Horsepower or Torque, what makes the car accelerate?
10-30-2005, 07:19 PM
#1
TECH Fanatic
Thread Starter
iTrader: (3)
Join Date: Jun 2004
Location: Dayton, OH
Posts: 1,844
Likes: 0
Received 0 Likes
on
0 Posts
Horsepower or Torque, what makes the car accelerate? Engineers and physics gurus, help me understand this!
After all of the physics classes I have taken, you'd think I would know this. After reading about every article on Howstuffworks.com I think I am even more bewildered than I was before.
I know the basics:
F=ma, a = F/m
To correctly calculate that equation, would Force be the torque or horsepower? For now, I am ignoring tire pressure, wind resistance, friction, and all of those other variables that can just be added in later anyways.
To convert torque to horsepower, hp = tq x rpms/5252.
Well, that explains why cars make more hp at higher rpms and more torque at lower rpms. What it doesn't explain is what is the Force in the above equation.
I use to think that torque was better for moving/towing heavy amounts of mass and getting you car moving from a stop, while horsepower was better for accelerating an already moving car faster and faster.
Howstuffworks says that you want to start from a stop at the RPM's that are close to your peak hp (which ofcourse you don't want to do for wheelspin, but let's say wheelspin is not a variable and you have a perfect launch). I thought you were supposed to launch from your max torque rpms to get the car moving.
The next confusing thing is it goes on to say that big diesel engines aren't used for their high torque, but that they are used because they can produce their peak hp at low rpms, allowing them to last longer. I thought the big trucks/bulldozers etc had big diesel engines for the torque to get their large masses moving?
Just curious about all this mess and have never truely understood it. Thanks in advance for those who help shine some light on the subject.
After all of the physics classes I have taken, you'd think I would know this. After reading about every article on Howstuffworks.com I think I am even more bewildered than I was before.
I know the basics:
F=ma, a = F/m
To correctly calculate that equation, would Force be the torque or horsepower? For now, I am ignoring tire pressure, wind resistance, friction, and all of those other variables that can just be added in later anyways.
To convert torque to horsepower, hp = tq x rpms/5252.
Well, that explains why cars make more hp at higher rpms and more torque at lower rpms. What it doesn't explain is what is the Force in the above equation.
I use to think that torque was better for moving/towing heavy amounts of mass and getting you car moving from a stop, while horsepower was better for accelerating an already moving car faster and faster.
Howstuffworks says that you want to start from a stop at the RPM's that are close to your peak hp (which ofcourse you don't want to do for wheelspin, but let's say wheelspin is not a variable and you have a perfect launch). I thought you were supposed to launch from your max torque rpms to get the car moving.
The next confusing thing is it goes on to say that big diesel engines aren't used for their high torque, but that they are used because they can produce their peak hp at low rpms, allowing them to last longer. I thought the big trucks/bulldozers etc had big diesel engines for the torque to get their large masses moving?
Just curious about all this mess and have never truely understood it. Thanks in advance for those who help shine some light on the subject.
10-30-2005, 07:38 PM
#2
Go look at the application of the forumla for work...
Multiplication of TQ is what counts, and to do that BETTER takes RPM which means HP is important. Actually the most average TQ at the highest RPM after it has been multiplied thru the gearing to achive the same speeds/times.
These threads always turn ugly, I'm just going to put this in at the begining so we get off on the right foot.
Bret
Multiplication of TQ is what counts, and to do that BETTER takes RPM which means HP is important. Actually the most average TQ at the highest RPM after it has been multiplied thru the gearing to achive the same speeds/times.
These threads always turn ugly, I'm just going to put this in at the begining so we get off on the right foot.
Bret
Last edited by SStrokerAce; 10-30-2005 at 08:05 PM.
10-30-2005, 08:07 PM
#4
TECH Enthusiast
Join Date: Apr 2005
Posts: 540
Likes: 0
Received 0 Likes
on
0 Posts
SStroker Ace did a good job explaining.
Max acceleration in any pre-set gear occurs at the TORQUE peak because torque is a force.
Max horsepower is the point at which for any given speed, the gearing can be optimized to produce the maximum force.
so basically you would choose your gearing depending on the profile of the engine.
Low torque, high rpm, the gearing would have large "torque multiplication" (i.e. numerically large gear ratios)
High torque, low rpm , the gearing would not need as much "torque multiplication"(i.e. numerically lower gear ratios) to get the same peak forward thrust.
