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help me understand torque and hp

B

BlahBlahYouToo

Lifer
i know torque is a twisting force measured in lb-ft.
i know hp is a measure of energy... work over time.

in the world of car engines, the way i conceptualize the two is very simple minded (and probably wrong), but i think of torque as power in the lower rev range, e.g. below 3000 rpm and hp in the higher 3000rpm to redline range.

i base this off of reading power curve graphs where the torque curve starts off higher at lower rpm's and decreases steadily as RPM climbs. the HP curve is the opposite.
(except in my honda where i have no torque anywhere)

help a noob understand!
 
BlahBlahYouToo said:
i know torque is a twisting force measured in lb-ft.
i know hp is a measure of energy... work over time.

in the world of car engines, the way i conceptualize the two is very simple minded (and probably wrong), but i think of torque as power in the lower rev range, e.g. below 3000 rpm and hp in the higher 3000rpm to redline range.

i base this off of reading power curve graphs where the torque curve starts off higher at lower rpm's and decreases steadily as RPM climbs. the HP curve is the opposite.
(except in my honda where i have no torque anywhere)

help a noob understand!
Click to expand...

the first part is right

Torque is a rotational moment.
HP is just power (1 hp = 745.699872 watts).

The reason you interpret hp and torque the way they are is because of their relationship (in converting ft*lb to hp)
hp = torque (in ft*lb) * rpm / 5252
022fd30474af9bf8c407992118eaabb2.png
 
i think of torque as power in the lower rev range, e.g. below 3000 rpm and hp in the higher 3000rpm to redline range.
Click to expand...
That's a pretty good way to interpret it.

For most people in most situations, peak power is the only value that matters. You don't need full power from the engine when you're cruising at 80mph in a compact car. Cruising takes very little power, very little torque. If you need to pass a car or quickly accelerate, flooring it will drop gears to get you into the max power range.

The torque is mostly for pulling things because it tells you how fast the engine will be spinning while pulling. A 200HP Honda Civic engine in a truck could theoretically pull just as much weight as a 200HP truck engine, but the civic engine is a low torque engine. The civic engine would need to be spinning around at 6k rpm, getting horrendous gas mileage, and it would put a lot of wear on the engine. An engine with torque at a lower rpm will spin slower, get better gas mileage while towing, and it won't wear out as quickly. If you have a truck, having lots of torque at low rpm is very important. In a car, torque is usually not that important.
 
ShawnD1 said:
In a car, torque is usually not that important.
Click to expand...

I'd rather have more torque than HP in my car.

I see torque used as pulling off the line or passing at highway speeds. I see HP more as a measure of maximum achievable/maintainable speed. I could be completely off my rocker though.
 
SithSolo1 said:
I'd rather have more torque than HP in my car.

I see torque used as pulling off the line or passing at highway speeds. I see HP more as a measure of maximum achievable/maintainable speed. I could be completely off my rocker though.
Click to expand...
Torque is how fast it accelerates without changing gears. Power is how fast it accelerates with changing gears.

We already had this argument in the thread where I said diesel was the worst thing ever. Some guy with a Jetta or Golf TDI was so proud of the fact that a Civic Si accelerated ridiculously slow when left in sixth gear but the diesel accelerated just fine. As soon as gears were thrown into the equation, the Si pulls away and never looks back.
 
SithSolo1 said:
I'd rather have more torque than HP in my car.

I see torque used as pulling off the line or passing at highway speeds. I see HP more as a measure of maximum achievable/maintainable speed. I could be completely off my rocker though.
Click to expand...

You're still on the rocker, don't worry.

OP: The easy way to think of it is: "power is how fast you can go, torque is how fast you get there" though this statement is very simplified.
 
HP is power not energy.

Torque is force, power is that force x distance / time.

Hp (power) = Torque (force) @ xxxx RPM (rotations = distance, per minute = time )
 
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JCH13 said:
OP: The easy way to think of it is: "power is how fast you can go, torque is how fast you get there" though this statement is very simplified.
Click to expand...
The above statement is true if there's no gear available to keep the engine near max power. Usually that means the first gear from a start.


Does anyone remember that game Gran Turismo for the Playstation 1? The cars that had 900HP with turbochargers but no torque had an interesting problem where you had to spin the tires at the start of the race or the car wouldn't accelerate properly. That's exactly what low torque does. Once the car is moving fast enough to redline in first gear, the car can be kept in the power range for all of the other gears. In a car in most situations, it's not really a big deal that your first gear has no acceleration for the few couple thousand rpm. In a truck, that's a huge problem.
 
exdeath said:
HP is power not energy.

Torque is force, power is that force x distance / time.

Hp (power) = Torque (force) @ xxxx RPM (rotations = distance, per minute = time )
Click to expand...

FTFY, power = force * speed = force * distance / time
 
Think of this way.. torque is a moment.. ie at any given moment an engine is putting out X ft-lbs of torque - ie. time isn't relevant to figure out torque.

Horsepower is work over time, 1 mechanical horsepower (yes, there's more than 1 definition for horsepower, mechanical is the common one) is 33,000 ft-lbs/min - ie it takes 1 minute to move 33,000lbs 1 foot.

