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Old 11-17-2008, 11:49 PM
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Horsepower Vs. Torque

So, my understanding of how HP is calulated is: Torque x RPM / 5252. Is this correct? If it is then all HP numbers are directly related to torque numbers. why is horsepower the main focus for so many hotrodders? I know a lot of people say to focus on the torque, but most look at HP. please give me the complex answer. Im not new to hotrodding by any means, but this little peice of knowledge has just slipped passed me.
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Old 11-18-2008, 12:08 AM
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Probably all boils down to the HP # being larger(higher) than the Torque #!
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Old 11-18-2008, 12:56 AM
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it isnt always higher, im pretty sure that most dyno sheets show the two curves crossing at or very close to 5252rpm, hence the 5252 in the equation.
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Old 11-18-2008, 01:58 AM
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Quote:
Originally Posted by CNC-Dude
Probably all boils down to the HP # being larger(higher) than the Torque #!
Bingo, that would be my thoughts on it too.

automods, I have seen some dyno bashes that result in the total torque being higher than the total horsepower, but they were centrifugally-blown motors. I don't think I've ever seen a naturally aspirated motor where this occurred.
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Old 11-18-2008, 05:03 AM
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As i understand it basically hp is how fast the engine can use the tq. in basic theory more hp will get your down the track faster but tq will maintain the speed for you.

Skeeter
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Old 11-18-2008, 06:08 AM
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Quote:
Originally Posted by automods
So, my understanding of how HP is calulated is: Torque x RPM / 5252. Is this correct?
Yes.

Quote:
Originally Posted by automods
If it is then all HP numbers are directly related to torque numbers. why is horsepower the main focus for so many hotrodders? I know a lot of people say to focus on the torque, but most look at HP. please give me the complex answer.
History. Before the steam engine came on the scene, the horse was understood by most folks who needed work done. James Watt needed to impress on folks using horses how many horses the steam engine would replace.
For a street engine, torque is important. Specialized vehicles which run in a narrow rpm band may be more interested in peak horsepower amount and location.
Here are two good articles on the subject as well as where the 5252 comes from.

http://auto.howstuffworks.com/horsepower.htm
http://auto.howstuffworks.com/question622.htm
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Old 11-18-2008, 10:56 AM
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Here's the way I see it, it might be opinion or fact:

Torque is just a twisting force. Give me a two foot wrench and if I hang on the end of it you get 360ft-lbs (at close to zero rpm in this case).

If you consider torque and rpm then you have power pretty much. (the ability to produce the torque at a certain speed).

Horsepower is a rating of power; 1HP =~745 Watts. If you actually want to do something fast you need to measure your needs in power.
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Old 11-18-2008, 12:33 PM
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Quote:
Originally Posted by redsdad
Yes.


History. Before the steam engine came on the scene, the horse was understood by most folks who needed work done. James Watt needed to impress on folks using horses how many horses the steam engine would replace.
For a street engine, torque is important. Specialized vehicles which run in a narrow rpm band may be more interested in peak horsepower amount and location.
Here are two good articles on the subject as well as where the 5252 comes from.

http://auto.howstuffworks.com/horsepower.htm
http://auto.howstuffworks.com/question622.htm
So horsepower is basically Ft-Lbs per minute. Its a unit of acceleration. so to put this into dumb dumb terms, if you were to take an engine like a 1978 Chrysler 400 rated at 190hp and i think close to 400ft-lbs if there was no load, this engine would take longer to accelerate to say 4000rpm than an engine the same dimensions that produced 400hp and the same torque.

so although torque is very important the horsepower is what will get the acceleration, am i close?
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Old 11-19-2008, 05:40 AM
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Quote:
Originally Posted by automods
So horsepower is basically Ft-Lbs per minute. Its a unit of acceleration.
No. Acceleration has a times squared unit. Acceleration due to gravity is approx. 32 ft/sec^2 on earth.
And it is not a velocity value as that would be distance per time.
Torque, measured in ft-lbf in English (American) is force applied at a distance around a central point. Twisting.
Work is torque per time. HP.
And here again, it relates to what people used to understand. They would get so much work out of a horse. If they had a 10 hp steam engine, it could replace 10 horses provided the work was applied properly.
On a steam engine or a race engine, you can design the rest of the system around the engine so you are at or near the peak hp rating most of the time. So, the higher the horsepower, the more work you are going to get. You can use it to plow a field or make low et. HP doesn't care what it is doing.
On the street, you go from idle to highway speeds. You are seldom at the peak hp point. The twisting gets the car moving and accelerating. So, having a broad, flat, high torque curve is desirable.
So, for me at least, if I am looking at a race engine, I am more interested in hp and where the peak is. For a street engine, I am more interested in what the torque curve looks like.
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Old 11-19-2008, 10:47 AM
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I've always viewed it this way - torque is a force applied to a load, regardless of whether the torque causes the load to move. Horsepower is a measure of work performed. Work performed in this case is moving the load. If you apply a force of 10 lb/ft to an object, and that force doesn't move the object, you still have a force of 10 lb/ft applied, but you have zero horsepower; ie no work has been performed. The same can be applied to a rotating crankshaft, although the mere fact that it is rotating indicated some work is being performed.

On an engine dyno, the only thing being measured is torque and rpm. Horsepower is merely a calculation.
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Old 11-19-2008, 10:45 PM
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Quote:
Originally Posted by techinspector1
I have seen some dyno bashes that result in the total torque being higher than the total horsepower, but they were centrifugally-blown motors. I don't think I've ever seen a naturally aspirated motor where this occurred.
An engine designed for low-rpm use will do that. For example, a 1988 T-bird, 302cid, 155hp @ 3200rpm, 275lb-ft @ 2000rpm. Not something you'd likely see in a typical dyno bash, I suspect.



Torque is twisting force, the ability to move something in a circular direction. Horsepower results from the rapid and repeated application of this force. The more times this force is applied in a set length of time, the more horsepower.

Torque is what moves the car. Horsepower is what gives the rate of acceleration of said car.
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Old 11-22-2008, 12:22 AM
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Horsepower is work accomplished

Torque is effort applied.

Everyone is always more impressed by work accomplished no matter how much effort was applied!
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Old 12-10-2008, 09:00 PM
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Quote:
Originally Posted by techinspector1
Bingo, that would be my thoughts on it too.

automods, I have seen some dyno bashes that result in the total torque being higher than the total horsepower, but they were centrifugally-blown motors. I don't think I've ever seen a naturally aspirated motor where this occurred.
If peak torque occurs below 5252 RPM it will be higher than peak torque, likewise peak torque above 5252 implies peak horsepower will be the larger of the numbers. (the rpm/5252 is greater than 1 if rpm>5252, similar for rpm<5252)

To really understand the notion of horsepower it might be helpful to look at the physics of it. I found this webpage http://hyperphysics.phy-astr.gsu.edu/hbase/work.html
joules are another measurement of the power.
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Old 12-11-2008, 06:25 AM
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I was always under the impression that you want your torque numbers higher than horsepower numbers. I know my engine, before the blower, torque was higher. I do not have anything now, just the estimated.

I have seen (read) turbocharged engines making awesome horsepower numbers, but their torque numbers are almost embarrassing compared to the horsepower.

This has always been something that is interesting to me - one higher or lower than the other.

Jason
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Old 12-13-2008, 02:29 PM
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The torque is actually the more important number overall. Thats why the newer VW diesel engines have decently good takeoff from a small 2.0L motor. They have 138 hp and 236 lb/ft of torque and still get 60 mpg highway. Its because the torque gets the car moving to help it be more efficient.
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