Max acceleration in any pre-set gear occurs at the TORQUE peak because torque is a force.
Max horsepower is the point at which for any given speed, the gearing can be optimized to produce the maximum force.
so basically you would choose your gearing depending on the profile of the engine.
Low torque, high rpm, the gearing would have large "torque multiplication" (i.e. numerically large gear ratios)
High torque, low rpm , the gearing would not need as much "torque multiplication"(i.e. numerically lower gear ratios) to get the same peak forward thrust.
10-30-2005, 08:21 PM
#6
LS1 Tech Veteran
iTrader: (1)
Join Date: Oct 2003
Location: Wichita, Ks
Posts: 1,170
Likes: 0
Received 0 Likes
on
0 Posts
Torque is required to accelerate or decellerate a mass and horsepower is the sustainer/perpetuator. Further, horse power (like any other power measurment such as watts) is a rate measurment per unit of time. In fact, horsepower (or kilowatts for metric measurers) is the rate at which force (measured in pound feet or Newton meters) can be sustained per minute. Horse power and torque are joined at the hip. The only real difference is that HP is a rate per unit of time measurment.
Steve
Steve
10-30-2005, 08:22 PM
#7
Originally Posted by DanO
SStroker Ace did a good job explaining.
Max acceleration in any pre-set gear occurs at the TORQUE peak because torque is a force.
Max horsepower is the point at which for any given speed, the gearing can be optimized to produce the maximum force.
so basically you would choose your gearing depending on the profile of the engine.
Low torque, high rpm, the gearing would have large "torque multiplication" (i.e. numerically large gear ratios)
High torque, low rpm , the gearing would not need as much "torque multiplication"(i.e. numerically lower gear ratios) to get the same peak forward thrust.
Max acceleration in any pre-set gear occurs at the TORQUE peak because torque is a force.
Max horsepower is the point at which for any given speed, the gearing can be optimized to produce the maximum force.
so basically you would choose your gearing depending on the profile of the engine.
Low torque, high rpm, the gearing would have large "torque multiplication" (i.e. numerically large gear ratios)
High torque, low rpm , the gearing would not need as much "torque multiplication"(i.e. numerically lower gear ratios) to get the same peak forward thrust.
I generalize as well because I tend to look at the situation as I'm stuck with XXX set of variables. XXX amount of cubic inches is only going to produce so much TQ, so if that TQ can be produced at higher RPM then it can be mulitplied more to get more to the ground.
Originally Posted by steve
Horse power and torque are joined at the hip. The only real difference is that HP is a rate per unit of time measurment.
Bret
Trending Topics
10-30-2005, 08:26 PM
#8
TECH Fanatic
Thread Starter
iTrader: (3)
Join Date: Jun 2004
Location: Dayton, OH
Posts: 1,844
Likes: 0
Received 0 Likes
on
0 Posts
Originally Posted by DanO
SStroker Ace did a good job explaining.
Max acceleration in any pre-set gear occurs at the TORQUE peak because torque is a force.
Max horsepower is the point at which for any given speed, the gearing can be optimized to produce the maximum force.
so basically you would choose your gearing depending on the profile of the engine.
Low torque, high rpm, the gearing would have large "torque multiplication" (i.e. numerically large gear ratios)
High torque, low rpm , the gearing would not need as much "torque multiplication"(i.e. numerically lower gear ratios) to get the same peak forward thrust.
Max acceleration in any pre-set gear occurs at the TORQUE peak because torque is a force.
Max horsepower is the point at which for any given speed, the gearing can be optimized to produce the maximum force.
so basically you would choose your gearing depending on the profile of the engine.
Low torque, high rpm, the gearing would have large "torque multiplication" (i.e. numerically large gear ratios)
High torque, low rpm , the gearing would not need as much "torque multiplication"(i.e. numerically lower gear ratios) to get the same peak forward thrust.
You would want to launch at peak tq because that is where max force is attained.
If your car has more hp than torque, then you would want a big gear ratio (like 4.10:1) so that your rpms would be up high most of the time in WOT.
If your car has more torque than hp, then you would want a lower gear ratio (like 2.70:1) so that your rpms sit down in the lower range a bit longer.