Think of using a torque wrench. A person can easily put out a lot of torque, especially if you have a really big torque wrench that gives you leverage advantage, you can put 900 ft-lbs of torque on something by yourself. When you are though, you're really cranking on the wrench to get it to budge so your not spinning it fast and you're putting out very little horsepower because you can only do 1 rpm with it. Now if you hooked up a pneumatic torque wrench instead, it's putting out the same amount of torque but it can do say 20rpm at 900ft-lbs - it's getting way more work done than you, which is to say it's delivering a lot more horsepower even though you're both putting out the same amount of torque.
 
IcePickFreak said:
Think of this way.. torque is a moment.. ie at any given moment an engine is putting out X ft-lbs of torque - ie. time isn't relevant to figure out torque.

Horsepower is work over time, 1 mechanical horsepower (yes, there's more than 1 definition for horsepower, mechanical is the common one) is 33,000 ft-lbs/min - ie it takes 1 minute to move 33,000lbs 1 foot.

Think of using a torque wrench. A person can easily put out a lot of torque, especially if you have a really big torque wrench that gives you leverage advantage, you can put 900 ft-lbs of torque on something by yourself. When you are though, you're really cranking on the wrench to get it to budge so your not spinning it fast and you're putting out very little horsepower because you can only do 1 rpm with it. Now if you hooked up a pneumatic torque wrench instead, it's putting out the same amount of torque but it can do say 20rpm at 900ft-lbs - it's getting way more work done than you, which is to say it's delivering a lot more horsepower even though you're both putting out the same amount of torque.
Click to expand...

pretty good analogy. i think i can live with that.
 
For any given engine in the world, I can extract ANY amount of torque I want from it. The final torque is a function of the gear ratio of the transmission. Torque is nothing more than the FORCE the wheels can apply to the ground.

Of course, a larger motor will put out more torque (force) at the flywheel (before the transmission), and thus require less gear reduction.

To simply it even further, torque is a force. Picture it as a force on a lever. If we move the fulcrum of the lever, a 2 pound weight at one end can pick up a 2000 pound weight at the other. It just depends on the configuration of the lever. That lever is your transmission.

Horsepower is work (Force * distance) over time. So if you know the horsepower, and you know the distance you're going to travel and the forces that will slow you down (air friction, rolling friction, etc) you can solve for the time it will take.

HP (horsepower) = Force * distance / time.

Time = Force * distance / horsepower

Of course this is an ideal solution, and you should consider losses from gear train, friction, etc, but that should help you understand it a bit. After solving the above equation for time, you can then figure out what torque (force) you need to apply to achieve that time, and then gear your transmission accordingly to deliver that torque.
 
horsepower really just means you don't have to change gears as much. that's good because you're not applying force to the ground when changing gears. 1000 hp at 20,000 rpm in an F1 car is basically ~250 ft lbs of torque, but it makes that 250 ft lbs for a 10,000 rpm band. they can wind that engine a long time before they need to change gears.
 
I like square engines. 700 hp AND 700 lb-ft torque from 2000 RPM to redline. 😀

Why pick just one?
 
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ShawnD1 said:
The above statement is true if there's no gear available to keep the engine near max power. Usually that means the first gear from a start.


Does anyone remember that game Gran Turismo for the Playstation 1? The cars that had 900HP with turbochargers but no torque had an interesting problem where you had to spin the tires at the start of the race or the car wouldn't accelerate properly. That's exactly what low torque does. Once the car is moving fast enough to redline in first gear, the car can be kept in the power range for all of the other gears. In a car in most situations, it's not really a big deal that your first gear has no acceleration for the few couple thousand rpm. In a truck, that's a huge problem.
Click to expand...

Wasn't that turbo lag? The cars in stock form would actually accelerate better than their tuned 900HP versions.
 
SithSolo1 said:
I'd rather have more torque than HP in my car.

I see torque used as pulling off the line or passing at highway speeds. I see HP more as a measure of maximum achievable/maintainable speed. I could be completely off my rocker though.
Click to expand...
Torque tells you far less about a car's ability to accelerate quickly than horsepower. Horsepower is more meaningful and is why it's the measure used most often. A hamster on a wheel can produce 15,000 ft lbs of torque if geared appropriately and assuming no frictional losses. But slap him in an engine bay and I bet he accelerates that car to 60 much slower than an actual engine producing only 100 lbs.
 
IcePickFreak said:
Think of using a torque wrench. A person can easily put out a lot of torque, especially if you have a really big torque wrench that gives you leverage advantage, you can put 900 ft-lbs of torque on something by yourself. When you are though, you're really cranking on the wrench to get it to budge so your not spinning it fast and you're putting out very little horsepower because you can only do 1 rpm with it. Now if you hooked up a pneumatic torque wrench instead, it's putting out the same amount of torque but it can do say 20rpm at 900ft-lbs - it's getting way more work done than you, which is to say it's delivering a lot more horsepower even though you're both putting out the same amount of torque.
Click to expand...

Nice analogy :thumbsup:

When in the proper gear, a car with more horsepower will always be the quicker and faster car because it turns the wrench faster and gets more work done. 😀
 
exdeath said:
I like square engines. 700 hp AND 700 lb-ft torque from 2000 RPM to redline. 😀

Why pick just one?
Click to expand...

Flat out putting out those specs constantly I wonder what would happen 1st on a full tank of gas, to your engine. Mechanical failure or run out of gas.....
 
MrWizzard said:
Flat out putting out those specs constantly I wonder what would happen 1st on a full tank of gas, to your engine. Mechanical failure or run out of gas.....
Click to expand...
Running full torque all the time? I think those are called trucks. No they don't fail every couple miles.
 
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