Power vs Torque, a Cars 1 Analogy
While trying to take a nap last Sunday afternoon, my mind wandered off to a scene in Cars 1 where Lightning McQueen was struggling to pull Bessie. I mean sure, Bessie is a huge, heavy, road paving machine. But isn't Lightning McQueen supposed to be a high powered race car? If so, why was he struggling to pull Bessie? Where did all his horsepower go? The answer I came up with (if true) helped me better understand the differences between horsepower and torque.
A quick search online will reveal hundreds of articles explaining the terms horsepower and torque. No matter how many times I read them though, I just don't get it at times. The easiest way I can sum them up is, horsepower determines how fast a car can go, while torque determines how strong a car feels when you step on the gas. However, the most important thing to note about the two though, is the formula for calculating horsepower; Horsepower = (Torque X RPM) / 5252. From that formula alone you can see that more torque, or higher RPM, or a combination of both produces more horsepower.
So how does that tie in to the scene in Cars 1 with McQueen struggling to pull Bessie? It goes like this.
Since Lightning McQueen is a race car based off stock car racing, I'm going to assume his engine can produce around 800 horsepower and has a redline of 9000 RPM. Since they race on high speed oval racetracks, his engine is most likely tuned to only make power at the higher RPM range, probably around the 7000-9000 RPM range. Which means, the reason he is struggling to pull Bessie, is because he just doesn't make enough horsepower at the lower RPM range. In other words, he doesn't make enough torque at low RPM. Also, at low RPM, he is nowhere near his engine's power band.
A car's engine feels strongest when driving in the engine's power band. The typical definition of an engine's power band is the RPM range where the engine makes peak torque, up to the RPM where the engine makes peak horsepower.
In our hypothetical example of Lightning McQueen, his engine's power band is at the 7000-9000 RPM range. In my own car, a 2013 Mazdaspeed3, my engine's power band is around 3000-5500 RPM. Any time you step on the gas while you are in the engine's power band, the car will feel like it has a lot of pickup. It will feel strong, it will feel torquey.
Going back to the scene where Lightning McQueen was struggling to pull Bessie and taking into account the formula for calculating horsepower, we can see that in that instance, Lightning McQueen's RPM was too low to generate enough horsepower. Since he has a high strung engine, his engine also most likely didn't make enough torque that low in the RPM range, which again means not enough horsepower. To generate enough horsepower to easily pull Bessie, he had to be spinning his engine at like 7000 RPM, but since he was starting from 0 mph, there was no way to get to 7000 RPM without first going through 1000, 2000, 3000 RPM where he didn't make enough horsepower. (Having a very short 1st gear would have solved this issue, but seeing as he was struggling to pull Bessie in that scene, he most likely didn't have a short enough 1st and 2nd gear.)
So, at the end of the day, all we (I) really care about is horsepower. Horsepower determines how fast a car can go and how strong it feels in everyday driving. Torque determines how much horsepower you make at a certain RPM.
As stated above, the peak torque number that is quoted for engines, determines when/where a car's power band starts. When people say that a car has great low end torque, what they are really saying is that the engine in that car generates more horsepower at the low RPM range compared to other cars. It also means that in a car that generates great low end torque, you are most likely already in the engine's power band even if you are just cruising along. A quick stab of the gas pedal will elicit a quick response from the engine, making the car feel a lot more powerful than the numbers suggest. This is what makes the Volkswagen GTI a very popular daily driver. It doesn't make a lot of horsepower, but it makes good enough torque low in the RPM range that it makes the car feel stronger than it ought to be.
Wow I just geeked out on a CG movie about cars talking about horsepower and torque. I feel like such a nerd but I blame Davin haha. If anything I've said about the topic of horsepower and torque is woefully inaccurate, please let me know by sending me a comment.
If you have not watched any of the Disney Pixar Cars movies, man you are missing out. They are great movies in their own right even if you are not a big fan of cars. The lessons they teach in the movies also make them great movies to watch with your kids.