The only sentence I'm not totally grasping is "Max horsepower is the point at which for any given speed, the gearing can be optimized to produce the maximum force." You just said that max acceleration is at peak torque. Now you are saying that max force is at max horsepower?
)
10-30-2005, 08:38 PM
#10
TECH Regular
Join Date: Oct 2005
Posts: 430
Likes: 0
Received 0 Likes
on
0 Posts
(OK, while you guys were 'fast fingering' I was pounding out the following technobabble, so I claim prior art.
MB
Hey Louie,
This is an apparently simple question that will get you a ton of conflicting answers, but since I seem to be first up to the plate, you're gonna get the truth first, instead of eventually!
The 'force' in your equation is neither torque nor horsepower, it is pounds force, just as written. This force is the reaction at the tire contact points which is generated by the axle torque. If the axle is transmitting 2,000 lb-ft of torque and the tire diameter is 24", or a radius of one foot, then the force is 2,000/1 = 2,000 pounds. (If the curb weight of the car is one ton and it has adequate traction, this would accelerate it at one 'g'.)
The next level of confusion often encountered is: "OK, since that's the physics, it is obviously torque, not horsepower that accelerates a car, so we are way off base talking about HP!" Not quite! We next need to introduce the concept of "Work", which is force times the distance of movement. I can easily develop 2,000 ft-lb of torque (more, alas!) just by hanging off the end of a ten foot pipe attached to the axle, but I can't accelerate the car worth a damn. In fact, until/unless the car moves, I haven't done any 'work' at all. If I can move the car, it will be a slow and painful process, which brings us to the next concept:time.
A very big diesel engine directly driving that axle could generate 2,000 ft-lb of torque while turning at say 1000 RPM. This means it has applied the 2,000 lb. force while moving the vehicle 1000 X the circumference of the wheel (1' x 3.14) or 3,140 ft. The work done is thus 2,000 lb. force applied over a distance of 3,140 feet, or 6,280,000 pound-feet of work in one minute.
Now we come to 'Power'. In the seventeen hundreds, James Watt, wanting to show how a steam engine could replace pit ponies for drawing ore up from mines, needed a standard. He observed that the average beast could work away all day at a rate of lifting (or applying a force of) 300 pounds a distance of 110 feet in each minute. Thus, his "Horse Power" equivalent became a rate of doing work of 300 x 110 or 33,000 pound feet per minute. Thus our Big Torque diesel, at 6,280,000 lb-ft./min, is producing 6,280,000/33,000 = 190 Horsepower.
Lastly, lets look at a very different scenario: Supposing instead of "Big Torque Diesel", with 2,000 lb-ft @ 1,000 RPM, we have a modern FI Grand Prix engine, making only 300 lb-ft but at 15,000 RPM: No torque, no acceleration, right? Wrong! "What a friend we have in gearing!" By hooking the Ferrari mill up to the axle through a 15:1 gear reduction, we turn the axle at the same 1,000 RPM, but the torque at the axle is now 300 x 15 = 4,500 lb-ft! Think that will accelerate the vehicle a little faster? It should, we are now applying work at the rate of 857 HP!
And that's why (as long as you have enough gears) Horsepower is King!
MB
Hey Louie,
This is an apparently simple question that will get you a ton of conflicting answers, but since I seem to be first up to the plate, you're gonna get the truth first, instead of eventually!
The 'force' in your equation is neither torque nor horsepower, it is pounds force, just as written. This force is the reaction at the tire contact points which is generated by the axle torque. If the axle is transmitting 2,000 lb-ft of torque and the tire diameter is 24", or a radius of one foot, then the force is 2,000/1 = 2,000 pounds. (If the curb weight of the car is one ton and it has adequate traction, this would accelerate it at one 'g'.)
The next level of confusion often encountered is: "OK, since that's the physics, it is obviously torque, not horsepower that accelerates a car, so we are way off base talking about HP!" Not quite! We next need to introduce the concept of "Work", which is force times the distance of movement. I can easily develop 2,000 ft-lb of torque (more, alas!) just by hanging off the end of a ten foot pipe attached to the axle, but I can't accelerate the car worth a damn. In fact, until/unless the car moves, I haven't done any 'work' at all. If I can move the car, it will be a slow and painful process, which brings us to the next concept:time.
