question about horsepower?

[chrisjiujitsu]

I just recently looked up horsepower for different cars. I notice they all give different rpms for the horse power specified. Can anyone enlighten me on what this means. For example. What is the difference between a car with 190 hp at 4500 rpm and one with 190 hp at 3500 rpm, is the lower rpm mean it is a faster car. And would it have more horsepower at 4500 rpm?

[del]

oh boy, every car makes a certain amount of hp at any given rpm. usually the hp figures you read are the peak hp output, just depending on what you're reading off of. some cars peak at lower rpms and some at higher rpms. both those hypothetical cars you mentioned make 190hp. the only difference is that they make that 190hp at different points in the rpm range.

peaking at a lower rpm doesn't necessarily make the car faster, and the other way around. the information you provided is too little to assess which car would be faster. how much does the car weigh, what kind of transmission is it, is it a 4 cylinder or 8 cylinder, etc etc etc.

check this site out:

http://auto.howstuffworks.com/horsepower.htm

[crayzayjay]

moved from car comparisons

How did this belong to the "car comparisons" forum?

Post in the appropriate forum please

[BeEfCaKe]

Well assuming same weight and everything else same except peak hp at diff range... i think the lower rpm w/ same hp would probably accelerate a little faster because u're reaching this peak hp faster.. but maybe top speed wise, the higher rpm version would be better?

[del]

Originally posted by BeEfCaKe
Well assuming same weight and everything else same except peak hp at diff range... i think the lower rpm w/ same hp would probably accelerate a little faster because u're reaching this peak hp faster.. but maybe top speed wise, the higher rpm version would be better?

in a racing sense, the higher rpm version would be better since your engine would probably be in the upper half of the rpm range anyway you see what i'm saying? low end torque is good when you're towing a trailer, or just driving around town. but for racing purporses, it's good to have that power towards the higher end. low end would help in drag racing but then so would high end.

in your example, a car with a lot of low end would get off the line quicker, but i would bet that by the end of the race the guy with more power in the high end would come out ahead coz by then, the guy with the low end power runs out of steam when the guy with the high end is just getting started. does that make any sense? there's a lot of variables that come in to play. so it's hard to explain.

and having horsepower in the higher end does help in top speed of course, but then you're limited only by your transmission. a car with 1000000hp can have a top speed of 90 if the gearing is set up that way, however a car with 10 hp can go twice that fast with the right gearing, you see?

[ivymike1031]

low end torque is good when you're towing a trailer, or just driving around town. but for racing purporses, it's good to have that power towards the higher end... a car with a lot of low end would get off the line quicker, but i would bet that by the end of the race the guy with more power in the high end would come out ahead coz by then, the guy with the low end power runs out of steam when the guy with the high end is just getting started.

I think you're probably confusing the subject here. The comparison is between two cars with equal peak power output, but it sounds like you're comparing two cars with equal peak torque output, one at lower rpm than the other.

In this case, we know that torque at 3500 rpm for the first car is 285 ft*lbf, and torque at 4500 rpm for the second car is 221 ft*lbf. We don't know what the torque curves look like off of those points, which means that we really can't say anything about the driveability of either vehicle, or which would better for racing, towing, or sitting in a parking lot. If I had to guess, I'd say that the high-torque, low-rev engine, if it had the same mass as the other, would be a much more driveable engine, and would be better for racing AND towing, if it was coupled to a good transmission. It's probably turbocharged, and probably has a nice torque rise at the low end. The other one is probably naturally aspirated, and has fairly "peaky" output, so the driver will have to be constantly shifting if he wants to get anywhere in it... ah, aren't make-believe engines fun?

and having horsepower in the higher end does help in top speed of course, but then you're limited only by your transmission. a car with 1000000hp can have a top speed of 90 if the gearing is set up that way, however a car with 10 hp can go twice that fast with the right gearing, you see?

A car with 10hp power output would have to be extremely aerodynamic to go over 60mph, let alone the 180mph you claim, no matter what the gearing is like. Ignoring the transmission for a second, peak speed depends only on peak power. When total drag on the vehicle equals the peak power output of the engine, the vehicle is going as fast as it can possibly make itself go. You can only make things worse through gearing choices. If both of the cars in the original example are equally aerodynamic, and they both have ideally-chosen transmissions, then they will have exactly the same top vehicle speed. (One will be running at a lower engine speed than the other)

[pod]

I suppose that a 10 hp car would go sloe , but how does this box of a super milage car do 80 with a 3.5hp briggs and stratton

[ivymike1031]

I suppose that a 10 hp car would go sloe , but how does this box of a super milage car do 80 with a 3.5hp briggs and stratton

Apparently it has very low drag. I'm not sure what vehicle you're talking about, though. As I said earlier, when total drag on the vehicle equals the peak power output of the engine, the vehicle is going as fast as it can possibly make itself go. That is, of course, based on the assumption that "the engine" is the only thing providing power to the wheels. A typical car takes about 15hp to keep it going at 65mph. That figure varies quite a bit depending on the size and shape of the vehicle, the type of tires installed, etc.

[SaabJohan]

Drag increase on the square of the velocity, simplified this means that if we want to double our top speed we must increase engine power four times.

It may be enough with 3,5 hp to reach 80 mph with a low drag car but if we want to go say 200 mph with a high drag car like the Porsche 962C we will then need 1200 hp!

Gearing can also lower the top speed. Say that we have a car which is producing enough power to reach 150 mph, but it doesn't reach the rpm of maximum power when going 150 mph the top speed can be lower since the engine will produce less than the needed at the current rpm.

[chrisjiujitsu]

Man you guys are into it. Thanks for the info, I think I got the idea. I decided that I shouldnt worry about it so much and drive carfully. I am not a street racer anyway. Maybe later in my life I will start drag racing. Thanks for the info guys.