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CNET On Cars: Car Tech 101: Horsepower vs. Torque

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CNET On Cars: Car Tech 101: Horsepower vs. Torque

4:32 /

Brian Cooley explains the sometimes tricky relationship between the two ways your engine's power is measured.

-Okay, aside from emissions and heat, the 2 big outputs of any engine are horsepower and torque. You know all about the first one. You've heard about since you were a little kid. It's in every automative advertisement, everything the automakers ever talked about as horsepower in there somewhere, but torque is far less well understood though extremely important. Let's break them down defined them first. Horsepower is a measure of work. Its definition makes that obvious. One horsepower is equal to 33,000 foot-pounds per minute. Now, torque is also a measure of work, but it describes work as twisting force. It's kind of like horsepower in a circle and without the per minute factor. Torque is measured in foot-pounds, not a certain number of them though. For example, it may take 27 foot-pounds of torque to loosen a particular bolt on this engine without regard for how long you have to apply that force. So at the risk of what we are simplifying, horsepower expresses how much work you can get down on a certain amount of time. Torque is about how hard you can twist something and that's key because how does a car move itself. The engine turns. It twists the gears in the transmission. They twist output for drive shafts and that moves the car. Torque really should be the star. Okay, seriously, let me show you on some charts how horsepower and torque work. I promise you won't fall asleep. Now, these are charts from cars that have been put on dynamometers, basically treadmills for cars. You'd seen this. Our partners over at Edmund's do a lot of this kind testing and gave us this data and it's very instructive. Here's the 2011 Ford Mustang. Here's who the chart works. On the left, you've got your vertical axis of either foot pounds of torque or amount of horsepower. Down across the right is basically your tachometer, it's RPM from 1 to 8,000 on this case. Now, look what happens, the torque line is this light blue one. You start building around 2000 RPM and you got more and more torque as you increase the revs. You peak right around here about 4200 with 365 foot-pounds of all the engine has and you stayed about that range until around 5200 rpm. This is the sweet spot. This is where you've got peak torque that just keeps coming. After a while, more RPMs, the torque begins to drop, horsepower keeps increasing. Let's look at another car. It's a very different engine in a McLaren MP4-12C super car. Here's our torque line again, builds gradually, and notice that about 42,000 all the way out to about 62,000. This guy stays flat and right about at the peak amount of torque around 415 foot-pounds. This is a wider what they call fatter torque band. You've got more RPMs when you have full torque from the engine. Mercedes-Benz S63, instructive because this is a twin turbo V8. Look what happens with turbos. This is the torque line, it's out of hand. It peaks really fast and it stays broad, chunky and 6011 foot-pounds of torque. That's a lot by the way for a very long time before it begins to degrade. And finally a very different car, Scion FR-S, peaks here early. It only 143 foot-pounds of torque, but then look what happens, you get this dip here. If you ever hearing me driving a car and say I feel it as flat spot in the torque curve. It's not flat at all. It's a actually a dip where the car feels kind of gutless and then up around 4800 rpm, it kicks back in again and stays nice and plat all the way out to the mid 6000. Now, you may have noticed in all these charts, torque tends to fader out as you get to the higher RPMs. That's up near your red line. That's because the engine is less able to breathe efficiently there. Secondly, notice that horsepower keeps climbing even after torque drops off. Why is that? It seems like it's two engines doing different things. While either as torque drops off the RPMs keep climbing and horsepower is largely a product of RPM times torque. So, we can put those 2 together and keep that horsepower line moving up because torque is dropping, but only modestly as RPMs go up in a linear fashion. Okay, I hope you get a better understanding now of horsepower, torque, the relationship and how they impact the driving experience of a car. Also, note, we do our car videos. I'll show you those 2 numbers and horsepower is usually the bigger. Torque is often the smaller unless that car has a turbo or a super charger, which artificially puts more in the engine and allows the torque to come higher or as high as the horsepower. That gives you another clue as to what that car is gonna be like to drive.

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