A very big diesel engine directly driving that axle could generate 2,000 ft-lb of torque while turning at say 1000 RPM. This means it has applied the 2,000 lb. force while moving the vehicle 1000 X the circumference of the wheel (1' x 3.14) or 3,140 ft. The work done is thus 2,000 lb. force applied over a distance of 3,140 feet, or 6,280,000 pound-feet of work in one minute.
Now we come to 'Power'. In the seventeen hundreds, James Watt, wanting to show how a steam engine could replace pit ponies for drawing ore up from mines, needed a standard. He observed that the average beast could work away all day at a rate of lifting (or applying a force of) 300 pounds a distance of 110 feet in each minute. Thus, his "Horse Power" equivalent became a rate of doing work of 300 x 110 or 33,000 pound feet per minute. Thus our Big Torque diesel, at 6,280,000 lb-ft./min, is producing 6,280,000/33,000 = 190 Horsepower.
Lastly, lets look at a very different scenario: Supposing instead of "Big Torque Diesel", with 2,000 lb-ft @ 1,000 RPM, we have a modern FI Grand Prix engine, making only 300 lb-ft but at 15,000 RPM: No torque, no acceleration, right? Wrong! "What a friend we have in gearing!" By hooking the Ferrari mill up to the axle through a 15:1 gear reduction, we turn the axle at the same 1,000 RPM, but the torque at the axle is now 300 x 15 = 4,500 lb-ft! Think that will accelerate the vehicle a little faster? It should, we are now applying work at the rate of 857 HP!
And that's why (as long as you have enough gears) Horsepower is King!
10-31-2005, 11:17 AM
#12
TECH Addict
iTrader: (10)
Join Date: Aug 2005
Location: Seminole County, Florida
Posts: 2,803
Likes: 0
Received 1 Like
on
1 Post
i didnt read any above posts. but torque is calcualated from where the horsepower is in the range. if the horsepower is high in the rpm range you will have low relative torque(and vice versa). so basicly torque is calcualted because most people dont understand that 300hp at 5000 rpm is different than 300hp at 9000rpm. horsepower at a lower rpm is what puts your *** in the seat.
im not scientist but imo thats kinda how it works
im not scientist but imo thats kinda how it works
10-31-2005, 05:06 PM
#14
TECH Fanatic
Thread Starter
iTrader: (3)
Join Date: Jun 2004
Location: Dayton, OH
Posts: 1,844
Likes: 0
Received 0 Likes
on
0 Posts
Originally Posted by Terry'sToy
MB, that was excellent. You have made that very simple to understand.
Thanks.
Thanks.
11-03-2005, 09:50 AM
#19
TECH Junkie
Join Date: Aug 2002
Location: Fort Collins, CO
Posts: 3,726
Likes: 0
Received 0 Likes
on
0 Posts
Originally Posted by Mike K.
I always think of it like this,
tq is how fast you can apply your horsepower.
tq is how fast you can apply your horsepower.
Here's another question. If I make 600HP @ 7,000 RPM and you make 600HP @ 10,000 RPM, who's car is faster? (Assuming both cars are geared properly and weigh the same and yada yada yada)
11-03-2005, 10:03 AM
#20
TECH Junkie
Join Date: Aug 2002
Location: Fort Collins, CO
Posts: 3,726
Likes: 0
Received 0 Likes
on
0 Posts
Originally Posted by MadBill
(OK, while you guys were 'fast fingering' I was pounding out the following technobabble, so I claim prior art.
MB
Hey Louie,
This is an apparently simple question that will get you a ton of conflicting answers, but since I seem to be first up to the plate, you're gonna get the truth first, instead of eventually!
The 'force' in your equation is neither torque nor horsepower, it is pounds force, just as written. This force is the reaction at the tire contact points which is generated by the axle torque. If the axle is transmitting 2,000 lb-ft of torque and the tire diameter is 24", or a radius of one foot, then the force is 2,000/1 = 2,000 pounds. (If the curb weight of the car is one ton and it has adequate traction, this would accelerate it at one 'g'.)
The next level of confusion often encountered is: "OK, since that's the physics, it is obviously torque, not horsepower that accelerates a car, so we are way off base talking about HP!" Not quite! We next need to introduce the concept of "Work", which is force times the distance of movement. I can easily develop 2,000 ft-lb of torque (more, alas!) just by hanging off the end of a ten foot pipe attached to the axle, but I can't accelerate the car worth a damn. In fact, until/unless the car moves, I haven't done any 'work' at all. If I can move the car, it will be a slow and painful process, which brings us to the next concept:time.
