Why do people get so caught up in horsepower?
04-15-2009, 10:07 PM
#1
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Why do people get so caught up in horsepower? I just do not understand why people talk about horsepower and not torque. I know that P=Tw so with engine speed and one you can find the other, but it just seems that Power can easily be manipulated by high engine speeds, so why talk about it? I don't know a single formula that uses power to calculated acceleration so why would you use power and weight to calculate et's? If I was going to go about it I would say,
Tractive Force=(Torque*Gear Ratios*Efficiencies)/Tire Radius
Then I would say
Acceleration=(Tractive Force-Drag Force-Rolling Resistance Force)/Mass
I just do not see where horsepower is needed. Is there something I am missing?
,Chase
Tractive Force=(Torque*Gear Ratios*Efficiencies)/Tire Radius
Then I would say
Acceleration=(Tractive Force-Drag Force-Rolling Resistance Force)/Mass
I just do not see where horsepower is needed. Is there something I am missing?
,Chase
04-16-2009, 05:34 AM
#4
FormerVendor
I just do not understand why people talk about horsepower and not torque. I know that P=Tw so with engine speed and one you can find the other, but it just seems that Power can easily be manipulated by high engine speeds, so why talk about it? I don't know a single formula that uses power to calculated acceleration so why would you use power and weight to calculate et's? If I was going to go about it I would say,
Tractive Force=(Torque*Gear Ratios*Efficiencies)/Tire Radius
Then I would say
Acceleration=(Tractive Force-Drag Force-Rolling Resistance Force)/Mass
I just do not see where horsepower is needed. Is there something I am missing?
,Chase
Tractive Force=(Torque*Gear Ratios*Efficiencies)/Tire Radius
Then I would say
Acceleration=(Tractive Force-Drag Force-Rolling Resistance Force)/Mass
I just do not see where horsepower is needed. Is there something I am missing?
,Chase
TQ can be easily "manipulated" by gearing etc. where as HP can not.
You have it backwards.
TQ means little on it's own at all and neither does RPM.
Put them together and then you can see what your rear wheel TQ is at some speed and gear and now you know how hard you can accelerate.
04-16-2009, 02:11 PM
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You are right that you don't understand.
TQ can be easily "manipulated" by gearing etc. where as HP can not.
You have it backwards.
TQ means little on it's own at all and neither does RPM.
Put them together and then you can see what your rear wheel TQ is at some speed and gear and now you know how hard you can accelerate.
TQ can be easily "manipulated" by gearing etc. where as HP can not.
You have it backwards.
TQ means little on it's own at all and neither does RPM.
Put them together and then you can see what your rear wheel TQ is at some speed and gear and now you know how hard you can accelerate.
,Chase
04-16-2009, 02:34 PM
#6
400HP at 12,000RPMs or 2,000RPMs is still simply 400HP. The motor running 2,000RPMs is running a lot more torque, yet realistically it will not make the same car accelerate faster. Basically, if two engines running the same power at different RPMs were geared to the same speed at their peak RPMs they would be applying the same torque to the wheels. This assumes no difference in frictional losses of course.
04-16-2009, 03:37 PM
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Average power to the wheels is what matters...
here was a quick story i made last time this topic came up... keep in mind power is a measurement of how often that torque is applied...
assume the lb/minute rate they are building the wall as thrust force moving your vehicle down the track. Becuase multiplying the torque by the amount of times you can apply it (rpm) will give you the resultant thrust force to move the vehicle (that is a very simple explanation)
here was a quick story i made last time this topic came up... keep in mind power is a measurement of how often that torque is applied...
You are trying to build a wall (aka drive down the drag strip) and you want to get it built as quickly as possible (ET). You have a few people to help (rpm) and they can each carry a certain size rock (torque/force) at a rate of one rock each per minute
Gasp.. you have one group that cant get along with the other to determine how to build this wall the fastest. One team of 3000 people big surly men think they can get it done faster than the team of 7000 women. The team of men cary big heavy rocks (350lbs) and each one can move one rock every minute. The women are weaker so they carry slightly smaller rocks (262 lbs) and they also can each get one rock placed every minute.
Which team is building at a faster pace?
A: The team of men are building their wall at a rate of 1,050,000 lbs per minute, and the team of women are building their wall at a rate of 1,834,000 lbs per minute. This is calculated by multiplying the weight of the rocks by the number of rocks being placed
Moral of story.. the size of your rocks dont matter unless you have alot of helping hands to build the wall. A single 4000lb rock is impressive but if nobody moves it.. no wall will be built
Gasp.. you have one group that cant get along with the other to determine how to build this wall the fastest. One team of 3000 people big surly men think they can get it done faster than the team of 7000 women. The team of men cary big heavy rocks (350lbs) and each one can move one rock every minute. The women are weaker so they carry slightly smaller rocks (262 lbs) and they also can each get one rock placed every minute.
Which team is building at a faster pace?
