Friday, October 21, 2011

What is the difference between horsepower and torque, and what's with all this RPM talk anyway?

     When you read anything about car shopping, everybody wants to talk about the power of an engine. But it gets very confusing making the comparisons. One car emphasizes how many horsepower the engine produces. Another talks about low-end torque. Some cars look like they should have the same power, but feel very different when I drive them. And then there is the @4000 RPM tag after the number. What does that mean?
Putting it simply, torque is the power an engine produces to get a vehicle moving, while horsepower is the ability to keep it moving. From there, it all depends what is important to you. If you like the “seat of your pants” thrust at the stop lights, or if you are purchasing a vehicle to pull a trailer or haul a heavy load, torque is your answer. If you are looking for more of a touring car, and passing power is more up your alley, then a high-horsepower sports car (2 or 4 doors – they make both these days) will be more appealing.
There are a number of other factors that can complicate matters, and that’s a topic for a different discussion, but the type of transmission, the gearing in the differential/transaxle is a factor (often discussed in pickups and large SUVs), and of course, that RPM term we see next to the horsepower and torque ratings.
RPM (Revolutions Per Minute) refers to how fast the engine is revving when it hits that peak horsepower or torque number. Low-end torque means that you don’t have to wind the motor up very far at all to get to the peak torque, and get the load moving. Many diesel trucks emphasize that you will be producing a lot of torque right at the engine speed the engine idles at. That means that when you first put it in gear, you have essentially all the “get it going” power you need to start towing or hauling. You don’t have to “step on it” to get into the power to get moving.
Horsepower, you will typically see with a relatively high RPM number. Some are “high-revving” motors, which mean that as you wind up for aggressive driving, the horsepower is there to finish the drag race. A daily commuter car will be just fine having the peak horsepower at a more moderate RPM, and may even provide a fuel efficiency advantage as a result of this design.
Which is more important to you will depend on what you need the car to do. I hope this brief explanation of the two is helpful as you research your purchase. Above all, however, remember that research is important, but nothing will tell you as much as a 10-15 minute test drive. Make sure the salesperson takes you on a route that will allow you to experience the elements of the car that best fit what you are going to need it for. If you need torque for carrying a load or pulling a trailer, make sure you are able to climb a hill on your drive. If you are more of a spirited driver, make sure you get the chance to accelerate up to highway speeds. If you are looking for a travel vehicle, make sure you get to drive on a variety of road surfaces, and at both city and highway speeds. Then combine your reading with your driving, and see if you are in the right car. From there, your decision will be easier. Happy shopping!

Mike Bidwell
Mankato Motor Co.

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