A very big diesel engine directly driving that axle could generate 2,000 ft-lb of torque while turning at say 1000 RPM. This means it has applied the 2,000 lb. force while moving the vehicle 1000 X the circumference of the wheel (1' x 3.14) or 3,140 ft. The work done is thus 2,000 lb. force applied over a distance of 3,140 feet, or 6,280,000 pound-feet of work in one minute.
Now we come to 'Power'. In the seventeen hundreds, James Watt, wanting to show how a steam engine could replace pit ponies for drawing ore up from mines, needed a standard. He observed that the average beast could work away all day at a rate of lifting (or applying a force of) 300 pounds a distance of 110 feet in each minute. Thus, his "Horse Power" equivalent became a rate of doing work of 300 x 110 or 33,000 pound feet per minute. Thus our Big Torque diesel, at 6,280,000 lb-ft./min, is producing 6,280,000/33,000 = 190 Horsepower.
Lastly, lets look at a very different scenario: Supposing instead of "Big Torque Diesel", with 2,000 lb-ft @ 1,000 RPM, we have a modern FI Grand Prix engine, making only 300 lb-ft but at 15,000 RPM: No torque, no acceleration, right? Wrong! "What a friend we have in gearing!" By hooking the Ferrari mill up to the axle through a 15:1 gear reduction, we turn the axle at the same 1,000 RPM, but the torque at the axle is now 300 x 15 = 4,500 lb-ft! Think that will accelerate the vehicle a little faster? It should, we are now applying work at the rate of 857 HP!
And that's why (as long as you have enough gears) Horsepower is King!
MB
Hey Louie,
This is an apparently simple question that will get you a ton of conflicting answers, but since I seem to be first up to the plate, you're gonna get the truth first, instead of eventually!
The 'force' in your equation is neither torque nor horsepower, it is pounds force, just as written. This force is the reaction at the tire contact points which is generated by the axle torque. If the axle is transmitting 2,000 lb-ft of torque and the tire diameter is 24", or a radius of one foot, then the force is 2,000/1 = 2,000 pounds. (If the curb weight of the car is one ton and it has adequate traction, this would accelerate it at one 'g'.)
The next level of confusion often encountered is: "OK, since that's the physics, it is obviously torque, not horsepower that accelerates a car, so we are way off base talking about HP!" Not quite! We next need to introduce the concept of "Work", which is force times the distance of movement. I can easily develop 2,000 ft-lb of torque (more, alas!) just by hanging off the end of a ten foot pipe attached to the axle, but I can't accelerate the car worth a damn. In fact, until/unless the car moves, I haven't done any 'work' at all. If I can move the car, it will be a slow and painful process, which brings us to the next concept:time.
A very big diesel engine directly driving that axle could generate 2,000 ft-lb of torque while turning at say 1000 RPM. This means it has applied the 2,000 lb. force while moving the vehicle 1000 X the circumference of the wheel (1' x 3.14) or 3,140 ft. The work done is thus 2,000 lb. force applied over a distance of 3,140 feet, or 6,280,000 pound-feet of work in one minute.
Now we come to 'Power'. In the seventeen hundreds, James Watt, wanting to show how a steam engine could replace pit ponies for drawing ore up from mines, needed a standard. He observed that the average beast could work away all day at a rate of lifting (or applying a force of) 300 pounds a distance of 110 feet in each minute. Thus, his "Horse Power" equivalent became a rate of doing work of 300 x 110 or 33,000 pound feet per minute. Thus our Big Torque diesel, at 6,280,000 lb-ft./min, is producing 6,280,000/33,000 = 190 Horsepower.
Lastly, lets look at a very different scenario: Supposing instead of "Big Torque Diesel", with 2,000 lb-ft @ 1,000 RPM, we have a modern FI Grand Prix engine, making only 300 lb-ft but at 15,000 RPM: No torque, no acceleration, right? Wrong! "What a friend we have in gearing!" By hooking the Ferrari mill up to the axle through a 15:1 gear reduction, we turn the axle at the same 1,000 RPM, but the torque at the axle is now 300 x 15 = 4,500 lb-ft! Think that will accelerate the vehicle a little faster? It should, we are now applying work at the rate of 857 HP!
And that's why (as long as you have enough gears) Horsepower is King!