Whenever you talk about cars, one of the most important characteristic is going to be the power that it makes. This usually is expressed as X amount of horsepower. If you delve deeper you’ll probably hear about a completely different torque figure as well, making it all very confusing for the casual observer, but what exactly are these things.
Let’s start with torque. Torque, expressed in Nm or Ft-lbs is the actual twisting force that an engine makes at the flywheel. The linear motion of the pistons is converted to a rotating motion and this is measured by x amount of torque, which is basically THE power of the engine.
So what about horsepower? Horsepower in simple terms is a function of torque and engine speed (rpm). If an engine produces the bulk of its torque at low RPMs you’ll get a low horsepower figure, on the other hand if most of the torque is available at high RPMs you’ll get a high horsepower figure.
Why is this important? Small engines especially are not capable of delivering enough torque at low rpms to pull a car along. To compensate for this, engineers design the engine in a way that it obtains the same small amount of torque at high rpm. This in effect makes it more powerful because of inertia, it moves faster which makes it more reluctant to stall once is up to speed.
So why not just make an engine with enough torque at low rpm? On an internal combustion engine torque is determined by far and large by its size or cubic capacity. Other characteristics like compression ratio, number of valves per cylinder, fuel delivery and so on will have an influence but not as much as capacity. Simply making an engine bigger is not always an option. This is where forced induction comes in. Turbocharging or supercharging will vastly increase the amount of torque delivered. Forced induction is a clever way of increasing an engines capacity without making it physically bigger.
So what is horsepower? Horsepower is an all in one representation of an engine’s performance, taking into account not only its twisting power, torque but also the inertial advantages of a fast moving object, rpm.
Depending on application engines can be designed to obtain torque at different engine speeds. Truck engines, where pulling power is a priority are designed to make torque very low in the rev range, because of this they also have a very low redline, often under 3000 rpm. The net effect being that it pulls well from idle but acceleration suffers. On the other end of the spectrum are motorbike engines. Their size limits the amount of torque that can be obtained, but by designing the engine to rev higher (10000 rpm or more) they can accelerate very fast.
The future. Electric motors produce their maximum amount of torque at all revs. From 0 to very high, limited only by the current supply. As such they can both pull very well and accelerate at the same time without having to change the design of the engine.
Q&A
I installed headers, exhaust, cold air intake, larger throttle body etc. to my car and I should have a million more horsepower but the car doesn’t seem to pull as it used to. What happened?
The one and only way to increase torque is by increasing engine capacity, either physically (stroker kit) or forced induction. All the modifications above create a more free flowing engine, which means the engine is now more rev happy, which in turn means that the bulk of the torque is now obtained at a higher rpm. So yes, you do have more horsepower on paper, but you lost low end torque, so in normal driving the engine feels less powerful. The maximum amount of torque is still the same but it comes in higher in the rev range. The same thing happens if you lighten the flywheel or internal components.