driven accessories and emission controls. Naturally, these horsepower ratings were significantly lower than those of the previous year.
Now that we have a better understanding of the different ways of measuring horsepower, I want you forget all about it. That’s because we haven’t taken the “fib factor” into account. And believe me, the “fib factor” accounts for just as much variation in quoted horsepower rating as parasitic loss, barometric pressure, or exhaust headers.
As hard as it may be to believe, manufacturers actually fibbed in order to increase sales! Prior to about 1970, during the “horsepower wars,” ratings were often inflated to entice buyers away from a competitive manufacturer. Then, beginning around 1970, real horsepower ratings were often deflated in an attempt to ensure the insurance industry that they would not sell vehicles with ridiculous amounts of horsepower (wink, wink) to the general public. Honestly, how many people really believed that a Boss 302 Mustang put out a mere 290 horsepower?
Variations in measurement methods, as well as the “fib factor,” make it very difficult to state with any certainty what the actual horsepower output of any classic car engine really was.
In 2005 the SAE (Society of Automotive Engineers) introduced a new rating called “SAE Certified Power.” The test is voluntary, so not all manufacturers use it. It gets even more confusing because some engines that have been tested to these standards showed an increase in horsepower, while others showed a decline. Cadillacs Northstar V8 went from 440HP to 469HP, while Toyotas 3.0 Liter engine went from 210HP to 190HP. Other manufacturers showed similar variations.
There is really no way that I know of to accurately compare the horsepower output of classic muscle cars with those of todays muscle cars. I think that in a general sense, todays engines are more efficient and produce more horsepower per cubic inch of displacement. However, I don’t think that there’s any substitute for the massive torque that pushes us back into the seat of our classic muscle cars whenever we step on the “go pedal.” Which kind of makes the point moot.