Almost everyone knows that maximum horsepower and maximum torque are the two most common measurements of engine performance. Here are some interesting facts about them:
- Why do most horsepower figures come in certain weird numbers?
If you read the specification table frequently, you may notice that most engines have output measured in PS at certain numbers: 163, 177, 184, 258, 272, 286, etc. You may then wonder why the engineers do not put them into more precise numbers, for example, multiples of 10. Actually, they do come in a more precise form in terms of kW. The numbers mentioned above are equal to 120, 130, 135, 190, 200, and 210 kW respectively. Nowadays engineers usually set the number in multiples of 5 in kW, but due to marketing needs, the numbers are converted into metric horsepower (PS, abbreviation of the German word Pferdestärke) for customers. It is probably more romantic to think of “how many horses do I have under the lid”!
2. What is the relation between power (PS) and torque (Nm)?
The power is the overall output of the engine, and the torque is the sudden turning strength of the engine. In fact, the torque is measurable, but the power is not. The power can only be calculated using the measured torque. A way to do so is to multiply the torque (Nm or kg-m) with the engine revolution (RPM) and then adjust the product with a constant. Thus, we can simply regard power as a function of Nm and RPM. Given a certain RPM, the higher the Nm is, the more the PS; given a certain Nm, the higher the RPM is, the more the PS.
Seeing from this, the way to increase power is either increasing the torque or increasing the engine revs. However, if you look closely into the specification table again, you can find that the torque of a N/A engine is limited given a certain displacement. According to my own observation, the max torque of a 2.0L N/A engine is about 200Nm, and 300Nm for a 3.0L, and so on. That is why N/A sports cars have higher redline revs (e.g. Porsche 911 GT3 and HONDA S2000), and why F1 engines have redlines over 10,000 RPM.
So how can the torque be increased, and therefore the power? To achieve this goal without significantly increasing the redline, both increasing the displacement and adding charger(s) are feasible. Under the downsizing trend, however, large displacement is considered not so eco-friendly. So the most common solution is adding charger(s), either turbochargers, superchargers, or electric turbochargers. A charged engine can achieve the same power with higher torque at lower revs. That is why the redline differs between N/A and charged. (Check the acceleration video of the 911 GT3 RS and 911 turbo S.)
3. So which one is more important? PS or Nm number?
I would say neither. The maximum PS and maximum Nm only come out at a certain range of revs, which is often at high RPMs that cannot be reached quickly. A max power at a very high RPM does not mean that engine can provide enough power when launching from still. Therefore, the output curve is more relevant to the overall performance of an engine.
And this is where the diesel engines shine. Although diesels usually have lower PS than petrols with the same displacement, the torque of diesels is larger and comes out earlier. The diesels may lose when racing at high speed, but at the launch they are rocket-fast. This is also why most trucks are diesels. The heavy-duty diesel engines do not have high PS numbers, but they provide enormous torque at only 1,000 or 2,000 RPM. After all, the purpose of trucks is to transport, not to race!
