What does it mean to know the power output at a certain rpm?

[Aro2220]

Whenever I look at cars, they are always rated for some horsepower and some torque at some rpm.

For example, Dodge Dart has 160hp at 6400rpm
and 145 lb*ft @ 4800rpm

What is the relevance of the rpm?

I mean, what can I conclude about the car if it was 160hp @ 6400rpm vs if it was 160 hp at 4000rpm or 160 hp at 8000rpm?

thanks

[BradyGarcia]

RPM in car stands for "revolution per minute" that is a car will have maximum speed at the given rpm in any gear . So when you will accelerate the more rpm it will climb and more fuel will be used.

[Oldbearcat]

If you look at a graph of both engine torque and HP (Y axes) versus engine RPM (X axis) the figures you mention are where both curves on the graph hit their peak. In your example, for instance, the engine has to be spooled up pretty good to get decent performance out of it - you'll notice that its peak HP doesn't occur until you're close to the engine's redline, and, peak torque occurs at fairly high RPM also. That means that, if the car is fairly heavy or loaded, it'll have to downshift and scream its guts out to climb modest grades at interstate speeds. As a different example, I have an antique car that makes 200 Ft lbs of torque at 1200 RPM, and 90 HP at 3000 RPM. Its a 64 year old Chevrolet 217 CID inline 6 in a 3400 pound car, and, it'll climb interstate grades at speed with no sweat in high gear. Some of the more sophisticated engines available today with VVT, multiple level induction systems, etc. have "mesa" shaped torque and HP curves. They make peak torque and HP over a wide range of engine RPM - like 2000 - 6000 RPM. Take a look at the graphs for a Mercedes Benz 3.5 V6, a Jaguar 4.2 V8, or some of the newer engines from GM (3.6 V6) as examples. These engines will drag a heavy vehicle up a interstate grade at 70 MPH and hardly ever need to downshift.

Regards:
Oldbearcat

[Gohan Ryu]

It's so you know where to hold the revs when you're getting ready to launch that bad boy down the 1/4 mile.