A: The team of men are building their wall at a rate of 1,050,000 lbs per minute, and the team of women are building their wall at a rate of 1,834,000 lbs per minute. This is calculated by multiplying the weight of the rocks by the number of rocks being placed
Moral of story.. the size of your rocks dont matter unless you have alot of helping hands to build the wall. A single 4000lb rock is impressive but if nobody moves it.. no wall will be built
Last edited by DanO; 04-16-2009 at 03:42 PM.
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04-16-2009, 04:35 PM
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I am sorry I just assumed that this was engineering related since in one of my upper level engineering classes we spent a chapter or two on simulating acceleration.
Last edited by Silver2000WS-6; 04-17-2009 at 09:11 AM.
04-16-2009, 04:38 PM
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Average power to the wheels is what matters...
here was a quick story i made last time this topic came up... keep in mind power is a measurement of how often that torque is applied...
assume the lb/minute rate they are building the wall as thrust force moving your vehicle down the track. Becuase multiplying the torque by the amount of times you can apply it (rpm) will give you the resultant thrust force to move the vehicle (that is a very simple explanation)
here was a quick story i made last time this topic came up... keep in mind power is a measurement of how often that torque is applied...
assume the lb/minute rate they are building the wall as thrust force moving your vehicle down the track. Becuase multiplying the torque by the amount of times you can apply it (rpm) will give you the resultant thrust force to move the vehicle (that is a very simple explanation)
,Chase
04-17-2009, 09:35 AM
#13
If your asking this question and you are in a upper engineering class you need to take a step back.
04-17-2009, 11:24 AM
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04-17-2009, 03:25 PM
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yeah.. thats pretty easy
F=ma
use power, engine rpm, gear ratio, vehicle weight and wheel speed..
the part that you are missing is that more power allows more torque multiplication via gear ratio
for a given vehicle speed your tire rpm is fixed, but if you can have your engine spinning 7000 rpm putting out a high toque level and then multiply it by a higher gear ratio, your tractive force will be higher than an engine with the same high torque at 3000 rpm cause they cant multiply the torque as much through the transmission.
this concept should be easy to grasp by an upper level engineering student..
F=ma
use power, engine rpm, gear ratio, vehicle weight and wheel speed..
the part that you are missing is that more power allows more torque multiplication via gear ratio
for a given vehicle speed your tire rpm is fixed, but if you can have your engine spinning 7000 rpm putting out a high toque level and then multiply it by a higher gear ratio, your tractive force will be higher than an engine with the same high torque at 3000 rpm cause they cant multiply the torque as much through the transmission.
this concept should be easy to grasp by an upper level engineering student..
Last edited by DanO; 04-17-2009 at 03:46 PM.
04-17-2009, 07:46 PM
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yeah.. thats pretty easy
F=ma
use power, engine rpm, gear ratio, vehicle weight and wheel speed..
the part that you are missing is that more power allows more torque multiplication via gear ratio
for a given vehicle speed your tire rpm is fixed, but if you can have your engine spinning 7000 rpm putting out a high toque level and then multiply it by a higher gear ratio, your tractive force will be higher than an engine with the same high torque at 3000 rpm cause they cant multiply the torque as much through the transmission.
this concept should be easy to grasp by an upper level engineering student..
F=ma
use power, engine rpm, gear ratio, vehicle weight and wheel speed..
the part that you are missing is that more power allows more torque multiplication via gear ratio
for a given vehicle speed your tire rpm is fixed, but if you can have your engine spinning 7000 rpm putting out a high toque level and then multiply it by a higher gear ratio, your tractive force will be higher than an engine with the same high torque at 3000 rpm cause they cant multiply the torque as much through the transmission.
this concept should be easy to grasp by an upper level engineering student..
Yeah you are going about it the same way as I did earlier but you using power and dividing by engine speed to get the torque # I use. I see what you mean about if an engine spins up really high you can stuff in some low gears to bump up the tractive force which obviously makes sense.
04-17-2009, 08:24 PM
#18
No that statement does not make any sense. There is so much mis-information regarding this. So many people say torque is what gets your going and power is what keeps you going. Anyone who says that just simply does not understand what they are talking about. I believe that phrase comes from people associating high torque numbers with wide powerbands. Like stated above, acceleration is about average power output. So obviously a 900hp motor with a wide powerband is going to generally accelerate better than a peaky 900hp motor.
04-18-2009, 10:12 AM
#19
Here's the deal. I can deliver 100 ft lbs of torque to a lug nut, or a wrench or whatever. What I cannot do is deliver a 100 ft lbs of torque at 7000 RPM. Do you see the difference in power and torque now?
The wall analogy is a good one above. There is no equation that says a car will accelerate with so much power. If you are a higher level engineer you should have already had a class for "dynamic systems" This would be a good class to solve a problem like this. You would need a little differential equations and to build a simulation to see how a car is going to accelerate.
04-18-2009, 01:05 PM
#20
FormerVendor
Actually if you know the weight and the power you can use several equations to find the acceleration very easily.
If you only have a static torque value though you can not figure anything out at all as far as acceleration.
If you only have a static torque value though you can not figure anything out at all as far as acceleration.