Horsepower and Torque

Which one really matters?

There is a lot of confusion about torque and horsepower out there. To have a fast car, some say you need torque, and others say you need horsepower. Then there are the advertised horsepower and torque numbers given by manufacturers, and the different engine speeds (RPM) that they can be rated at. It helps to understand how torque relates to horsepower, and how this data can translate into driveability and performance. Torque and horsepower are not independent values, that is, at a given rpm, there is only value of horsepower for a given value of torque. But wait, you say how come all the small engine guys brag about horsepower numbers and all the large displacement engine guys brag about great torque numbers? For most people, this is due to a lack of understanding the benefits and limitations of one engine design over another. The reality is that at 5252 rpm, every engine produces the same amount of horsepower as it does torque (in ft-lbs.). Take a look at the hp calculator and play with some of the numbers.

So why do engines normally have different torque numbers than horsepower numbers? Well, that is because although the horsepower and torque are the same value at 5252 rpm (assuming the engine can operate at that speed), the peak numbers for horsepower and torque typically occur at different points. It is these peak numbers that are used for advertising purposes, and most people don’t really care where the peak occurs, but they look at the number. The reality is that engines that produce peak power at a lower rpm have their ideal applications, and those that produce peak power at a higher rpm have their applications. For instance, no competitive race car can put up a good race if everyone else is running 2000 rpm higher than he is. This is due to the simple reality that horsepower and torque exhibit a linear relationship with engine speed. The ticket to the biggest horsepower numbers is to design the engine to operate at the highest speeds practical.

Of course, there is no free lunch, and there is to be some tradeoff to achieve these higher operating speeds. Not the least of which is the intended application of the vehicle. You see, the same design that allows huge horsepower and torque numbers and high engine speed will also increase wear and tear on engine components, result in smaller power bands, require more shifting, and generally decrease engine efficiency. Of course, there are many clever ways to minimize these negative aspects, such as Variable Valve Timing, Variable volume intake manifolds, and many others. At the cost of increased complexity and bulk, these methods can allow one engine to operate effectively over a wider rpm range than might otherwise be possible.

By the same token, a lower rpm motor will last longer, generally be more efficient, have a wider effective operating range, require less gears and less shifting, and produce less power than a similar engine capable of a higher operating speed.

Torque is a simple concept. Anybody who has ever used a torque wrench will understand. If you put 50 lbs of force at the end of a 1 foot long wrench, you have applied 50 ft-lbs. of torque. Horsepower is simply the rate at which that torque is applied to the wrench. For instance, if you were to turn the wrench at 1 revolution per second (60 rpm) and maintain that 50 ft-lbs of torque during the process, you would be putting out 50 ft-lbs of torque at 60 rpm, which corresponds to 0.57 hp. Now apply that same 50 ft-lbs of torque at 6000 rpm, and you have 57 hp. It can also be seen that above 5252 rpm, every engine will be producing a higher amount of horsepower than torque, and below 5252 rpm the opposite will be true. So the next time somebody tells you that the horsepower is what really matters, you now know you can’t change one without changing the other.

So what is the bottom line, what is most important? Well, simply put, the horsepower is the rate at which an engine is able to do work, i.e. accelerate the car. But don’t get caught up in peak numbers, always look at the curve, because the torque numbers are represented in the horsepower vs. rpm curve. If an engine produces peak horsepower at 7000 or 8000 rpm, don’t expect to have much horsepower available before about 5000 rpm. By the same measure, if peak horsepower occurs at 4000 rpm, don’t be expecting to make any screaming redline shifts at 7000 rpm.