Horsepower and torque are terms frequently used, without any real explanation for what they mean. We all have a reasonable sense of meaning, but there is more to it to understand the relationship these words have to your classic car especially if you plan on changing your engine.
Horsepower is a made-up measurement. Steam engine pioneer James Watt wanted to compare the effectiveness of his engines against the horse that previously did their work. Watt defined one horsepower as the equivalent of the energy expended by a single horse lifting 33,000 pounds one foot in the air from the surface of the Earth in one minute. Mathematically speaking, horsepower is the force needed to move 550 pounds one foot in a second, or 33,000 pounds one foot in a minute. Engine horsepower is measured using a dynamometer, but what the dynamometer really does is measure the torque output of the engine, as well as the RPM – or “revolutions per minute”. These numbers are plugged into a formula (torque x RPM / 5,252) to determine the horsepower. Horsepower is found by measuring torque because torque is easier to calculate.
Not an easy thing to imagine. But it illustrates the point about horsepower being a measurement of the ‘work’ an engine can do. The main thing to remember is that, in general, the higher the horsepower, however it is defined, the faster the car.
Basics of Horsepower and Torque – What You Need to Know
Not many people understand what horsepower and torque really mean, let alone how it affects the performance of the classic car. Nevertheless, almost every ad for a heavy-duty truck will mention those specs at some point. If you have never noticed, try listening for it next time you see one.
Here, we take you through everything about engine power you need to know.
In essence, HP, BHP, PS, and CV all refer to the amount of power that an engine produces. Or, in less technical terms, how much ‘work’ it can do. As a rule of thumb, the higher the horsepower, the faster a car can ultimately go.
Power also affects acceleration. Again, as a rule of thumb, the higher the horsepower, the quicker a car will gain speed.
The power produced by an engine is called its horsepower. In physics, power is defined as the rate at which something does work. For cars, horsepower translates into speed. So, if you want to go faster, and get up to speed quicker, you need more horsepower.
Horsepower (HP) vs Brake Horsepower (BHP)
Traditionally ‘brake horsepower’ (bhp) has been used as the definitive measurement of engine power. It is distinct from horsepower because it considers power loss due to friction – it is measured by running an engine up to full revs, then letting it naturally slow down to a dead stop.
If that is still not clear, it is nothing like as complex as the math involved in working it all out.
To further complicate matters, European and Japanese car manufacturers use a metric measurement of horsepower. Most quote their engines’ power using the German term PS (pferdestarke, the literal translation of ‘horsepower’), though some use the French term CV (chevaux vapeur).
One metric horsepower is slightly more than one brake-horsepower. That is why you will see, say, an Audi RS 3 marketed as producing 400PS, but a British car magazine will quote a power figure of 394bhp.
Why the difference? Well, on the part of the car magazines, old habits die hard. And on Audi’s part, it keeps its marketing consistent across the world. And 400 just sounds better than 394, doesn’t it?
Talking Torque
You might be wondering where torque comes into all this. In simple terms, torque is a different measurement of the work an engine can do, taken from a different part of the engine. It is possible for an engine to have high power but low torque, or high torque but low power. The ideal is an engine with lots of both.
Torque figures tell you how strongly an engine ‘pulls’, particularly at low revs. If you have driven a diesel car you will be familiar with the sensation of the car surging forward from, say 50 to 70mph in a way that gas-engine cars struggle to match. It is the innate high torque of diesel engines that produces that surge.
Think of the difference between power and torque as the difference between an explosion and a fire.
Torque, on the other hand, is the expression of a rotational or twisting force. In vehicles, the engines rotate around an axis, thus creating torque. Torque can be viewed as the “strength” of a vehicle. It is the force that rockets a sports car from 0-60 in seconds and pushes you back in the seat. It is also what powers big trucks hauling heavy loads into motion. Torque, as previously mentioned, is the expression of a twisting force and is measured in units of force times distance from the axis of rotation. So, for instance, if you use a 1-foot-long wrench to exert a force of 10 pounds on the end of a bolt, then you are applying a torque of 10 pound-feet (10 lb-ft).
These are the basics of horsepower and torque, but how are these concepts measured and how are they interrelated?
Relationship Between Horsepower and Torque
Both horsepower and torque affect a car’s overall speed, so you can see why people mix the two up. However, in the real world of driving and hauling, their differences—along with vehicle’s design—have a significant impact.
For instance, the greater the horsepower an engine produces, the higher the torque potential. The way this “potential” torque translates into real-world applications is through the vehicle’s axel differentials and transmission. This explains why a race car and a tractor that the same amount of horsepower has can vary so drastically. In a race car, all the torque is used for speed through the gearing, whereas the tractor converts the horsepower into pushing and pulling extremely heavy weights.
Another way to understand horsepower vs. torque is twisting off the cap on a new pickle jar. When you are using all your strength to open the jar, you are applying torque—regardless of whether the cap comes off. Horsepower, however, only exists with movement. So, you need torque to loosen the lid first, and then you can apply horsepower with your hand by spinning the lid of quickly.
So, which is best to have more of in your classic vehicle—horsepower or torque? It all depends on how you intend to use your classic car or classic truck. A lightning-fast Dodge Charger for example will have more horsepower, while a Cummins Diesel truck will have more torque to help pull those heavy loads.
At the end of the day what does all this math and science mean? For most of us classic car owners – nothing; unless you are using your classic car for racing or an engine builder, because for most owners the difference is not meaningful. For instance, I have 400 horsepower in my ‘65 Cobra which is more than enough to make me a bit uncomfortable and about the same in my ’68 Chevelle which is ample to drive to my various cruises and events.
