During the time that I’ve been writing this column I’ve received many questions that were more general in nature than those that I usually answer. Rather than answer all of these similar questions individually, I thought that it would be interesting to more fully explore some of these topics.
This week I’m going to take one subject and look at it in more detail than usual. Hopefully you’ll enjoy it. Depending on your feedback, I might continue this format for a few weeks. This week's topic is Horsepower.
The ability to move 33,000 pounds one foot in one minute. That is 1 HP. But I don’t own any cars that weigh 33,000 pounds. The engine in my 1972 Cougar produces about 163 HP, so some simple math dictates that it should be able to move 5,379,000 pounds one foot in one minute. All of my worldly possessions combined, including my house, don’t even come close to weighing over 5 million pounds, and I can’t imagine any situation in which I would need to move them one foot in one minute.
Horsepower is a measurement of work done over a period of time. In fact, the formula is HP = Work/Time. Going back to our original example, 1HP = Move 33,000 lbs 1 foot (work) /1minute (time).
The amount of horsepower produced by the engines in our cars are measured on an engine dynamometer, but the dynamometer cannot measure horsepower directly. It must first measure torque, and then convert the torque to horsepower using the formula Torque x RPM / 5252 = horsepower. Using this formula we see that an engine that produces 300 pound-feet of torque at 4000 RPM makes 228 HP, or 300 x 4000 /5252 = 228. Keep in mind that as the RPM changes, so does the torque, and therefore, so does the horsepower. So it’s a little misleading to say that an engine makes 300HP, or 400, or 500. All engines make different amounts of horsepower at different RPMs.
Which leads me to my next point. What does all of this mean, and how does it relate to the performance of our cars?
I think that most of us understand that, in general, the more horsepower any given car has, the faster it will be. But it’s not that simple.
Let’s take a make-believe 1970 Camaro with a 300 horsepower engine. Don’t forget that it only makes that 300 horsepower at a specific RPM. The RPM at which it makes the 300 horsepower is dependent on many different variables including, but not limited to the engines bore, stroke, camshaft profile, intake and exhaust design, and compression ratio. The variables are determined by the manufacturer, or engine builder depending on the performance needs.
If the Camaro is going to be raced at a dragstrip, it will need to make as much horsepower as possible very low in the RPM range because the car will be starting from a standstill every time it races. If it will be raced on a road course, the engine will need to be built in such a way as to make the maximum horsepower in the middle and upper RPM ranges because that is where the engine will be operating during most of the race.
Suddenly it becomes apparent that the published horsepower figures that we see really mean very little. A simple graph showing how much horsepower an engine makes as the engines RPM increases would be much more useful, and that is exactly the information that a “dyno chart” provides.
Now that we’re all experts on horsepower, forget everything I just told you, because during the time that our classic cars were built, the manufacturers umm…err, fibbed when it was convenient for them.
During the “horsepower wars” of the 1960’s, manufacturers could get higher horsepower figures by stripping the test engines of accessories such as alternators, or even water pumps! After all, the engine only had to run for a short time. This backfired when the insurance companies began to notice some very high horsepower figures, and perceiving a higher risk, they charged a higher premium, resulting in decreased sales for the manufacturers. In order to reverse this loss of sales, the manufacturers went in the opposite direction by under-reporting actual horsepower figures and leaving it up to buyers to look at test reports in the enthusiast magazines of the era. An example of this would be the 1969 Camaro Z-28 or the 1969 Boss 302 Mustang. It is generally accepted that the reported horsepower of both of these engines was grossly underrated at only 290 horsepower.
Finally, in the early 1970s the SAE (Society of Automotive Engineers) stepped in with standardized test procedures and horsepower ratings were more consistent.
So what does all of this mean? Nothing. Unless you’re a professional racer or engine builder, because you can’t really feel horsepower. The force that pushes you back in your seat when you step on the gas pedal is the “torque” that the engine produces. But that’s a story for another day.